EP2284828A1 - Liquid crystal display device - Google Patents

Abstract

A liquid crystal display device according to the present invention includes a plurality of pixels, each including first and second subpixels. When a predetermined grayscale tone is displayed continuously through four or more consecutive even number of vertical scanning periods, the first and second subpixels have different luminances in at least two of the even number of vertical scanning periods, first polarity periods that are included in the vertical scanning periods and that maintain a first polarity are as long as second polarity periods that are also included in the vertical scanning periods and that maintain a second polarity for each of the first and second subpixels, and in each of the first and second polarity periods, the difference between the average of effective voltages applied to the liquid crystal layer of the first subpixel and that of effective voltages applied to the liquid crystal layer of the second subpixel is substantially equal to zero.

Description

TECHNICAL FIELD
• The present invention relates to a liquid crystal display device and more particularly relates to a liquid crystal display device that can reduce the viewing angle dependence of the γ characteristic thereof.
BACKGROUND ART
• A liquid crystal display (LCD) is a flat-panel display that has a number of advantageous features including high resolution, drastically reduced thickness and weight, and low power dissipation. The LCD market has been rapidly expanding recently as a result of tremendous improvements in its display performance, significant increases in its productivity, and a noticeable rise in its cost effectiveness over competing technologies.
• A twisted-nematic (TN) mode liquid crystal display device, which used to be used extensively in the past, is subjected to an alignment treatment such that the major axes of its liquid crystal molecules, exhibiting positive dielectric anisotropy, are substantially parallel to the respective principal surfaces of upper and lower substrates and are twisted by about 90 degrees in the thickness direction of the liquid crystal layer between the upper and lower substrates. When a voltage is applied to the liquid crystal layer, the liquid crystal molecules change their orientation directions into a direction that is parallel to the electric field applied. As a result, the twisted orientation disappears. The TN mode liquid crystal display device utilizes variation in the optical rotatory characteristic of its liquid crystal layer due to the change of orientation directions of the liquid crystal molecules in response to the voltage applied, thereby controlling the quantity of light transmitted.
• The TN mode liquid crystal display device allows a broad enough manufacturing margin and achieves high productivity. However, the display performance (e.g., the viewing angle characteristic, in particular) thereof is not fully satisfactory. More specifically, when an image on the screen of the TN mode liquid crystal display device is viewed obliquely, the contrast ratio of the image decreases significantly. In that case, even an image, of which the grayscales ranging from black to white are clearly observable when the image is viewed straightforward, loses much of the difference in luminance between those grayscales when viewed obliquely. Furthermore, the grayscale characteristic of the image being displayed thereon may sometimes invert itself. That is to say, a portion of an image, which looks darker when viewed straight, may look brighter when viewed obliquely. This is a so-called "grayscale inversion phenomenon".
• To improve the viewing angle characteristic of such a TN mode liquid crystal display device, an inplane switching (IPS) mode liquid crystal display device, a multi-domain vertical aligned (MVA) mode liquid crystal display device, an axisymmetric aligned (ASM) mode liquid crystal display device, and other types of liquid crystal display devices were developed recently. Liquid crystal displays employing any of the novel modes described above (wide viewing angle modes) solve the concrete problems with viewing angle characteristics, specifically, the problems that the display contrast ratio decreases considerably or the grayscales invert when the display surface of the display is viewed obliquely.
• Although the display qualities of LCDs have been further improved nowadays, a viewing angle characteristic problem in a different phase has arisen just recently. Specifically, the γ characteristic of LCDs would vary with the viewing angle. That is to say, the γ characteristic when an image on the screen is viewed straight is different from the characteristic when it is viewed obliquely. As used herein, the characteristic" refers to the grayscale dependence of display luminance. That is why if the γ characteristic when the image is viewed straight is different from the characteristic when the same image is viewed obliquely, then it means that the grayscale display state changes according to the viewing direction. This is a serious problem particularly when a still picture such as a photo is presented or when a TV program is displayed.
• According to a known method, such viewing angle dependence of the γ characteristic can be reduced by providing two or more subpixels for each single pixel and by making the luminance of one of the two subpixels different from that of the other when a moderate luminance is displayed (see Patent Documents Nos. 1 and 2, for example).
• Specifically, the liquid crystal display device disclosed in Patent Document No. 1 applies a different effective voltage to the liquid crystal layer of a second subpixel from the one applied to the liquid crystal layer of a first subpixel when a moderate luminance is displayed, thereby making the luminances of the first and second subpixels different from each other and reducing the viewing angle dependence of the γ characteristic. The transmittance of the liquid crystal layer changes with the absolute value of the effective voltage irrespective of the direction of the electric field applied to the liquid crystal layer (i.e., the direction of the electric line of force). Thus, the liquid crystal display device disclosed in Patent Document No. 1 inverts the direction of the electric field applied to the liquid crystal layer alternately every vertical scanning period, thereby flattening the uneven distribution of DC levels and overcoming residual image and other reliability-related problems.
• Meanwhile, the liquid crystal display device disclosed in Patent Document No. 2 inverts the brightness levels of first and second subpixels every vertical scanning period (e.g., makes the luminance of the first subpixel higher than that of the second subpixel in a first vertical scanning period but makes the luminance of the second subpixel higher than that of the first subpixel in a second vertical scanning period). In addition, the device also inverts the direction of the electric field applied to the liquid crystal layer every vertical scanning period, too. If one of multiple subpixels were always bright, then the image on the screen would look non-smooth. However, the liquid crystal display device disclosed in Patent Document No. 2 minimizes such non-smoothness of the image on the screen by inverting the brightness levels of the first and second subpixels one vertical scanning period after another.
• It should be noted that such a display or driving method that reduces the viewing angle dependence of the γ characteristic by making the luminances of multiple subpixels different from each other will be referred to herein as a "multi-pixel display", a "multi-pixel drive", an "area grayscale display" or an "area grayscale drive".
• Patent Document No. 1: Japanese Patent Application Laid-Open Publication No.2004-62146
• Patent Document No. 2: Japanese Patent Application Laid-Open Publication No.2003-295160 (corresponding to United States Patent No.6,958,791)
DISCLOSURE OF INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION
• In the liquid crystal display device disclosed in Patent Document No. 1, as the luminance of the first subpixel is always higher than that of the second subpixel when a moderate luminance is displayed, the difference in brightness level between those subpixels may be quite sensible and the image presented may sometimes look non-smooth.
• On the other hand, in the liquid crystal display device disclosed in Patent Document No. 2, as the direction of the electric field applied to the liquid crystal layer and the brightness levels of the subpixels are inverted every vertical scanning period, the direction of the electric field applied to the liquid crystal layer is always the same when one of the two subpixels is brighter than the other subpixel.
• For example, in the liquid crystal display device disclosed in Patent Document No. 2, if the absolute value of the effective voltage applied to the first subpixel is greater than that of the effective voltage applied to the second subpixel to make the first subpixel look brighter than the second one in a vertical scanning period, the electric field applied to the liquid crystal layer is directed from a subpixel electrode toward a counter electrode. The electric field with such a direction is supposed to have a first polarity. In the next vertical scanning period, as the absolute value of the effective voltage applied to the second subpixel becomes greater than that of the effective voltage applied to the first subpixel to make the second subpixel look brighter than the first one, the electric field applied to the liquid crystal layer is directed from the counter electrode toward the subpixel electrode. The electric field with such a direction is supposed to have a second polarity. In the next vertical scanning period, as the absolute value of the effective voltage applied to the first subpixel becomes greater than that of the effective voltage applied to the second subpixel to make the first subpixel look brighter than the second subpixel, the electric field has the first polarity. And in the next vertical scanning period, as the absolute value of the effective voltage applied to the second subpixel becomes greater than that of the effective voltage applied to the first subpixel to make the second subpixel look brighter than the first one, the electric field has the second polarity.
• In this manner, in the liquid crystal display device disclosed in Patent Document No. 2, the electric field always has the first polarity when the effective voltage applied to the first subpixel has the greater absolute value and always has the second polarity when the effective voltage applied to the second subpixel has the greater absolute value. That is why the average effective voltages applied to the first and second subpixels have the first and second polarities, respectively.
• In a normal liquid crystal display device, if the same image continues to be presented for a long time with the average of the voltages applied to a pixel kept unequal to zero (i.e., with a DC voltage component left in the voltage applied to the pixel), then that image that has been presented for a long time will still remain on the screen even when the images on the screen are changed after that. That is to say, a so-called "residual image" phenomenon will occur. To avoid such a residual image phenomenon, a normal liquid crystal display device performs an AC drive on (i.e., applies voltages with two different polarities but with the same absolute value to) pixels, thereby making the average of the voltages applied to the liquid crystal layer equal to zero. Furthermore, if the average of the voltages applied does not become equal to zero even by the AC drive, then the normal liquid crystal display device further regulates the counter voltage, thereby setting the average voltage equal to zero.
• In the liquid crystal display device disclosed in Patent Document No. 2, however, the respective effective voltages applied to the first and second subpixels have mutually different averages. That is why even if the counter voltage is regulated, only the average voltage applied to one of the two subpixels can be made equal to zero and the average voltage applied to the other subpixel cannot be zero. In that case, the residual image phenomenon will occur in the subpixel with the non-zero average voltage. As a result, the residual image phenomenon cannot be eliminated from the overall display device. Consequently, in the liquid crystal display device disclosed in Patent Document No. 2, not both of the average voltages applied to the first and second subpixels can be equal to zero, and therefore, the residual image and other reliability-related problems should arise.
• In order to overcome the problems described above, the present invention has an object of providing a liquid crystal display device that can resolve those reliability-related problems such as non-smoothness of the image on the screen and the residual image phenomenon.
MEANS FOR SOLVING THE PROBLEMS
• A liquid crystal display device according to the present invention includes a plurality of pixels, each including a first subpixel and a second subpixel. Each of the first and second subpixels includes: a counter electrode; a subpixel electrode; and a liquid crystal layer interposed between the counter electrode and the subpixel electrode. The subpixel electrodes of the first and second subpixels are provided separately from each other as first and second subpixel electrodes, respectively, while the first and second subpixels share the same counter electrode with each other. When a predetermined grayscale tone is displayed continuously through four or more consecutive even number of vertical scanning periods, the first and second subpixels have mutually different luminances in at least two of the even number of vertical scanning periods, first polarity periods that are included in the even number of vertical scanning periods and that maintain a first polarity are as long as second polarity periods that are also included in the even number of vertical scanning periods and that maintain a second polarity for each of the first and second subpixels, and in each of the first and second polarity periods, the difference between the average of effective voltages applied to the liquid crystal layer of the first subpixel and that of effective voltages applied to the liquid crystal layer of the second subpixel is substantially equal to zero.
• In one preferred embodiment, if the effective voltages applied to the respective liquid crystal layers of the first and second subpixels of each said pixel are represented by VLspa and VLspb, respectively, then two of the four consecutive vertical scanning periods are the first polarity periods and the other two vertical scanning periods are the second polarity periods. In at least one of the first polarity periods and the second polarity periods, one of the two vertical scanning periods thereof satisfies | VLspa | > | VLspb | and the other vertical scanning period satisfies | VLspa | < | VLspb | .
• In another preferred embodiment, if the effective voltages applied to the respective liquid crystal layers of the first and second subpixels of each said pixel are represented by VLspa and VLspb, respectively, then two of the four consecutive vertical scanning periods are the first polarity periods and the other two vertical scanning periods are the second polarity periods. In at least one of the first polarity periods and the second polarity periods, the | VLspa | and | VLspb | values of one of the two vertical scanning periods thereof are equal to those of the other vertical scanning period.
• In this particular preferred embodiment, of the four vertical scanning periods, the number of vertical scanning periods that satisfy | VLspa | > |VLspb |is equal to that of vertical scanning periods that satisfy | VLspb | < | VLspb |.
• In still another preferred embodiment, the plurality of the pixels are arranged in column and row directions so as to form a matrix pattern, and in each of the plurality of the pixels, the first and second subpixels are arranged in the column direction.
• In yet another preferred embodiment, in each of the plurality of the pixels, voltages applied to the first and second subpixel electrodes change as voltages on their associated storage capacitor lines vary.
• In this particular preferred embodiment, in each of the plurality of the pixels, a voltage on a storage capacitor line associated with the first subpixel electrode and a voltage on a storage capacitor line associated with the second subpixel electrode change mutually differently.
• In yet another preferred embodiment, a voltage applied to the second subpixel electrode of a particular one of the plurality of the pixels and a voltage applied to the first subpixel electrode of another pixel that is adjacent to the particular pixel in the column direction change as the voltage on their common storage capacitor line varies.
• In an alternative preferred embodiment, a voltage applied to the second subpixel electrode of a particular one of the plurality of the pixels and a voltage applied to the first subpixel electrode of another pixel that is adjacent to the particular pixel in the column direction change as voltages on their associated storage capacitor lines vary.
• In yet another preferred embodiment, in each of the plurality of the pixels, the first and second subpixel electrodes are connected to the same signal line by way of their associated switching element.
• In yet another preferred embodiment, in each of the plurality of the pixels, the first and second subpixel electrodes are respectively connected to first and second signal lines by way of first and second switching elements, respectively.
• In yet another preferred embodiment, in each of the first and second polarity periods, one of the two vertical scanning periods satisfies |VLspa|>|VLspb| and the other vertical scanning period satisfies |VLspa|<|VLspb|.
• In yet another preferred embodiment, in each of the plurality of the pixels, |VLspa| and |VLspb| switch their magnitudes every vertical scanning period and the polarities of the first and second subpixels are inverted every other vertical scanning period.
• In yet another preferred embodiment, the frame frequency is 60 Hz.
• In yet another preferred embodiment, in each of the plurality of the pixels, |VLspa|and |VLspb| switch their magnitudes every other vertical scanning period and the polarities of the first and second subpixels are inverted every vertical scanning period.
• In yet another preferred embodiment, the frame frequency is 120 Hz.
• In yet another preferred embodiment, in each of the plurality of the pixels, |VLspa| and |VLspb| switch their magnitudes every other vertical scanning period and the polarities of the first and second subpixels are inverted every other vertical scanning period. |VLspb| and |VLspb| switch their magnitudes non-synchronously with the inversion of the polarities of the first and second subpixels.
• In yet another preferred embodiment, in either the first polarity periods or the second polarity periods, one of the two vertical scanning periods satisfies |VLspa|>|VLspb | and the other vertical scanning period satisfies |VLspa|< |VLspb|. In the other polarity periods, VLspa is equal to VLspb in each of the two vertical scanning periods.
• In this particular preferred embodiment, voltages on storage capacitor lines associated with the first and second subpixel electrodes change between a first level, a second level that is higher than the first level, and a third level that is higher than the second level.
• In yet another preferred embodiment, the first and second subpixel electrodes have the same display area.
EFFECTS OF THE INVENTION
• The present invention provides a liquid crystal display device that can minimize the occurrence of reliability problems such as non-smoothness of image displayed or residual images.
BRIEF DESCRIPTION OF DRAWINGS
• FIG. 1 is a schematic representation illustrating the structure of a liquid crystal display device as a first preferred embodiment of the present invention.
• FIG. 2 is a schematic block diagram illustrating a liquid crystal panel for the liquid crystal display device of the first preferred embodiment.
• FIG.3(a) is a schematic plan view illustrating a single pixel in the liquid crystal display device of the first preferred embodiment andFIG.3(b) is a schematic cross-sectional view illustrating a single subpixel thereof.
• FIG.4 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in a conventional liquid crystal display device, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
• FIG. 5 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in another conventional liquid crystal display device, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
• FIG. 6 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in the liquid crystal display device as the first preferred embodiment of the present invention, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
• FIG. 7 is a schematic representation illustrating an exemplary pixel structure for the liquid crystal display device of the first preferred embodiment.
• FIG. 8 is an equivalent circuit diagram of a single pixel in the liquid crystal display device of the first preferred embodiment.
• FIG. 9 shows exemplary waveforms of voltages that are applied to drive the liquid crystal display device of the first preferred embodiment.
• FIG.10 shows a relation between the effective voltages applied to the respective liquid crystal layers of subpixels in the liquid crystal display device of the first preferred embodiment.
• FIGS.11 (a) and11(b) show the γ characteristics of the liquid crystal display device of the first preferred embodiment at a right 60 degree viewing angle and at an upper right 60 degree viewing angle, respectively.
• FIG.12 shows exemplary waveforms of various voltages to be applied over a number of vertical scanning periods to the liquid crystal display device of the first preferred embodiment.
• FIG.13 shows an exemplary equivalent circuit diagram of the liquid crystal display device of the first preferred embodiment.
• FIG.14 is a schematic representation illustrating the arrangement, brightness levels and polarities of multiple subpixels in the liquid crystal display device of the first preferred embodiment.
• FIG.15 shows exemplary waveforms of various voltages to be applied to the liquid crystal display device of the first preferred embodiment.
  FIG.16 shows exemplary waveforms of various voltages to be applied over a number of vertical scanning periods to the liquid crystal display device of the first preferred embodiment.
  FIG.17 is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the liquid crystal display device of the first preferred embodiment.
  Portions(a) and(b) ofFIG.18 show exemplary waveforms of various voltages to be applied over a number of vertical scanning periods to the liquid crystal display device of the first preferred embodiment.
  Portions(a) to(c) ofFIG.19 show exemplary waveforms of various voltages to be applied over a number of vertical scanning periods to the liquid crystal display device of the first preferred embodiment.
  FIG.20 shows exemplary waveforms of various voltages to be applied over a number of vertical scanning periods to the liquid crystal display device of the first preferred embodiment.
  FIG.21 shows exemplary waveforms of various voltages to be applied over a number of vertical scanning periods to the liquid crystal display device of the first preferred embodiment.
  FIG.22 is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the liquid crystal display device of the first preferred embodiment.
  FIG.23 shows an exemplary equivalent circuit diagram of the liquid crystal display device of the first preferred embodiment.
  FIG.24 shows exemplary waveforms of various voltages to be applied to the liquid crystal display device of the first preferred embodiment.
  FIG.25 is a schematic representation illustrating an exemplary pixel structure for the liquid crystal display device of the first preferred embodiment.
  FIG.26 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in a liquid crystal display device as a second preferred embodiment of the present invention, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
  FIG.27 is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the liquid crystal display device of the second preferred embodiment.
  FIG.28 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in a liquid crystal display device as a third preferred embodiment of the present invention, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
  FIG.29 shows exemplary waveforms of various voltages to be applied to the liquid crystal display device of the third preferred embodiment.
  FIG.30 is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the liquid crystal display device of the third preferred embodiment.
  FIG.31 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in a liquid crystal display device as a fourth preferred embodiment of the present invention, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
  FIG.32 is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the liquid crystal display device of the fourth preferred embodiment.
  FIG.33 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in a liquid crystal display device as a fifth preferred embodiment of the present invention, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
  FIG.34 shows exemplary waveforms of various voltages to be applied to the liquid crystal display device of the fifth preferred embodiment.
  FIG.35 is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the liquid crystal display device of the fifth preferred embodiment.
  FIG.36 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in a liquid crystal display device as a sixth preferred embodiment of the present invention, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and (c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
  FIG.37 is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the liquid crystal display device of the sixth preferred embodiment.
  FIG.38 schematically shows how first and second subpixels change their brightness levels, polarities and effective voltages in a liquid crystal display device as a seventh preferred embodiment of the present invention, wherein portion(a) schematically shows how the first and second subpixels change their brightness levels and polarities and portions(b) and(c) schematically show how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change.
• FIG.39A is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in one frame for the liquid crystal display device of the seventh preferred embodiment.
• FIG.39B is a schematic representation illustrating the brightness levels and polarities of respective subpixels and the first change of storage capacitor voltages in respective vertical scanning periods of each subpixel in the next frame for the liquid crystal display device of the seventh preferred embodiment.
• FIG.40 shows exemplary waveforms of various voltages to be applied to the liquid crystal display device of the seventh preferred embodiment.
DESCRIPTION OF REFERENCE NUMERALS

10

pixel

10a, 10b

subpixel

13

liquid crystal layer

17

counter electrode

18a, 18b

subpixel electrode

100

liquid crystal display device

100A

liquid crystal panel

BEST MODE FOR CARRYING OUT THEINVENTIONEMBODIMENT 1
• Hereinafter, a first preferred embodiment of a liquid crystal display device according to the present invention will be described with reference to the accompanying drawings.
• First of all, the configuration of a liquidcrystal display device 100 as the first preferred embodiment of the present invention will be outlined with reference toFIGS. 1 to 3.FIG. 1 illustrates the liquidcrystal display device100 of this preferred embodiment. Theliquid crystal panel100A of the liquidcrystal display device100 includes adisplay section110 in which a number of pixels are arranged in columns and rows to define a matrix pattern and adriver120 for driving thedisplay section110 as shown inFIG. 2. In thedisplay section110, each pixel includes a liquid crystal layer and a plurality of electrodes for applying a voltage to the liquid crystal layer. Thedriver120 generates a drive signal based on an input video signal.
• FIG.3(a) is a schematic plan view illustrating the electrode structure of a single pixel, whileFIG.3(b) is a schematic cross-sectional view of a single subpixel as viewed on theplane3B-3B' shown inFIG.3(a). As shown inFIG.3(a), eachpixel10 includes first andsecond subpixels 10a and10b that are arranged in the column direction. As shown inFIG.3(b), thefirst subpixel10a includes aliquid crystal layer13, afirst subpixel electrode18a, and acounter electrode17 that faces thefirst subpixel electrode18a with theliquid crystal layer13 interposed between them. AlthoughFIG.3(b) illustrates the configuration of only thefirst subpixel10a, thesecond subpixel 10b has the same configuration as the one illustrated inFIG.3(b). Thecounter electrode17 is typically provided as a single common electrode for everypixel10. In the liquidcrystal display device100 of this preferred embodiment, mutually different voltages are applicable to the first andsecond subpixel electrodes18a and18b, thus making the effective voltage applied to the liquid crystal layer of thefirst subpixel10a different from the one applied to that of thesecond subpixel10b.
• Next, it will be described with reference toFIGS.4 through6 and in comparison with the liquid crystal display devices disclosed in Patent Documents Nos. 1 and 2 how the brightness levels of the subpixels and the directions of the electric field (or electric line of force) change in the liquidcrystal display device100 of this preferred embodiment. In the following description, each pixel is supposed to display a predetermined grayscale tone for several frames on end for the sake of simplicity.
• First of all, it will be described with reference toFIG.4 how the brightness levels of the subpixels and the directions of the electric field change and how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change in the liquid crystal display device disclosed in Patent Document No. 1. In portion(a) ofFIG.4, thereference numerals1 through6 denote respective vertical scanning periods. As used herein, one "vertical scanning period" is defined to be an interval between a point in time when one scan line is selected to write a display signal voltage and a point in time when that scan line is selected to write the next display signal voltage. Also, each of one frame period of a non-interlaced drive input video signal and one field period of an interlaced drive input video signal will be referred to herein as "one vertical scanning period of the input video signal". Normally, one vertical scanning period of a liquid crystal display device corresponds to one vertical scanning period of the input video signal. In the example to be described below, one vertical scanning period of the liquid crystal panel is supposed to correspond to that of the input video signal for the sake of simplicity. However, the present invention is in no way limited to that specific preferred embodiment. The present invention is also applicable to a so-called "2x drive" with a vertical scanning frequency of 120 Hz in which two vertical scanning periods of the liquid crystal panel (that lasts 2×1/120 sec, for example) are allocated to one vertical scanning period of the input video signal (that lasts 1/60 sec, for example). Also, in this example, the lengths of the respective vertical scanning periods are supposed to be equal to each other. Furthermore, in each vertical scanning period, the interval between a point in time when one scan line is selected and a point in time when the next scan line is selected will be referred to herein as one horizontal scanning period (1H).
• In portion(a) ofFIG.4, the upper and lower rectangles represent the first and second subpixels, respectively. Of these two subpixels, the one with the higher luminance is plain, while the other with the lower luminance is shadowed. Also, in portion(a) ofFIG. 4, "+" and "-" represent the polarities of the display signal voltages when the associated scan line is selected with respect to the common voltage applied to the counter electrode. In this case, "+" indicates that the potential at the first and second subpixel electrodes is higher than the one at the counter electrode and that the electric field is directed from the subpixel electrodes toward the counter electrode. On the other hand, "-" indicates that the potential at the first and second subpixel electrodes is lower than the one at the counter electrode and that the electric field is directed from the counter electrode toward the subpixel electrodes. In the following description, "+" and "-" will be referred to herein as a "first polarity" and a "second polarity", respectively, and will also be collectively referred to herein as "polarities". Also, a period with the "+" polarity and a period with the "-" polarity will be referred to herein as a "first polarity period" and a "second polarity period", respectively.
• As shown in portion(a) of.FIG.4, the first, third and fifth periods are first polarity periods, the second, fourth and sixth periods are second polarity periods, and the polarity inverts every vertical scanning period in the liquid crystal display device disclosed in Patent Document No. 1. As also shown in portion(a) ofFIG.4, in any of the first through sixth periods, the first subpixel has a higher luminance than the second subpixel in the device of Patent Document No. 1.
• Portions(b) and (c) ofFIG.4 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods in the liquid crystal display device of Patent Document No. 1. The levels of these voltages are indicated by the bold lines. The effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels are the effective values of the differences between the voltages applied to the first and second subpixel electrodes and the voltageVc applied to the counter electrode. In this example, the voltageVc applied to the counter electrode is shown as being constant. Although not shown in portions(b) and(c) ofFIG.4, the voltages applied to the respective liquid crystal layers of the first and second subpixels may also be changed within the same vertical scanning period by varying the voltage on the storage capacitor line as disclosed in Patent Document No. 1.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|) For that reason, as shown in portion(a) ofFIG.4, the first period is a first polarity period and the first subpixel is brighter than the second subpixel. However, on the transition from the first period into the second period, the effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels change. In the second period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|) For that reason, as shown in portion(a) ofFIG. 4, the second period is a second polarity period and the first subpixel is brighter than the second subpixel.
• From the third period on, the same brightness levels and polarities of the first and second subpixels as those of the first and second periods just appear repeatedly. Consequently, in the liquid crystal display device disclosed in Patent Document No. 1, the luminance of the first subpixel is always higher than that of the second subpixel, the difference in brightness level between those subpixels is quite sensible, and the image on the screen looks non-smooth as can be seen from portion(a) ofFIG.4.
• Next, it will be described with reference toFIG. 5 how the brightness levels of the subpixels, the directions of the electric field, and the effective voltages applied to the respective liquid crystal layers of the first and second subpixel change in the liquid crystal display device disclosed in Patent Document No. 2.
• As shown in portion(a) ofFIG.5, in the liquid crystal display device disclosed in Patent Document No. 2, the first, third and fifth periods are also first polarity periods, the second, fourth and sixth periods are second polarity periods, and the polarity inverts every vertical scanning period. Meanwhile, in the liquid crystal display device of Patent Document No. 2, the luminance of the first subpixel is higher than that of the second subpixel in the first, third and fifth periods but the luminance of the second subpixel is higher than that of the first subpixel in the second, fourth and sixth periods.
• Portions(b) and(c) ofFIG.5 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. Although not shown in portions(b) and(c) ofFIG.5, the voltages applied to the respective liquid crystal layers of the first and second subpixels may also be changed within the same vertical scanning period by varying the voltage on the storage capacitor line as disclosed in Patent Document No. 1.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.5, the first period is a first polarity period and the first subpixel is brighter than the second subpixel. However, on the transition from the first period into the second period, the effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels change. In the second period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspb|<|VLspb|). For that reason, as shown in portion(a) ofFIG.5, the second period is a second polarity period and the second subpixel is brighter than the first subpixel.
• From the third period on, the same brightness levels and polarities of the first and second subpixels as those of the first and second periods just appear repeatedly. In the liquid crystal display device disclosed in Patent Document No. 2, since not only the polarity but also the brightness levels of the subpixels are inverted every vertical scanning period, the first subpixel is sometimes brighter, but sometimes less bright, than the second subpixel unlike the liquid crystal display device disclosed in Patent Document No. 1. Consequently, the degree of non-smoothness on the screen can be reduced. In the liquid crystal display device disclosed in Patent Document No. 2, however, the period in which the first subpixel is brighter than the second subpixel is always the first polarity period and the period in which the second subpixel is brighter than the first subpixel is always the second polarity period. That is why as can be seen from portions(b) and(c) ofFIGS.5, the average of the effective voltages VLspa applied to the liquid crystal layer of the first subpixel over multiple vertical scanning periods (e.g., the first through fourth periods) is higher than the voltage Vc applied to the counter electrode, and the average of the effective voltagesVLspb applied to the liquid crystal layer of the second subpixel over multiple vertical scanning periods (e.g., the first through fourth periods) is lower than the voltageVc applied to the counter electrode. Thus, in the liquid crystal display device disclosed in Patent Document No. 2, the uneven distribution of DC levels among the respective subpixels still remains to produce residual image and other reliability-related problems.
• Next, it will be described with reference toFIG.6 how the brightness levels of the subpixels, the directions of the electric field, and the effective voltages applied to the respective liquid crystal layers of the first and second subpixel change in the liquidcrystal display device100 of this preferred embodiment.
• As shown in portion(a) ofFIG.6, in the liquidcrystal display device100 of this preferred embodiment, the first, second, fifth and sixth periods are first polarity periods, while the third and fourth periods are second polarity periods. As described above, the first polarity period is a period in which the voltages applied to the first and second subpixel electrodes are higher than the one applied to the counter electrode, while the second polarity period is a period in which the voltages applied to the first and second subpixel electrodes are lower than the one applied to the counter electrode. Look at four consecutive vertical scanning periods, and it can be seen that two out of the four periods are first polarity periods and the other two are second polarity periods. For example, in the first through fourth periods shown in portion(a) ofFIG.6, the first and second periods are first polarity periods and the third and fourth periods are second polarity periods.
• Portions(b) and(c) ofFIG.6 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. In this preferred embodiment, the voltages applied to the respective liquid crystal layers of the first and second subpixels may also be changed within the same vertical scanning period by varying the voltage on the storage capacitor line just as disclosed in Patent Documents Nos. 1 and 2. Also, since the voltageVc applied to the counter electrode is used as a reference voltage in portions(b) and(c) ofFIG.6, the voltageVc applied to the counter electrode is illustrated as being constant irrespective of time. However, the voltageVc applied to the counter electrode may also vary with time.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|), For that reason, as shown in portion(a) ofFIG.6, the first period is a first polarity period and the first subpixel is brighter than the second subpixel.
• However, on the transition from the first period into the second period, the effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels change. In the second period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer. of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (|VLspb|<|VLspb|) For that reason, as shown in portion(a) ofFIG.6, the second period is a first polarity period and the second subpixel is brighter than the first subpixel.
• In the third period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.6, the third period is a second polarity period and the first subpixel is brighter than the second subpixel.
• In the fourth period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspb|<|VLspb|). For that reason, as shown in portion(a) ofFIG.6, the fourth period is a second polarity period and the second subpixel is brighter than the first subpixel. After that, from the fifth period on, the brightness levels and polarities of the first and second subpixels just repeat those of the first and second subpixels in the first through fourth periods.
• As described above, in the liquidcrystal display device100 of this preferred embodiment, two out of four consecutive vertical scanning periods are first polarity periods, one of which satisfies |VLspa|>|VLspb| (e.g., the first period) and the other of which satisfies |VLspa|<| VLspb| (e.g., the second period). The two other ones of the four consecutive vertical scanning periods are second polarity periods, one of which satisfies |VLspa|>|VLspb| (e.g., the third period) and the other of which satisfies |VLspa|<| VLspb| (e.g., the fourth period). As can be seen from portion(a) ofFIG.6, in the liquidcrystal display device 100 of this preferred embodiment, the brightness levels of the subpixels are inverted every vertical scanning period and the polarity is inverted every other vertical scanning period. Specifically, the (brightness, polarity) combination of the first subpixel changes in the order of (B(right), +), (D(ark), +), (B, -) and (D, -), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (B, +), (D, -) and (B, -), where "B" indicates that the pixel is brighter than the other pixel and "D" indicates that the pixel is darker than the other. Since the effective voltages of the subpixels change in this manner, the difference between the average of the effective voltage applied to the liquid crystal layer of the first subpixel and that of the effective voltages applied to that of the second subpixel in each of the first and second polarity periods becomes substantially equal to zero.
• Unlike the liquid crystal display device of Patent Document No. 1, the liquidcrystal display device 100 of this preferred embodiment inverts the brightness levels of the subpixels every vertical scanning period, thus minimizing the degree of non-smoothness of the image on the screen. Also, in the liquidcrystal display device100 of this preferred embodiment, each pair of first and second polarity periods has a period that satisfies |VLspb|>|VLspb| and a period that satisfies |VLspa|<|VLspb| unlike the liquid crystal display device disclosed in Patent Document No. 2. Thus, as can be seen from portions(b) and(c) ofFIG.6, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be both equal to zero. Furthermore, even if the averages of the effective voltagesVLspa andVLspb do not become equal to zero, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage because the average of the effective voltagesVLspa is approximately equal to that of the effective voltagesVLspb. By controlling the averages of the effective voltages to zero in this manner, the residual image and other reliability-related problems can be overcome. It should be noted that various configurations could be used to apply mutually different voltages to the respective liquid crystal layers of the first and second subpixels such that the relations described above are satisfied.
• This preferred embodiment is preferably applied to a liquid crystal display device that uses a vertical alignment liquid crystal layer including a nematic liquid crystal material with negative dielectric anisotropy. Specifically, the liquid crystal layer of each subpixel preferably has four domains in which the liquid crystal molecules tilt in respective azimuth directions that are different from each other by approximately 90 degrees under a voltage applied (i.e., may operate in the MVA mode). Alternatively, the liquid crystal layer of each subpixel may also have axisymmetric alignment at least when a voltage is applied thereto (i.e., may operate in the ASM mode).
• Hereinafter, an MVA mode liquidcrystal display device100 according to this preferred embodiment will be described in further detail.
• As shown inFIG. 1, the liquidcrystal display device100 includes aliquid crystal panel100A, a pair of phase compensators (typically phase plates)20a and20b arranged on both sides of theliquid crystal panel100A, a pair ofpolarizers30a and30b arranged to sandwich these members between them, and abacklight40. Thepolarizers30a and30b are arranged as crossed Nicols such that their axes of transmission (which will also be referred to herein as "axes of polarization") cross each other at right angles. While no voltage is applied to theliquid crystal layer13 of theliquid crystal panel100A (seeFIG.3(b)), i.e., in a vertical alignment state, this device conducts black display. That is to say, this liquidcrystal display device100 is a normally black mode liquid crystal display device. Thephase compensators20a and20b are provided to improve the viewing angle characteristic of the liquid crystal display device and may be designed as best ones by known technologies. Specifically, thephase compensators20a and20b may be optimized such that the difference in luminance between when the image is viewed obliquely and when the image is viewed straight in the black display mode (i.e., the difference in black luminance) is minimized in every azimuth direction.
• As shown inFIG.3(a), ascan line12 is arranged between the first andsecond subpixel electrodes18a and18b. Naturally, scanlines12, signal lines, TFTs (not shown inFIG.3) and circuits for driving them are arranged on thesubstrate11a to apply predetermined voltages to the first andsecond subpixel electrodes 18a and18b at prescribed timings. On theother substrate11b, color filters and other members are arranged as needed.
• Next, the structure of a single pixel in the MVA mode liquidcrystal display device100 will be described with reference toFIGS.3(a) and3(b). The basic configuration and operation of an MVA mode liquid crystal display device are disclosed in Japanese Patent Application Laid-Open Publication No.11-242225.
• As shown inFIG.3(b), thesubpixel electrode18a on theglass substrate11a has aslit18s, and thesubpixel electrode18a and thecounter electrode17 together generate an oblique electric field in theliquid crystal layer13. On the other hand, on the surface of theglass substrate11b with thecounter electrode17, arranged areribs19 that protrude toward theliquid crystal layer13, which is made of a nematic liquid crystal material with negative dielectric anisotropy. And by providing a vertical alignment film (not shown) that covers thecounter electrode17, theribs19 and thesubpixel electrodes18a and18b, theliquid crystal layer13 exhibits a substantially vertically aligned state when no voltages are applied thereto. That is to say, the vertically aligned liquid crystal molecules can be tilted toward a predetermined direction with stability by using the sloped side surfaces of theribs19 and the oblique electric field in combination.
• As shown inFIG.3(b), theribs19 have sloped side surfaces that are raised toward their center, and the liquid crystal molecules are aligned substantially perpendicularly to those tilted side surfaces. Consequently, theribs19 produce a distribution of tilt angles of the liquid crystal molecules. As used herein, the tilt angle of a liquid crystal molecule means the angle defined by the long axis of the molecules with respect to the surface of the substrate. Also, theslit18s changes the directions of the electric field applied to the liquid crystal layer regularly. Due to the combined effects of theseribs19 and theslit18s, when an electric field is applied, the liquid crystal molecules are aligned in the four directions indicated by the arrows inFIG.3(a), i.e., upper rightward, upper leftward, lower rightward and lower leftward. As a result, a good viewing angle characteristic that is symmetrical both vertically and horizontally is realized. The rectangular display area of theliquid crystal panel100A is typically arranged such that its longitudinal direction is defined horizontally and the transmission axis of thepolarizer30a is defined to be parallel to the longitudinal direction. On the other hand, thepixels10 are arranged such that the longitudinal direction of thepixels10 intersects with that of theliquid crystal panel100A at right angles.
• As shown inFIG.3(a), the first andsecond subpixels10a and10b preferably have the same area. Each of these subpixels preferably has a first rib that runs in a first direction and a second rib that runs in a second direction that intersects with the first direction substantially at right angles, and the first and second ribs are preferably arranged symmetrically to each other within each subpixel with respect to a centerline that is defined parallel to thescan line12. And the arrangement of the ribs in one of the two subpixels and that of the ribs in the other subpixel are preferably symmetrical to each other with respect to a centerline that is drawn perpendicularly to thescan line12. By adopting such an arrangement, the liquid crystal molecules are aligned upper rightward, upper leftward, lower rightward and lower leftward within each subpixel and the respective liquid crystal domains come to have substantially the same area in the entire pixel including the first and second subpixels. As a result, a good viewing angle characteristic that is symmetrical both vertically and horizontally is realized. This effect is achieved particularly significantly when a pixel has a small area. Furthermore, it is preferred to adopt a configuration in which the interval between the respective centerlines of the two subpixels that are drawn parallel to the scan line is approximately equal to a half of the arrangement pitch of the scan lines.
• Next, the specific structure of eachpixel10 in the liquidcrystal display device100 of this preferred embodiment and application of mutually different voltages to the respective liquid crystal layers of the twosubpixels10a and10b included in thispixel10 will be described with reference toFIGS.7 through9.
• As shown inFIG.7, thepixel10 includes twosubpixels10a and10b. To thesubpixel electrodes18a and18b of the subpixels.10a and10b, connected are their associatedTFTs16a and16b and their associated storage capacitors (CS)22a and22b, respectively. The gate electrodes of theTFTs16a and16b are both connected to thesame scan line12. And the source electrodes of theTFTs16a and16b are connected to thesame signal line14. Thestorage capacitors22a and 22b are connected to their associated storage capacitor lines (CS bus lines)24a and24b, respectively. Thestorage capacitor22a includes a storage capacitor electrode that is electrically connected to thesubpixel electrode18a, a storage capacitor counter electrode that is electrically connected to thestorage capacitor line24a, and an insulating layer (not shown) arranged between the electrodes. Thestorage capacitor22b includes a storage capacitor electrode that is electrically connected to thesubpixel electrode18b, a storage capacitor counter electrode that is electrically connected to thestorage capacitor line24b, and an insulating layer (not shown) arranged between the electrodes. The respective storage capacitor counter electrodes of thestorage capacitors22a and22b are independent of each other and can receive mutually different storage capacitor counter voltages from thestorage capacitor lines24a and24b, respectively.
• FIG.8 schematically shows the equivalent circuit of onepixel10 of the liquidcrystal display device100. In this electrical equivalent circuit, the liquid crystal layers of thesubpixels10a and10b are identified by thereference numerals13a and13b, respectively. A liquid crystal capacitor formed of thesubpixel electrode18a, theliquid crystal layer13a, and thecounter electrode17 will be identified by Clca. On the other hand, a liquid crystal capacitor formed of thesubpixel electrode18b, theliquid crystal layer13b, and thecounter electrode17 will be identified by Clcb. Thesame counter electrode17 is shared by these twosubpixels10a and10b. The liquid crystal capacitorsClca andClcb are supposed to have the same electrostatic capacitance CLC (V). The value of CLC (V) depends on the effective voltages (V) applied to the liquid crystal layers of therespective subpixels10a and10b. Also, thestorage capacitors22a and22b that are connected independently of each other to the liquid crystal capacitors of therespective subpixels10a and10b will be identified herein by Ccsa and Ccsb, respectively, which are supposed to have the same electrostatic capacitanceCCS.
• In thesubpixel10a, one electrode of the liquid crystal capacitorClca and one electrode of the storage capacitorCcsa are connected to the drain electrode of theTFT16a, which functions as a switching element for thesubpixel10a. The other electrode of the liquid crystal capacitorClca is connected to thecounter electrode17. And the other electrode of the storage capacitorCcsa is connected to thestorage capacitor line24a. In thesubpixel10b, one electrode of the liquid crystal capacitorClcb and one electrode of the storage capacitorCcsb are connected to the drain electrode of theTFT16b, which functions as a switching element for thesubpixel10b. The other electrode of the liquid crystal capacitorclcb is connected to thecounter electrode17. And the other electrode of the storage capacitorCcsb is connected to thestorage capacitor line24b. The gate electrodes of theTFTs16a and16b are both connected to thescan line12 and the source electrodes thereof are both connected to thesignal line14.
• FIG.9 schematically shows how the respective voltages that are applied to drive the liquidcrystal display device 100 of this preferred embodiment vary within a vertical scanning period. Specifically, inFIG.9, Vs represents the voltage on thesignal line14; Vcsa represents the voltage on thestorage capacitor line24a; Vcsb represents the voltage on thestorage capacitor line24b; Vg represents the voltage on thescan line 12; Vlca represents the voltage to thefirst subpixel electrode18a; and Vlcb represents the voltage to thesecond subpixel electrode18b. InFIG.9, the dashed line indicates the voltageCOMMON (Vc) to thecounter electrode17. The voltageVcsa on the storage capacitor line.24a varies periodically within the range of Vc - Vad to Vc + Vad. Likewise, the voltageVcsb on thestorage capacitor line24b also varies periodically within the range of Vc - Vad to Vc + Vad. The waveform of the voltageVcsb on thestorage capacitor line24b has a phase that is different by 180 degrees from that of the voltageVcsa on thestorage capacitor line24a.
• Hereinafter, it will be described with reference toFIG.9 how the equivalent circuit shown inFIG.8 operates.
• First, at a timeT1, the voltageVg on thescan line12 rises from VgL to VgH to turn theTFTs16a and16b ON simultaneously. As a result, the voltageVs on thesignal line14 is transmitted to thesubpixel electrodes18a and18b of thesubpixels10a and10b to charge the liquid crystal capacitorsClca andClcb of thesubpixels10a and10b. In the same way, the storage capacitors Csa and Csb of the respective subpixels are also charged with the voltage on thesignal line14.
• Next, at a timeT2, the voltageVg on thescan line12 falls from VgH to VgL to turn theTFTs16a and16b OFF simultaneously and electrically isolate the liquid crystal capacitorsClca andClcb of thesubpixels10a and10b and the storage capacitorsCcsa andCcsb from thesignal line14. It should be noted that immediately after that, due to the feedthrough phenomenon caused by a parasitic capacitance of theTFTs16a and16b, for example, the voltagesVlca andVlcb applied to the first andsecond subpixel electrodes18a and18b decrease by approximately the same voltageVd to:$$\mathrm{Vlca}=\mathrm{Vs}-\mathrm{Vd}$$

  

  $$\mathrm{Vlcb}=\mathrm{Vs}-\mathrm{Vd}$$

  

  respectively. Also, in this case, the voltagesVcsa andVcsb on the storage capacitor lines are:$$\mathrm{Vcsa}=\mathrm{Vc}-\mathrm{Vad}$$

  

  $$\mathrm{Vcsb}=\mathrm{Vc}+\mathrm{Vad}$$

  

  respectively.
• Next, at a timeT3, the voltageVcsa on thestorage capacitor line24a connected to the storage capacitorCcsa rises from Vc - Vad to Vc + Vad and the voltageVcsb on thestorage capacitor line24b connected to the storage capacitor Ccsb falls from Vc + Vad to Vc - Vad. That is to say, these voltagesVcsa andVcsb both change twice as much as Vad. As the voltages on thestorage capacitor lines24a and24b change in this manner, the voltagesVlca andVlcb applied to the first and second subpixel electrodes change into:$$\mathrm{Vlca}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{<figure-callout\; label="Vd\; +"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">Vd</figure-callout>}\mathrm{+}\mathrm{}\mathrm{2}\mathrm{\times }\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  $$\mathrm{Vlcb}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{Vd}-\mathrm{2}\mathrm{\times }\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  respectively, where K=CCS/ (CLC (V) +CCS) .
• Next, at a timeT4, the voltageVcsa on thestorage capacitor line24a falls from Vc + Vad to Vc - Vad and the voltageVcsb on thestorage capacitor line24b rises from Vc-Vad to Vc+Vad. That is to say, these voltagesVcsa andVcsb both change twice as much as Vad again. In this case, the voltagesVlca andVlcb applied to the first and second subpixel electrodes also change from$$\mathrm{Vlca}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{<figure-callout\; label="Vd\; +"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">Vd</figure-callout>}\mathrm{+}\mathrm{}\mathrm{2}\mathrm{\times }\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  $$\mathrm{Vlcb}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{Vd}-\mathrm{2}\mathrm{\times }\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  into$$\mathrm{Vlca}=\mathrm{Vs}-\mathrm{Vd}$$

  

  $$\mathrm{Vlcb}=\mathrm{Vs}-\mathrm{Vd}$$

  

  respectively.
• Next, at a timeT5, the voltageVcsa on thestorage capacitor line24a rises from Vc - Vad to Vc + Vad and the voltageVcsb on thestorage capacitor line24b falls from Vc + Vad to Vc - Vad. That is to say, these voltagesVcsa andVcsb both change twice as much as Vad again. In this case, the voltagesVlca andVlcb applied to the first and second subpixel electrodes also change from$$\mathrm{Vlca}=\mathrm{Vs}-\mathrm{Vd}$$

  

  $$\mathrm{Vlcb}=\mathrm{Vs}-\mathrm{Vd}$$

  

  into$$\mathrm{Vlca}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{<figure-callout\; label="Vd\; +"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">Vd</figure-callout>}\mathrm{+}\mathrm{}\mathrm{2}\mathrm{\times }\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  $$\mathrm{Vlcb}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{Vd}-\mathrm{2}\mathrm{\times }\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  respectively.
• After that, every time a period of time that is an integral number of times as long as one horizontal scanning period 1H has passed, the voltagesVcsa, Vcsb, Vlca andVlcb alternate their levels at the timesT4 andT5. The alternation interval between T4 and T5 may be appropriately determined to be one, two, three or more times as long as 1H according to the driving method of the liquid crystal display device (such as the polarity inversion method) or the display state (such as the degree of flicker or non-smoothness of the image displayed). This alternation is continued until thepixel10 is rewritten next time, i.e., until the current time becomes equivalent to T1. Consequently, the average voltagesV1ca andVlcb applied to the first and second subpixel electrodes become:$$\mathrm{Vlca}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{Vd}\mathrm{+}\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  $$\mathrm{Vlcb}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{Vd}-\mathrm{K}\mathrm{\times }\mathrm{Vad}$$

  

  respectively.
• Therefore, the effective voltagesV1 (=VLspa) andV2 (=VLspb) applied to theliquid crystal layers13a and13b of thesubpixels10a and10b become the difference between the voltage at thefirst subpixel electrode18a and the voltage at thecounter electrode17 and the difference between the voltage at thesecond subpixel electrode18b and the voltage at thecounter electrode17. That is to say,$$\mathrm{V}\mathrm{1}\mathrm{=}\mathrm{VLspa}\mathrm{=}\mathrm{Vlca}\mathrm{-}\mathrm{Vcom}$$

  

  $$\mathrm{V}\mathrm{2}\mathrm{=}\mathrm{VLspb}\mathrm{=}\mathrm{Vlcb}\mathrm{-}\mathrm{Vcom}$$

  

  That is to say,$$\mathrm{V}\mathrm{1}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{Vd}\mathrm{+}\mathrm{K}\mathrm{\times }\mathrm{Vad}-\mathrm{Vc}$$

  

  $$\mathrm{V}\mathrm{2}\mathrm{=}\mathrm{Vs}\mathrm{-}\mathrm{Vd}-\mathrm{K}\mathrm{\times }\mathrm{Vad}-\mathrm{Vc}$$

  

  respectively. As a result, the difference Δ v (=V1-V2) between the effective voltages applied to theliquid crystal layers13a and13b of thesubpixels10a and10b becomes ΔV= 2 × K × Vad (where K = CCS/ (CLC (V) + CCS)). Thus, mutually different voltages can be applied to theliquid crystal layers13a and13b.
• FIG.10 schematically shows the relation between V1 and V2 in the liquidcrystal display device100 of this preferred embodiment. As can be seen fromFIG.10, the smaller the V1 value, the bigger ΔV in the liquidcrystal display device100 of this preferred embodiment. The ΔV value varies with V1 or V2 because the static capacitance CLC(V) of the liquid crystal capacitor varies with the voltage.
• FIG.11(a) shows the γ characteristic of the liquidcrystal display device100 of this preferred embodiment at a right 60 degree viewing angle, andFIG.11(b) shows the γ characteristic of the liquidcrystal display device 100 of this preferred embodiment at an upper right 60 degree viewing angle.FIGS.11(a) and11(b) also show the γ characteristics that were observed when the same voltage was applied to thesubpixels10a and10b for the purpose of comparison. As can be seen fromFIGS.11(a) and11(b), the grayscale characteristic of the liquidcrystal display device100 of this preferred embodiment is closer to the grayscale characteristic in the frontal viewing direction in which the ordinate is equal to the abscissa (and in which γ=2.2) than the situation where the same voltage was applied to the two subpixel electrodes. That is to say, the γ characteristic is improved by this preferred embodiment. As described above, by varying the respective voltages as shown inFIG.9 within a single vertical scanning period, mutually different effective voltages are applicable to the respective liquid crystal layers of different subpixels, and the γ characteristic in an oblique viewing direction is improved as a result.
• Hereinafter, it will be described with reference toFIG.12 how the voltage applied to thesingle pixel10 that has already been described with reference toFIGS.7 and8 changes through a number of vertical scanning periods.
• InFIG.12, Vg represents the voltage on thescan line12, Vcsa and Vcsb represent the voltages on the first and secondstorage capacitor lines24a and24b, respectively, and VLspa and VLspb represent the effective voltages applied to the respectiveliquid crystal layers13a and13b of the first andsecond subpixel electrodes10a and10b. As described above, one vertical scanning period is an interval between a point in time when a scan line is selected and a point in time when the next scan line is selected, and is represented by V-Total inFIG.12. It should be noted that the variation in the voltageVd caused by the feedthrough phenomenon that has already been described with reference toFIG.9 is not shown inFIG.12.
• Also, the voltagesVcsa andVcsb on the first and second storage capacitor lines each have display periodsAH and regulation periodsBH. Each of these voltagesVcsa andVcsb on the first and second storage capacitor lines varies periodically in different cycles through the display and regulation periodsAH andBH. In this example, the voltagesVcsa andVcsb vary in regular cycles of 20H through the display periodsAH and in different regular cycles of either 36H or 26H through the regulation periodsBH. The sum of one display periodAH and one regulation periodBH is equal to one vertical scanning period (V-Total). Furthermore, in this example, the display periodAH begins when the voltagesVcsa andVcsb on the first and second storage capacitor lines change after a vertical scanning period for a certain frame has started. On the other hand, the regulation periodBH ends when the voltagesVcsa andVcsb on the first and second storage capacitor lines change after the vertical scanning period for that frame has terminated. In this preferred embodiment, the frame frequency may be 60 Hz, for example.
• FIG.12 shows how the voltages change through four vertical scanning periods. In the following description, those four vertical scanning periods will be referred to herein as first, second, third and fourth vertical scanning periods, respectively, and the display periodsAH and regulation periodsBH associated with those vertical scanning periods will be referred to herein as first, second, third and fourth display periodsAH and first, second, third and fourth regulation periodsBH, respectively. Also, in this example, when the voltageVcsa on thestorage capacitor line24a rises to a higher voltageVcH, the voltageVcsb on thestorage capacitor line24b falls to a lower voltageVcL. Conversely, when Vcsa falls to a lower voltageVcL, Vcsb rises to a higher voltageVcH. The difference between VcH and VcL is equal to 2 × Vad that has already been described with reference toFIG.9.
• At a time when the voltageVcsa on the firststorage capacitor line24a is VcL and when the voltageVcsb on the secondstorage capacitor line24b is VcH, the voltageVg on thescan line12 changes from VgL into VgH. In response to the change of the voltageVg into VgH, the first vertical scanning period begins and they first andsecond subpixel electrodes18a and18b are charged. While the voltageVg on thescan line12 is VgH, the voltageVs on thesignal line14 is higher than the voltageVc at thecounter electrode17. That is why as a result of the charge, the voltages at the first andsecond subpixel electrodes18a and18b become higher than the voltageVc at thecounter electrode17. Thereafter, when the voltageVg on thescan line12 falls from VgH to VgL again, the first andsecond subpixel electrodes18a and18b finish being charged.
• After that, the voltageVcsa on the firststorage capacitor line24a rises to VcH and the voltageVcsb on the secondstorage capacitor line24b falls to VcL. In this example, it is when the voltageVcsa on the firststorage capacitor line24a increases and the voltageVcsb on the secondstorage capacitor line24b decreases that the first display periodAH begins. Through the first display periodAH, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b increase or decrease every 10H period and vary periodically in regular cycles of 20H. When the first display periodAH ends, the first regulation periodBH begins. Through the first regulation periodBH, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b increase or decrease every 18H period. The voltages at the first andsecond subpixel electrodes18a and18b change as the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b vary. That is why in the first vertical scanning period, the absolute value of the effective voltage applied to theliquid crystal layer13a of thefirst subpixel10a becomes greater than that of the effective voltage applied to theliquid crystal layer13b of thesecond subpixel10b and thefirst subpixel10a becomes brighter than thesecond subpixel10b.
• In the first regulation periodBH, at a time when the voltageVcsa on the firststorage capacitor line24a is VcH and when the voltageVcsb on the secondstorage capacitor line24b is VcL, the voltageVg on thescan line12 changes from VgL into VgH. In response to the change of the voltageVg into VgH, the first vertical scanning period ends and the second vertical scanning period begins and the first andsecond subpixel electrodes18a and18b are charged. While the voltageVg on thescan line12 is VgH, the voltageVs on thesignal line14 is higher than the voltageVc at thecounter electrode17. That is why as a result of the charge, the voltages at the first andsecond subpixel electrodes18a and18b become higher than the voltageVc at thecounter electrode17. Thereafter, when the voltageVg on thescan line12 falls from VgH to VgL again, the first andsecond subpixel electrodes18a and18b finish being charged.
• After that, the voltageVcsa on the firststorage capacitor line24a falls to VcL and the voltageVcsb on the secondstorage capacitor line24b rises to VcH. In this example, it is when the voltageVcsa on the firststorage capacitor line24a decreases and the voltageVcsb on the secondstorage capacitor line24b increases that the first regulation period ends and the second display periodAH begins. Through the second display periodAH, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b also increase or decrease every 10H period and vary periodically in regular cycles of 20H. And through the second regulation periodBH, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b will increase or decrease every 13H period. The voltages at the first andsecond subpixel electrodes18a and18b change as the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b vary. That is why in the second vertical scanning period, the absolute value of the effective voltage applied to theliquid crystal layer13b of thesecond subpixel10b becomes greater than that of the effective voltage applied to theliquid crystal layer13a of thefirst subpixel10a and thesecond subpixel10b becomes brighter than thefirst subpixel10a.
• Next, in the second regulation periodBH, at a time when the voltageVcsa on the firststorage capacitor line24a is VcH and when the voltageVcsb on the secondstorage capacitor line24b is VcL, the voltageVg on thescan line12 changes from VgL into VgH. In response to the change of the voltageVg into VgH, the second vertical scanning period ends and the third vertical scanning period begins and the first andsecond subpixel electrodes18a and18b are charged. While the voltageVg on thescan line12 is VgH, the voltageVs on thesignal line14 is lower than the voltageVc at thecounter electrode17. That is why as a result of the charge, the voltages at the first andsecond subpixel electrodes18a and18b become lower than the voltageVc at thecounter electrode 17. Thereafter, when the voltage Vg on thescan line12 falls from VgH to VgL again, the first andsecond subpixel electrodes18a and18b finish being charged.
• After that, the voltageVcsa on the firststorage capacitor line24a falls to VcL and the voltageVcsb on the secondstorage capacitor line24b rises to VcH. In this example, it is when the voltageVcsa on the firststorage capacitor line24a decreases and the voltageVcsb on the secondstorage capacitor line24b increases that the second regulation periodBH ends and the third display periodAH begins. Through the third display periodAH, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b also increase or decrease every 10H period and vary periodically in regular cycles of 20H. And through the third regulation periodBH, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b will increase or decrease every 18H period. The voltages at the first andsecond subpixel electrodes18a and18b change as the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b vary. That is why in the third vertical scanning period, the absolute value of the effective voltage applied to theliquid crystal layer13a of thefirst subpixel10a becomes greater than that of the effective voltage applied to theliquid crystal layer13b of thesecond subpixel10b and thefirst subpixel10 becomes brighter than thesecond subpixel10.
• Next, in the third regulation periodBH, at a time when the voltageVcsa on the firststorage capacitor line24a is VcL and when the voltageVcsb on the secondstorage capacitor line24b is VcH, the voltageVg on thescan line12 changes from VgL into VgH. In response to the change of the voltageVg into VgH, the third vertical scanning period ends and the fourth vertical scanning period begins and the first andsecond subpixel electrodes18a and18b are charged. While the voltageVg on thescan line12 is VgH, the voltageVs on thesignal line 14 is lower than the voltageVc at thecounter electrode17. That is why as a result of the charge, the voltages at the first andsecond subpixel electrodes18a and18b become lower than the voltageVc at thecounter electrode17. Thereafter, when the voltageVg on thescan line12 falls from VgH to VgL again, the first andsecond subpixel electrodes18a and18b finish being charged.
• After that, the voltageVcsa on the firststorage capacitor line24a rises to VcH and the voltageVcsb on the secondstorage capacitor line24b falls to VcL. In this example, it is when the voltageVcsa on the firststorage capacitor line24a increases and the voltageVcsb on the secondstorage capacitor line24b decreases that the third regulation periodBH ends and the fourth display periodAH begins. Through the fourth display periodAft, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b also increase or decrease every 10H period and vary periodically in regular cycles of 20H. And through the fourth regulation periodBH, the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b will increase or decrease every 13H period. The voltages at the first andsecond subpixel electrodes18a and18b change as the voltagesVcsa andVcsb on the first and secondstorage capacitor lines24a and24b vary. That is why in the fourth vertical scanning period, the absolute value of the effective voltage applied to theliquid crystal layer13b of thesecond subpixel10 becomes greater than that of the effective voltage applied to theliquid crystal layer13a of thefirst subpixel10a and thesecond subpixel10b becomes brighter than thefirst subpixel10a. From the fifth vertical scanning period on, the respective voltages will vary in quite the same way as in the first through fourth vertical scanning periods shown inFIG.12.
• As described above, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (D, +), (B, -) and (D, -), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (B, +), (D, -) and (B, -). That is to say, the brightness levels and polarities of the first and second subpixels change just as shown in portion(a) ofFIG. 6. By changing the voltagesVcsa andVcsb on the first and second storage capacitor lines in this manner, the deterioration of display quality can be minimized in a liquid crystal display device, of which the γ characteristic has reduced viewing angle dependence.
• As described above, the liquid crystal display device of this preferred embodiment is designed such that the potentials at the pixel electrode and at the counter electrode switch their levels at regular intervals and that the direction of the electric field applied to the liquid crystal layer is also inverted at regular intervals. In this case, in a typical liquid crystal display device including a counter electrode and pixel electrodes on two different substrates, the directions of the electric field applied to the liquid crystal layer change from toward the light source side into toward the viewer side, and vice versa. Such a drive method that sets an alternating current voltage is called an "AC drive method". In the liquid crystal display device of this preferred embodiment, the inversion interval of the direction of the electric field applied to the liquid crystal layer may be 66.667 ms, which is twice as long as two frame periods of 33.333 ms, for example. That is to say, in the liquid crystal display device of this preferred embodiment, the direction of the electric field applied to the liquid crystal layer is inverted every time two frame pictures are presented. That is why in presenting a still picture, unless the electric field strengths (i.e., the magnitudes of applied voltages) exactly matched with each other in respective electric field directions (i.e., if the electric field intensities changed every time the directions of the electric field change), the pixel luminances would change and a flicker would be produced on the screen whenever the electric field intensities change.
• To eliminate such a flicker, the electric field intensities (or the magnitudes of applied voltages) in the respective electric field directions need to be exactly matched with each other. In liquid crystal display devices that are manufactured on an industrial basis, however, it is difficult to exactly match the electric field intensities with each other in respective electric field directions. That is why the flicker is reduced by arranging pixels with mutually different electric field directions adjacent to each other within a display area and spatially averaging the luminances of those pixels. Such a method is generally called either a "dot inversion" or a "line inversion". It should be noted that there are various "inversion drive" methods that include not just a method in which the polarities of those pixels are inverted in a checkered pattern on a pixel-by-pixel basis (i.e., the polarities are inverted both every row and every column, which is a so-called "dot inversion drive") and a method in which the polarities are inverted on a line-by-line basis (i.e., the polarities are inverted every row, which is a so-called "line inversion drive") but also a method in which the polarities are inverted every other row and every column (which is a so-called "two-row, one-column dot inversion drive"). And an appropriate one of those methods is selected as needed.
• In view of these considerations, to avoid the flicker, the following three conditions are preferably satisfied:
• First of all, in respective electric field directions (and in both of the two polarities of respective applied voltages), the absolute values of the effective voltages applied to the liquid crystal layer should agree with each other as closely as possible. That is to say, as in resolving the reliability-related problem described above, the average of the voltages applied to the liquid crystal layer should be as close to zero as possible.
• Secondly, pixels, among which the electric field is applied to the liquid crystal layer in respectively different directions in each frame period, should be arranged adjacent to each other.
• And a third condition is that one type of subpixels that are brighter than subpixels of the other type be arranged as randomly as possible within the same frame. To achieve the maximum display effect on the screen, those subpixels are preferably arranged such that the one type of subpixels, which are brighter than the subpixels of the other type, are adjacent to each other in neither the column direction nor the row direction. In other words, the one type of subpixels that are brighter than the other type are preferably arranged in a checkered pattern.
• Hereinafter, it will be described how and why the liquid crystal display device of this preferred embodiment satisfies these three conditions. But before describing exactly how the device satisfies those conditions, it will be described with reference toFIGS.13 and14 that the liquidcrystal display device 100 of this preferred embodiment has a pixel arrangement that can be used effectively to get the one-dot inversion drive done with those conditions satisfied.
• FIG.13 illustrates an equivalent circuit of the liquidcrystal display device100. InFIG.13, each pixel is supposed to have the structure shown inFIGS.7 and8. Those pixels are arranged in a matrix pattern. In the following description, a pixel located at an n^th row and an m^th column will be referred to herein as "pixeln-m" and the two subpixels that form the pixeln-m will be referred to herein as "subpixeln-m-A" and "subpixeln-m-B", respectively.
• The liquidcrystal display device100 includes ten storage capacitor trunksCS1 throughCS10, and each subpixel is connected to one of those storage capacitor trunksCS1 throughCS10 by way of a storage capacitor line (CS bus line). For example, the storage capacitor trunkCS2 is connected to subpixels1-a-B, 1-b-B, 1-c-B, etc. on the first pixel row and to subpixels2-a-A, 2-b-A, 2-c-A, etc. on the second pixel row. In this configuration, each subpixel and another subpixel included in a different pixel that is adjacent to the former subpixel are connected to the same storage capacitor trunk by way of the same storage capacitor line.
• Hereinafter, the configurations of first and second subpixels1-a-A and1-a-B included in a pixel1-a that is specified by a scan lineG1 and a signal lineSa will be described. The first and second subpixels1-a-A and1-a-B include liquid crystal capacitorsCLC1-a-A andCLC1-a-B and storage capacitorsCCS1-a-A andCCS1-a-B, respectively. Each of the liquid crystal capacitors is formed by a subpixel electrode, the counter electrodeComLC and the liquid crystal layer interposed between them. Each of the storage capacitors is formed by a storage capacitor electrode, an insulating film and a storage capacitor counter electrodeComCS1 orComCS2.
• The first and second subpixels1-a-A and1-a-B are connected in common to the same signal lineSa by way of their associated TFTs1-a-A and1-a-B, respectively. The TFTs1-a-A and1-a-B have their ON/OFF states controlled with a voltage supplied onto their common signal lineG1. And when these two TFTs are ON, voltages are applied through the same signal lineSa to the respective subpixel electrodes and respective storage capacitor electrodes of the first and second subpixels1-a-A and1-a-B. The storage capacitor counter electrode of the subpixel1-a-A is connected to the storage capacitor trunkCS1 by way of its associated storage capacitor line (CS bus line)CS1. Meanwhile, the storage capacitor counter electrode of the subpixel1-a-B is connected to the storage capacitor trunkCS2 by way of its associated storage capacitor line (CS bus line)CS2. In this manner, the configuration shown inFIG.13, either a single storage capacitor line or a single scan line is shared by two subpixels, thus increasing the aperture ratio of each pixel, which is beneficial.
• FIG.14 shows the brightness levels and polarities of respective subpixels that have changed within the effective scanning period of a certain frame. Specifically, inFIG.14, illustrated are pixels on the first through twelfth rows and the a^th through, f^th columns.FIG.15 shows the waveforms of respective voltages (or signals) to drive a liquid crystal display device with the configuration shown inFIG.13. InFIG.15, Vsa and Vsb represent the voltages on the signal linesSa andSb, Vg1 through Vg12 represent the voltages on the scan linesG1 throughG12, Vcs1 through Vcs10 represent the voltages on the storage capacitor trunksCS1 throughCS10 and VLsp1-a-A through VLsp2-b-B represent the effective voltages applied to the liquid crystal layer of associated subpixels, respectively. What is shown inFIG.15 is voltage waveforms within one vertical scanning period.
• The liquid crystal display device with the configuration shown inFIG.13 is driven with voltages having the waveforms shown inFIG.15. In the following description, every pixel is supposed to display the same grayscale tone to avoid complicating the description excessively. In a situation where every pixel displays the same grayscale tone, the voltagesVsa andVsb on the signal linesSa andSb oscillate in regular cycles and with a predetermined amplitude as shown inFIG.15. One cycle time of oscillation of these voltagesVsa andVsb is two horizontal scanning periods (2H).
  Specifically, the voltageVsb on the signal lineSb varies with a phase difference of 180 degrees with respect to the voltageVsa on the signal lineSa. InFIG.15, a period in which the voltageVsa orVsb is higher than the voltage at the counter electrode is identified by "+" and a period in which the former is lower than the latter is identified by "-". As already described with reference toFIG.9, in a liquid crystal display device that uses TFTs, a voltage on a signal line is transmitted to a subpixel electrode by way of one of the TFTs and then changes due to a variation in the voltageVg on a scan line, thus producing a feedthrough phenomenon. The voltage at the counter electrode is determined in view of this feedthrough phenomenon. Also, although not shown inFIG.15, the voltages on other signal linesSc andSe also vary in the same way as the voltageVsa on the signal lineSa and the voltages on other signal linesSd andSf also vary in the same way as the voltageVsb on the signal lineSb. Furthermore, as described above, an interval between a point in time when a voltageVg on a certain scan line rises from Low level (VgL) to High level (VgH) and a point in time when the voltageVg on the next scan line rises from VgL to VgH is one horizontal scanning period (1H).
• As shown inFIG.15, the voltagesVcs1 throughVcs10 on the storage capacitor trunks CS1 throughCS10 oscillate with the same amplitude and in the same regular cycles. In this example, one oscillation cycle time is 20 H. For example, the voltagesVcs1 andVcs2 have such a relation that if one of these two voltages changes into VcH, the other voltage will change into VcL and that if one of these two voltages changes into VcL, the other voltage will change into VcH . The other four pairs of voltagesVcs3 andVcs4, Vcs5 andVcs6, Vcs7 andVcs8, andVcs9 andVcs10 too have the same relation as that pair of voltagesVcs1 andVcs2. Also, the voltagesVcs3 andVcs4 change 2H after the voltagesVcs1 andVcs2 have changed. In the same way, there is a time lag of 2H between the changes of the voltagesVcs5 andVcs6, the voltagesVcs7 andVcs8 and the voltagesVcs9 andVcs10.
• When a voltageVg on a scan line changes from VgL into VgH, the TFTs that are connected to that scan line are turned ON and a voltageVs on the associated scan line is applied to the subpixels that are connected to those TFTs. Next, after the voltage on the scan line changes into VgL, the voltages on the storage capacitor trunks will vary. And the magnitudes of the changes in voltages on those storage capacitor trunks (including the directions and signs of the changes) are different from each other between the respective subpixels. As a result, the effective voltages applied to the respective liquid crystal layers of those subpixels become different from each other.
• Hereinafter, it will be described how the voltages at the subpixels1-a-A and1-a-B change as an example. When the voltageVg1 on the scan lineG1 changes from VgL into VgH, the liquid crystal capacitorsCLC1-a-A andCLC1-a-B of the subpixels1-a-A and1-a-B are charged. If the voltageVg1 on the scan lineG1 is VgH, the voltageVsa on the signal lineSa is positive "+" and the liquid crystal capacitorsCLC1-a-A andCLC1-a-B of the subpixels1-a-A and1-a-B are charged to a higher potential level than the one at the counter electrode. Thereafter, when the voltageVg1 on the scan lineG1 changes from VgH into VgL, the liquid crystal capacitorsCLC1-a-A andCLC1-a-B of the subpixels.1-a-A and1-a-B get electrically isolated from the signal lineSa and finish being charged. After the voltageVg1 on the scan lineG1 has changed from VgH into VgL, the first change of the voltageVcs1 on the storage capacitor trunkCS1 is increase but the first change of the voltageVcs2 on the storage capacitor trunkCS2 is decrease. After that, these voltagesVcs1 andVcs2 will alternately increase and decrease a number of times on a 10H basis. Consequently, in the pixel1-a specified by the scan lineG1 and the signal lineSa, the absolute value of the effective voltage applied to the liquid crystal layer of the subpixel 1-a-A that is electrically connected to the storage capacitor trunk CS1 becomes greater than that of the effective voltage applied to that of the subpixel1-a-B that is electrically connected to the storage capacitor trunkCS2.
• As described above, if the first change in voltage on a storage capacitor trunk associated with a given subpixel is increase after the voltage on its associated scan line has changed from VgH into VgL, the effective voltage applied to the liquid crystal layer of that subpixel becomes higher than the voltage on its associated signal line when the voltage on its associated scan line is VgH. On the other hand, if the first change in voltage on its associated storage capacitor trunk is decrease, the effective voltage applied to the liquid crystal layer of that subpixel becomes lower than the voltage on its associated signal line when the voltage on its associated scan line is VgH. Consequently, if the sign of the voltage on the signal line when the associated scan line is selected is positive "+" and if the variation in the voltage on the storage capacitor trunk is increase, then the absolute value of the effective voltage applied to the liquid crystal layer increases compared to a situation where the voltage variation is decrease. On the other hand, if the sign of the voltage on the signal line when the associated scan line is selected is negative "-" and if the variation in the voltage on the storage capacitor trunk is increase, then the absolute value of the effective voltage applied to the liquid crystal layer decreases compared to a situation where the voltage variation is decrease.
• As described above,FIG. 14 shows the brightness levels and polarities of subpixels that have changed during the effective scanning period of a certain frame. InFIG.14, the sign "B" indicates that the given subpixel is brighter than the other subpixel (i.e., the absolute value of the effective voltage applied to the liquid crystal layer of that subpixel is greater than that of the effective voltage applied to the liquid crystal layer of the other). On the other hand, the sign "D" indicates that the given subpixel is darker than the other subpixel (i.e., the absolute value of the effective voltage applied to the liquid crystal layer of that subpixel is smaller than that of the effective voltage applied to that of the other). InFIG.14, the sign "+" also indicates that the voltage at the subpixel electrode is higher than the one at the counter electrode and the sign "-" also indicates that the voltage at the subpixel electrode is lower than the one at the counter electrode. Two subpixels included in each pixel are adjacent to a pixel with a smaller row number and a pixel with a bigger row number. In this example, of the two subpixels included in a single pixel, the subpixel adjacent to the pixel with the smaller row number will be identified herein by "A" and the subpixel adjacent to the pixel with the bigger row number will be identified herein by "B".
• Hereinafter, the brightness levels and polarities of respective subpixels will be described with reference toFIG.14 and15.
• First of all, the brightness levels and polarities of the subpixels1-a-A and1-a-B included in the pixel1-a will be described. As can be seen fromFIG.15, while the voltageVg1 on the scan lineG1 is VgH, the voltageVsa on the signal lineSa is higher than the voltage at the counter electrode. Therefore, the polarities of the subpixels1-a-A and1-a-B are both positive "+". On the other hand, when the voltageVg1 on the scan lineG1 changes from VgH into VgL, the voltagesVcs1 andVcs2 on the storage capacitor trunksCS1 andCS2 associated with the respective subpixels are as indicated by the leftmost arrows inFIG.15. That is why as can be seen fromFIG.15, after the voltageVg1 on the scan lineG1 has changed from VgH into VgL, the first change in the voltageVcs1 associated with the subpixel1-a-A is increase as indicated by "U" inFIG.15 and the first change in the voltageVcs2 on the storage capacitor trunkCS2 associated with the subpixel1-a-B is decrease as indicated by "D" inFIG.15. Consequently, the effective voltage applied to the subpixel1-a-A increases, the one applied to the subpixel1-aB decreases, and the subpixel1-a-A becomes brighter than the subpixel1-a-B.
• Next, the brightness levels and polarities of subpixels2-a-A and2-a-B included in the pixel2-a will be described. As can be seen fromFIG.15, while the voltageVg2 on the scan lineG2 is VgH, the voltageVsa on the signal lineSa is lower than the voltage at the counter electrode. Thus, the polarities of the subpixels2-a-A and2-a-B are both negative "-". On the other hand, when the voltageVg2 on the scan lineG2 changes from VgH into VgL, the voltagesVcs2 andVcs3 on the storage capacitor trunksCS2 andCS3 associated with the respective subpixels2-a-A and2-a-B are as indicated by the second leftmost arrows inFIG.15. That is why as can be seen fromFIG.15, after the voltageVg1 on the scan lineG1 has changed from VgH into VgL, the first change in the voltageVcs2 on the storage capacitor trunkCS2 associated with the subpixel2-a-A is decrease as indicated by "D" inFIG. 15 and the first change in the voltageVcs3 on the storage capacitor trunkCS3 associated with the subpixel2-a-B is increase as indicated by "U" inFIG.15. Consequently, the effective voltage applied to the subpixel2-a-A increases, the one applied to the subpixel2-a-B decreases, and the subpixel2-a-A becomes brighter than the subpixel2-a-B.
• Next, the brightness levels and polarities of subpixels1-b-A and1-b-B included in the pixel1-b will be described. While the voltageVg1 on the scan lineG1 is VgH, the voltageVsb on the signal lineSb is lower than the voltage at the counter electrode. Thus, the polarities of the subpixels1-b-A and1-b-B are both negative "-". On the other hand, when the voltageVg1 on the scan lineG1 changes from VgH into VgL, the voltagesVcs1 andVcs2 on the storage capacitor trunksCS1 andCS2 associated with the respective subpixels1-b-A and1-b-B are as indicated by the leftmost arrows inFIG.15. That is why as can be seen fromFIG.15, after the voltageVg1 on the scan lineG1 has changed from VgH into VgL, the first change in the voltage on the storage capacitor trunkCS1 associated with the subpixel1-b-A is increase as indicated by "U" inFIG.15 and the first change in the voltageVcs2 on the storage capacitor trunkCS2 associated with the subpixel1-b-B is decrease as indicated by "D" inFIG.15. Consequently, the effective voltage applied to the liquid crystal layer of the subpixel1-b-A decreases, the one applied to the subpixel1-b-B increases, and the subpixel1-b-B becomes brighter than the subpixel1-b-A.
• Next, the brightness levels and polarities of subpixels2-b-A and2-b-B included in the pixel2-b will be described. As can be seen fromFIG.15, while the voltageVg2 on the scan lineG2 is VgH, the voltageVsb on the signal lineSb is higher than the voltage at the counter electrode. Thus, the polarities of the subpixels2-b-A and2-b-B are both positive "+". On the other hand, when the voltageVg2 on the scan lineG2 changes from VgH into VgL, the voltagesVcs2 and²Vcs3 on the storage capacitor trunksCS2 andCS3 associated with the respective subpixels2-b-A and2-b-B are as indicated by the second leftmost arrows inFIG. 15. That is why as can be seen fromFIG.15, after the voltageVg1 on the scan lineG1 has changed from VgH into VgL, the first change in the voltageVcs2 on the storage capacitor trunkCS2 associated with the subpixel2-b-A is decrease as indicated by "D" inFIG.15 and the first change in the voltageVcs3 on the storage capacitor trunk CS3 associated with the subpixel2-b-B is increase as indicated by "U" inFIG.15. Consequently, the effective voltage applied to the subpixel2-b-A decreases, the one applied to the subpixel2-b-B increases, and the subpixel2-b-B becomes brighter than the subpixel2-b-A. As a result, the brightness levels and polarities of the respective subpixels become as shown inFIG.14.
• Hereinafter, it will be described how and why the liquid crystal display device of this preferred embodiment satisfies the three conditions mentioned above. First of all, the liquid crystal display device of this preferred embodiment satisfies the first condition for the following reasons.
• At first, it will be described that the liquid crystal display device of this preferred embodiment satisfies the first condition, i.e., the absolute values of the effective voltages applied to the liquid crystal layers of respective subpixels agree with each other in respective electric field directions. In the liquid crystal display device of this preferred embodiment, each pixel includes two subpixels, of which the liquid crystal layers are supplied with mutually different effective voltages. However, it is the brighter subpixel (i.e., the subpixel marked "B" inFIG.14) that will have a decisive effect on the display quality such a flicker on the screen. For that reason, this first condition is imposed on the subpixels marked "B", in particular.
• The first condition will be discussed with reference to the respective voltage waveforms shown inFIG.15, which shows the voltagesVLsp1-a-A andVLsp2-a-A to be applied to the liquid crystal layers of the "B" subpixels1-a-A and2-a-A with mutually different electric field directions (or polarities). In VLsp1-a-A and VLsp2-a-A shown inFIG.15, the solid line represents the voltages applied to the subpixel electrodes of the subpixels1-a-A and2-a-A and the dashed line represents the voltage applied to the counter electrode. The effective voltage applied to the liquid crystal layer is a difference between the voltages represented by the solid and dashed lines. That is why if the effective voltages applied to the liquid crystal layer in respective electric field directions (or the quantities of charge stored in the liquid crystal capacitors) are matched with each other as closely as possible by appropriately defining the voltage applied to the counter electrode, the first condition can be satisfied.
• Next, it will be described that the liquid crystal display device of this preferred embodiment satisfies the second condition, i.e., pixels with mutually different polarities are arranged adjacent to each other in each frame period. In the liquid crystal display device of this preferred embodiment, however, each pixel includes two subpixels, of which the liquid crystal layers are supplied with different effective voltages. That is why this second condition is imposed on not only on each pixel but also subpixels with the same effective voltage as well. Among other things, it is particularly important for bright subpixels, i.e., the subpixels marked "B" inFIG.14, to satisfy this second condition as in the first condition described above.
• As shown inFIG.14, the signs "+" and "-" representing the polarities (or electric field directions) of respective subpixels are inverted every other pixel (i.e., every second column) in the row direction (i.e., in the horizontal direction) in the order of (+, -), (+, -), (+, -), and so on, and also inverted every other pixel (i.e., every second row) in the column direction (i.e., in the vertical direction) in the order of (+, -), (+, -), (+, -), (+, -), and so on. That is to say, looking on a pixel-by-pixel basis, this device achieves the so-called "dot inversion" state, and therefore, satisfies the second condition.
• Next, the bright subpixels, i.e., the subpixels marked "B" inFIG.14, will be checked out. As shown inFIG.14, looking at subpixels on the same row (e.g., the subpixels1-a-A, 1-b-A, 1-c-A, etc., on the first row), it can be seen that the polarity of every "B" subpixel is positive "+". However, looking at subpixels on the same column (e.g., the subpixels1-a-A, 1-a-B, 2-a-A, 2-a-B, 3-a-A, 3-a-B, etc., on the first column), it can be seen that the polarities of the "B" subpixels are inverted every other pixel (i.e., every second row) in the order of "+", "-", "+", "-" and so on. That is to say, looking at subpixels with high-order luminances, which are particularly important ones, this device achieve the so-called "line inversion" state, and therefore, satisfies the second condition. Likewise, the "D" subpixels are also arranged with the same regularity, thus satisfying the second condition, too.
• Next, it will be described how the device of this preferred embodiment satisfies the third condition. To satisfy the third condition, multiple subpixels, of which the luminance levels are intentionally different from each other, should be arranged such that subpixels with the same luminance level are adjacent to each other at as small a number of locations as possible. InFIG.14, looking at a total of four subpixels that are arranged on two rows and two columns (e.g., the subpixels1-a-A, 1-a-B, 1-b-A and1-b-B), it can be seen that "B" and "D" subpixels are arranged in this order along the first column and then "D" and "B" subpixels are arranged in this order along the next column. Supposing these four subpixels form a "group of subpixels", the subpixels are arranged such that the entire screen is filled with such groups of subpixels with no gap left at all. That is to say, the "B" and "D" signs are arranged in a checkered pattern on a subpixel-by-subpixel basis as shown inFIG.14. Consequently, it can be seen that this device satisfies the third condition, too.
• As described above, the liquid crystal display device of this preferred embodiment that has just been described with reference toFIGS.14 and15 satisfies all of the three conditions mentioned above, and therefore, realizes a display of quality images with a flicker eliminated.
• The brightness levels and polarities of subpixels that have changed within the effective scanning period of a certain frame and the voltage waveforms are shown inFIGS.14 and15. In the next frame, however, the voltages on the signal lines change according to the waveforms shown inFIG.15 with respect to the voltages on the scan lines but the voltages on the storage capacitor trunks change inversely to the waveforms shown inFIG.15. That is why in that frame, the polarities of the respective subpixels are the same as those of the subpixels shown inFIG.14 but the brightness levels of the respective subpixels are inverted compared to the counterparts shown inFIG.14.
• In the frame after that next frame, with respect to the voltages on the scan lines, not only the voltages on the signal lines but also the voltages on the storage capacitor trunks change in the patterns opposite to the waveforms shown inFIG.15. Consequently, in that frame, the brightness levels of the respective subpixels are the same as those of the subpixels shown inFIG.14 but the polarities of the respective subpixels are inverted compared to the counterparts shown inFIG.14.
• And in the frame next to that frame, with respect to the voltages on the scan lines, the voltages on the signal lines change in the patterns opposite to the waveforms shown inFIG.15 but the voltages on the storage capacitor trunks change according to the waveforms shown inFIG.15. Consequently, in that frame, the brightness levels and polarities of the respective subpixels are inverted compared to the counterparts shown inFIG.14.
• Next, it will be described with reference toFIG.16 how the voltages change in multiple pixels of the liquid crystal display device of this preferred embodiment. InFIG.16, Vcs1 through Vcs6 represent the voltages on the storage capacitor trunksCS1 throughCS6, Vg1 through Vg3 represent the voltages on the scan linesG1 throughG3, and VLsp1-a-A through VLsp3-a-B represent the effective voltages applied to the respective liquid crystal layers of the subpixels1-a-A through3-a-B. In the following example, the four consecutive frames will be identified herein by n, n+1, n+2 and n+3, respectively.
• FIG.16 also shows vertical scanning periods of an input video signal. Each vertical scanning period of the input video signal consists of an effective scanning periodV-Disp during which pixels in theliquid crystal panel100A (seeFIG.1) are selected on a row-by-row basis and a vertical-blanking intervalV-Blank during which no pixels in theliquid crystal panel100A are selected at all. The duration of the effective scanning period is determined by the display area (or the number of rows of effective pixels) of theliquid crystal panel100A.
• In this description, when simply a "vertical scanning period" is mentioned, the "vertical scanning period" refers to a "vertical scanning period of a liquid crystal panel". That is to say, a "vertical scanning period" (i.e., a "vertical scanning period of the liquid crystal panel") is used herein in a different sense from a "vertical scanning period of an input video signal". A "vertical scanning period of an input video signal" is either a one-frame period or a one-field period, which begins and ends simultaneously for every pixel. On the other hand, a "vertical scanning period" means an interval between a point in time when a scan line is selected to write a display signal voltage and a point in time when that scan line is selected to write the next display signal voltage as described above. The vertical scanning periods start at different timing and end at different timing according to the associated scan line.
• InFIG.16, the oblique lines indicate that the start and end times of a vertical scanning period change according to the row of pixels selected. As can be seen fromFIG. 16, within each frame, scan lines are sequentially selected one after another from the first one. And when a scan line is selected, a voltage applied to its associated subpixel electrode changes to start a vertical scanning period for that subpixel. As described above, one vertical scanning period of an input video signal consists of an effective scanning periodV-Disp and a vertical-blanking intervalV-Blank. However, the vertical scanning period of a certain subpixel begins in the middle of the effective scanning period of a frame n, continues through the vertical-blanking interval, and then ends halfway through the effective scanning period of the nextframe n+1. After that, when its associated scan line is selected next time, the next vertical scanning period will begin for that subpixel. It should be noted that in any pixel, the length of the "vertical scanning period" is equal to that of the "vertical scanning period of the input video signal".
• As can be seen fromFIG.16, in the frames n to n+3, the (brightness, polarity) combinations of the subpixel1-a-A change in the order of (B, +), (D, +), (B, -), and (D, - ); the (brightness, polarity) combinations of the subpixel1-a-B change in the order of (D, +), (B, +), (D, -), and (B, -); the (brightness, polarity) combinations of the subpixel2-a-A change in the order of (B, -), (D, -), (B, +), and (D, +); and the (brightness, polarity) combinations of the subpixel2-a-B change in the order of (D, -), (B, -), (D, +), and (B, +).
• FIG.17 shows the brightness levels and polarities of the subpixels1-a-A and1-a-B and the first change of voltages on the storage capacitor lines at the vertical scanning period of the subpixels1-a-A and1-a-B. As shown inFIG.17, in framen, the polarity of the subpixels1-a-A and1-a-B is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-A is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-B is decrease "↓". In the next framen+1, the polarity of the subpixels1-a-A and 1a-B is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-A is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-B is increase "↑".
• In the framen+2, the polarity of the subpixels1-aA and1-a-B is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-A is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-B is increase "↑". In the next framen+3, the polarity of the subpixels1-a-A and1a-B is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-A is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the subpixel1-a-B is decrease "↓".
• As described above, the (polarity, first change of voltages on storage capacitor line) combinations of the subpixel1-a-A from framen through framen+3 change (+, ↑), (+, ↓), (-, ↓) and (-, ↑) in this order. That is to say, mutually different combinations appear one after another. On the other hand, the (polarity, first change of voltages on storage capacitor line) combinations of the subpixel1-a-B from frame n through framen+3 change (+, ↓), (+, ↑), (-, ↑) and (-, ↓) in this order. That is to say, these combinations of the subpixel1-a-B have the same polarity change pattern as, but a different storage capacitor line voltage variation pattern from, those of the subpixel1-a-A.
• In the preferred embodiment described above, the voltage on each storage capacitor line is supposed to change periodically in regular cycles of 20H during the display period. However, the present invention is in no way limited to that specific preferred embodiment. The voltage on each storage capacitor line may also change in regular cycles of 16H during the display period as shown in portion(a) ofFIG.18. In that case, the storage capacitor line voltage changes every 13H in the first and third regulation periodsBH but changes every 9H in the second and fourth regulation periodsBH, for example. Alternatively, the storage capacitor line voltage may also change in regular cycles of 24H during the display period as shown in portion(b) ofFIG.18. In that case, the storage capacitor line voltage changes every 15H in the first and third regulation periodsBH but changes every 21H in the second and fourth regulation periodsBH, for example. The intervals of the variation in storage capacitor line voltage during the BH period may be appropriately changed according to the V-total value.
• Also, in the preferred embodiment described above, the voltage on each storage capacitor line is supposed to complete one cycle of change during each regulation period. However, the present invention is in no way limited to that specific preferred embodiment. The voltage on each storage capacitor line may also change periodically during each regulation period either in a cycle time of 2H as shown in portion(a) ofFIG.19 or in a cycle time of 1H as shown in portion(b) ofFIG.19. Alternatively, the voltage on each storage capacitor line may also be maintained at the average of VcH and VcL during each regulation period as shown in portion(c) ofFIG.19.
• Furthermore, in the preferred embodiment described above, one regulation period is supposed to be included in each vertical scanning period for one frame. However, the present invention is in no way limited to that specific preferred embodiment. One regulation period may be provided for every two vertical scanning periods for two frames as shown inFIG.20. In the example illustrated inFIG.20, each vertical scanning period has a duration of 810H and the storage capacitor voltagesVcs1 throughVcs3 change periodically in regular cycles of 20H during the display period but changes every 5H during the regulation period. If two vertical scanning periods (e.g., 810H × 2 = 1,620H in this example) are an integral number of times as long as one cycle time (e.g., 20H in this example) of the display period in this manner, then a half-cycle period may be provided as a regulation period for the storage capacitor line voltage and the polarity may be inverted every other vertical scanning period. Then, as already described with reference toFIG. 17, the first change of storage capacitor voltages at the beginning of the third vertical scanning period can be different from the first change of storage capacitor voltages at the beginning of the first vertical scanning period. As a result, the brightness levels and polarities of subpixels can be changed as shown in portion(a) ofFIG.6.
• Furthermore, in the preferred embodiment described above, each regulation period is supposed to be an even number of times as long as one horizontal scanning period. However, the present invention is in no way limited to that specific preferred embodiment. Each regulation period may also be an odd number of times as long as one horizontal scanning period. Even if the first and third regulation periods have a cycle time of 37H and if the second and fourth regulation periods have a cycle time of 27H as shown inFIG.21, the degree of non-smoothness of the image on the screen can also be reduced by inverting the brightness levels and polarities of respective subpixels as in a situation where each regulation period is an even number of times as long as one horizontal scanning period.
• Furthermore, in the preferred embodiment described above, the same storage capacitor line is supposed to be connected to two subpixels belonging to two different adjacent pixels. However, the present invention is in no way limited to that specific preferred embodiment. Two different storage capacitor lines may also be provided for two subpixels belonging to two different adjacent pixels and the voltages on those two storage capacitor lines may be changed independently of each other.
• FIG.22 shows the brightness levels and polarities of respective subpixels that have changed within the effective scanning period of a certain frame. Specifically, inFIG.22, illustrated are pixels on the first through sixth rows and the a^th through f^th columns. In this example, the liquidcrystal display device100 also has ten storage capacitor trunksCS1 throughCS10. As shown inFIG.22, the storage capacitor trunkCS1 is connected to subpixels1-a-A,1-b-A,1-c-A; etc. on the first row of pixels and to subpixels6-a-A, 6-b-A,6-c-A, etc. on the sixth row of pixels. The storage capacitor trunkcs2 is connected to subpixels1-a-B,1-b-B, 1-c-B, etc. on the first row of pixels and to subpixels6-a-B,6-b-B,6-cB, etc. on the sixth row of pixels. And the storage capacitor trunkcs3 is connected to subpixels2-a-A,2-b-A,2-c-A, etc. on the second row of pixels. In this manner, in the liquidcrystal display device 100 with the configuration shown inFIG.22, a given subpixel and a subpixel belonging to another pixel adjacent to the former subpixel are connected to two different storage capacitor trunks and are electrically independent of each other.
• FIG.23 illustrates an equivalent circuit of the liquidcrystal display device100 with the configuration shown inFIG.22. AndFIG.24 shows the waveforms of various voltages (or signals) to drive the liquid crystal display device. InFIG.24, Vsa and Vsb represent the voltages on the signal linesSa andSb, Vg1 through Vg12 represent the voltages on the scan linesG1 throughG12, Vcs1 through Vcs10 represent the voltages on the storage capacitor trunksCS1 throughCS10 and VLsp1-a-A through VLsp2-b-B represent the effective voltages applied to the liquid crystal layers of the subpixels1-a-A through2-b-B, respectively. What is shown inFIG.24 is voltage waveforms within one vertical scanning period.
• As shown inFIG.24, the voltagesVcs1 throughVcs10 on the storage capacitor trunksCS1 throughCS10 oscillate with the same amplitude and in the same regular cycles. In this example, one oscillation cycle time is 10H. For example, the voltagesVcs1 andVcs2 have such a relation that if one of these two voltages changes into VcH, the other voltage will change into VcL and that if one of these two voltages changes into VcL, the other voltage will change into VcH. The other four pairs of voltagesVcs3 andVcs4, Vcs5 andVcs6,Vcs7 andVcs8, andVcs9 andVcs10 too have the same relation as that pair of voltagesVcs1 andVcs2. As can be seen fromFIG.24, after the voltageVg1 on the scan lineG1 has become VgL, the voltageVcs1 increases (↑) and the voltageVcs2 decreases (↓). As also can be seen fromFIG.24, after the voltageVg2 on the scan lineG2 has become VgL, the voltageVcs3 decreases (↓) and the voltageVcs4 increases (↑).
• In the configuration shown inFIG.22, subpixels belonging to two different rows are connected to mutually different storage capacitor trunks, and therefore, in each of multiple pixels, the voltages applied to the liquid crystal layer of the subpixels can be increased or decreased at the same time. In this case, all of the three conditions mentioned above can be satisfied by driving the liquid crystal display device having the configuration shown inFIG.22 with the voltage waveforms shown inFIG.24. As a result, a display of a quality image is realized with a flicker eliminated.
• The brightness levels and polarities of subpixels that have changed within the effective scanning period of a certain frame and the voltage waveforms have been described with reference toFIGS. 22 to24. In the next frame, however, the voltages on the signal lines change according to the waveforms shown inFIG.24 with respect to the voltages on the scan lines but the voltages on the storage capacitor trunks change inversely to the waveforms shown inFIG.24. That is why in that frame, the polarities of the respective subpixels are the same as those of the subpixels shown inFIG.22 but the brightness levels of the respective subpixels are inverted compared to the counterparts shown inFIG.22.
• In the frame after that next frame, with respect to the voltages on the scan lines, not only the voltages on the signal lines but also the voltages on the storage capacitor trunks change in the patterns opposite to the waveforms shown inFIG.24. Consequently, in that frame, the brightness levels of the respective subpixels are the same as those of the subpixels shown inFIG.22 but the polarities of the respective subpixels are inverted compared to the counterparts shown inFIG.22.
• And in the frame next to that frame, with respect to the voltages on the scan lines, the voltages on the signal lines change in the patterns opposite to the waveforms shown inFIG.24 but the voltages on the storage capacitor trunks change according to the waveforms shown inFIG.24. Consequently, in that frame, the brightness levels and polarities of the respective subpixels are inverted compared to the counterparts shown inFIG.22. In this manner, the liquid crystal display device with the configuration shown inFIG.22 can also reduce the viewing angle dependence of the γ characteristic and minimize the deterioration of display quality.
• Furthermore, in the preferred embodiment described above, asingle signal line14 is provided as a common line for twosubpixels10a and10b included in thesame pixel10 as shown inFIG.8. However, the present invention is in no way limited to that specific preferred embodiment. Two different signal lines may also be provided for two subpixels included in the same pixel. In that case, even if the voltages on storage capacitor lines are not changed subpixel by subpixel, mutually different effective voltages can also be applied to the liquid crystal layers of subpixels by varying the voltages on the signal lines.
• FIG.25 illustrates apixel10, of which the twosubpixels 10a and 10b are provided withsignal lines14a and14b, respectively. As shown inFIG.25, thepixel10 includes twosubpixel electrodes18a and18b that are connected to the twodifferent signal lines14a and14b via their associatedTFTs16a and16b, respectively. As these twosubpixels 10a and10b form onepixel10, theTFTs16a and16b have their gates connected to the same scan line (i.e., gate bus line)12 in common and have their ON/OFF states controlled using the same scan signal. On the other hand, signal voltages (or grayscale voltages) are supplied to the signal lines (i.e., source bus lines)14a and14b so as to satisfy the relation described above. It is preferred that the gates of theTFTs16a and16b be used in common.
• In the above description, the voltage applied to the counter electrode is shown to be constant. However, the present invention is in no way limited to that specific preferred embodiment. The voltage applied to the counter electrode may be changed with time.
• Furthermore,FIG.10 shows that the effective voltages applied to the first and second subpixels are different from each other in a broad grayscale range. However, the present invention is in no way limited to that specific preferred embodiment. The effective voltages applied to the subpixels could be different from each other only in a particular grayscale range (e.g., in the range of 36^th through 128^th grayscales in a 256 grayscale display in which the grayscale range from black to white is divided into 256 levels consisting of 0^th through 255^th grayscales).
• Furthermore, although it has been described how effectively the present invention contributes to improving the display quality of a normally black mode liquid crystal display device (e.g., an MVA mode LCD, among other things), the present invention is in no way limited to that specific preferred embodiment. If necessary, this invention is also applicable for use in an IPS mode liquid crystal display device. The viewing angle dependence of the γ characteristic is more significant in the MVA and ASM modes than in the IPS mode. In the IPS mode, however, it is more difficult to manufacture panels that can have a high contrast ratio in the frontal viewing direction than in the MVA and ASM modes. In view of these considerations, it can be seen that it is a more urgent task to overcome the viewing angle dependence problem of the γ characteristic of the MVA and ASM mode liquid crystal display devices.
EMBODIMENT 2
• Hereinafter, a second preferred embodiment of a liquidcrystal display device100 according to the present invention will be described. The liquidcrystal display device100 of this preferred embodiment is different from the counterpart of the first preferred embodiment described above in the brightness levels and polarities of subpixels and the order of change of the effective voltages in the four consecutive vertical scanning periods. In the following description, the similar description as that of theEmbodiment 1 is omitted for avoiding redundancy.
• It will be described with reference toFIG.26 how the brightness levels and electric field directions change in the subpixels and how the effective voltages applied to the liquid crystal layers of the first and second subpixels change in the liquidcrystal display device100 of this preferred embodiment.
• As shown in portion(a) ofFIG.26, the first, fourth and fifth periods are first polarity periods, while the second, third and sixth periods are second polarity periods. Looking at any series of four vertical scanning periods, it can be seen that two out of the four are first polarity periods and the rest is second polarity periods. For example, in the first through fourth periods shown in portion(a) of
  FIG.26, the first and fourth periods are first polarity periods and the second and third periods are second polarity periods. In the liquidcrystal display device100 of this preferred embodiment, however, the first polarity periods include a period that satisfies |VLspa|>|VLspb| (e.g., the first period in this example) and a period that satisfies | VLspa|<|VLspb| I (e.g., the fourth period in this example). Also, in this liquidcrystal display device100, the second polarity periods include a period that satisfies |VLspa|>| VLspb| (e.g., the third period in this example) and a period that satisfies |VLspa|<|VLspb| (e.g., the second period in this example).
• Portions(b) and(c) ofFIG.26 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. The effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels are the effective values of the differences between the voltages applied to the first and second subpixel electrodes and the voltageVc applied to the counter electrode. In this example, the voltageVc applied to the counter electrode is shown as being constant.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.26, the first period is a first polarity period and the first subpixel is brighter than the second subpixel.
• In the second period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.26, the second period is a second polarity period and the second subpixel is brighter than the first subpixel.
• In the third period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspb|>|VLspb|). For that reason, as shown in portion(a) ofFIG.26, the third period is a second polarity period and the first subpixel is brighter than the second subpixel.
• In the fourth period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspb|<|VLspb|). For that reason, as shown in portion(a) ofFIG.26, the fourth period is a first polarity period and the second subpixel is brighter than the first subpixel. From the fifth period on, the brightness levels and polarities of the first and second subpixels will vary in quite the same pattern as the first and second subpixels in the first through fourth periods.
• Thus, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (D, -), (B, -) and (D, +), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (B, -), (D, -) and (B, +) as shown in portion(a) ofFIG.26. In this manner, the liquid crystal display device of this preferred embodiment inverts the brightness levels of the subpixels every vertical scanning period and also inverts their polarities every other vertical scanning period. In the liquid crystal display device of this preferred embodiment, since the brightness levels of the subpixels are inverted every vertical scanning period as in the liquid crystal display device of the first preferred embodiment, the degree of non-smoothness of the image on the screen can be reduced. Also, in the liquid crystal display device of this preferred embodiment, each set of first and second polarity periods has a period in which the first subpixel is brighter than the second subpixel as in the liquid crystal display device of the first preferred embodiment. Thus, as can be seen from portions(b) and(c) ofFIG.26, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be equal to each other. Furthermore, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage. As a result, the residual image and other reliability-related problems can be overcome.
• FIG.27 shows the brightness levels and polarities of the first and second subpixels and the first change of voltages on the storage capacitor lines at the vertical scanning period of the first and second subpixels. InFIG.27, the four consecutive frames are identified by n, n+1, n+2 and n+3, respectively.
• As shown inFIG.27, in frame n, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓". In the next framen+1, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓".
• In the framen+2, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑". In the next framen+3, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑".
• If the first and second subpixels shown in portion(a) ofFIG.6, which have been referred to for the description of the first preferred embodiment, were interchanged with each other, the brightness levels and polarities of the subpixels in the second through fifth periods would correspond with those of the subpixels in the first through fourth periods shown in portion(a) ofFIG.26. That is why if the display area of the first subpixel electrode is as large as that of the second subpixel electrode, then the liquid crystal display device of this preferred embodiment will achieve substantially the same effects as the counterpart of the first preferred embodiment described above.
EMBODIMENT 3
• Hereinafter, a third preferred embodiment of a liquidcrystal display device100 according to the present invention will be described. The liquidcrystal display device100 of this preferred embodiment is different from the counterparts described above in the brightness levels and polarities of subpixels and the order of change of the effective voltages in the four consecutive vertical scanning periods. In the following description, the repeated description is omitted for avoiding redundancy.
• It will be described with reference toFIG.28 how the brightness levels and polarities change in the subpixels and how the effective voltages applied to the liquid crystal layers of the first and second subpixels change in the liquidcrystal display device100 of this preferred embodiment.
• As shown in portion(a) ofFIG.28, the first, third and fifth periods are first polarity periods, while the second, fourth and sixth periods are second polarity periods in the liquidcrystal display device100 of this preferred embodiment. Looking at any series of four vertical scanning periods, it can be seen that two out of the four are first polarity periods and the rest is second polarity periods. For example, in the first through fourth periods shown in portion(a) ofFIG.28, the first and third periods are first polarity periods and the second and fourth periods are second polarity periods. In the liquidcrystal display device 100 of this preferred embodiment, however, the first polarity periods include a period that satisfies |VLspa|>|VLspb| I (e.g., the first period in this example) and a period that satisfies | VLspa|<|VLspb| (e.g., the third period in this example). Also, in this liquidcrystal display device100, the second polarity periods include a period that satisfies |VLspb|>| VLspb| (e.g., the second period in this example) and a period that satisfies |VLspa|<|VLspb| (e.g., the fourth period in this example).
• Portions(b) and(c) ofFIG.28 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. The effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels are the effective values of the differences between the voltages applied to the first and second subpixel electrodes and the voltageVc applied to the counter electrode. In this example, the voltage Vc applied to the counter electrode is shown as being constant.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspb|>|VLspa|). For that reason, as shown in portion(a) ofFIG.28, the first period is a first polarity period and the first subpixel is brighter than the second subpixel.
• In the second period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspb|>|VLspb|). For that reason, as shown in portion(a) ofFIG.28, the second period is a second polarity period and the first subpixel is brighter than the second subpixel.
• In the third period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.28, the third period is a first polarity period and the second subpixel is brighter than the first subpixel.
• In the fourth period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.28, the fourth period is a second polarity period and the second subpixel is brighter than the first subpixel. From the fifth period on, the brightness levels and polarities of the first and second subpixels will vary in quite the same pattern as the first and second subpixels in the first through fourth periods. In the liquid crystal display device of this preferred embodiment, the frame frequency may be 120 Hz, for example.
• Thus, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (B, -), (D, +) and (D, -), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (D, -), (B, +) and (B, -) as shown in portion(a) ofFIG.28. In this manner, the liquid crystal display device of this preferred embodiment inverts the brightness levels of the subpixels every other vertical scanning period and also inverts their polarities every vertical scanning period. In the liquid crystal display device of this preferred embodiment, since the brightness levels of the subpixels are inverted every other vertical scanning period unlike the liquid crystal display device disclosed in Patent Document No. 1, the degree of non-smoothness of the image on the screen can be reduced. Also, in the liquid crystal display device of this preferred embodiment, the brightness levels of the first and second subpixels are inverted in any of the first and second polarity periods unlike the liquid crystal display device disclosed in Patent Document No. 2. Thus, as can be seen from portions(b) and(c) ofFIG.28, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be approximately equal to each other. Furthermore, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage. As a result, the residual image and other reliability-related problems can be overcome.
• Next, it will be described with reference toFIG.29 how the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change over multiple vertical scanning periods. InFIG.29, Vg represents the voltage on the scan line, Vcsa and Vcsb represent the voltages on the first and second storage capacitor lines, respectively, and VLspa and VLspb represent the effective voltages applied to the respective liquid crystal layers of the first and second subpixels. In this example, the voltages on the first and second storage capacitor lines vary in regular cycles of 20H by increasing or decreasing every 10H through the display periodsAH. On the other hand, the voltages on the first and second storage capacitor lines increase or decrease every 18H during the first and third regulation periodsBH and increase or decrease every 13H during the second and fourth regulation periodsBH.
• The effective voltages applied to the respective liquid crystal layers of the first and second subpixels change as the voltages on the first and second storage capacitor lines vary. As a result, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (B, -), (D, +) and (D, -), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (D, -), (B, +) and (B, -). In this manner, the brightness levels and polarities of the first and second subpixels change as shown in portion(a) ofFIG.28. Consequently, the liquid crystal display device of this preferred embodiment can minimize the deterioration of display quality with the viewing angle dependence of the γ characteristic reduced.
• FIG. 30 shows the brightness levels and polarities of the first and second subpixels and the first change of voltages on the storage capacitor lines at the vertical scanning period of the first and second subpixels. InFIG.30, the four consecutive frames are identified by n, n+1, n+2 and n+3, respectively.
• As shown inFIG.30, in framen, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓". In the next framen+1, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑".
• In the framen+2, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑". In the next framen+3, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓".
• ComparingFIGS.17 and30 to each other, it can be seen that the first change of voltages on the storage capacitor line at the vertical scanning period of the first or second subpixel in the liquid crystal display device of this preferred embodiment is the same as in the counterpart of the first preferred embodiment described above. However, the polarities change differently in the liquid crystal display device of this preferred embodiment from in the first preferred embodiment described above.
• Hereinafter, the difference in the brightness inversion interval of the subpixels between the liquid crystal display device of this preferred embodiment and the counterpart of the first preferred embodiment will be described. Specifically, in the liquid crystal display device of this preferred embodiment, the brightness levels of the subpixels invert every other vertical scanning period as shown inFIG.28. On the other hand, in the liquid crystal display device of the first preferred embodiment described above, the brightness levels of the subpixels invert every vertical scanning period as shown inFIG.6. That is to say, the subpixel brightness inversion interval of the liquid crystal display device of this preferred embodiment is twice as long as that of the liquid crystal display device of the first preferred embodiment. The non-smoothness of the image on the screen can be reduced by inverting the brightness levels of the subpixels as described above. In this case, the shorter the subpixel brightness inversion interval, the more significantly the non-smoothness can be reduced. Nevertheless, if one vertical scanning period became too short, then the orientations of the liquid crystal molecules could not change so much within one vertical scanning period that the luminance could fall short of a predetermined value. That is to say, if one vertical scanning period were too short for the response speed of liquid crystal molecules, the difference in luminance between the subpixels would not be so much as to reduce the viewing angle dependence of the γ characteristic significantly.
• The following Table 1 summarizes how the display qualities of the liquid crystal display devices disclosed in Patent Documents Nos. 1 and 2 and the device of the first and this preferred embodiments of the present invention were affected when the frame frequencies were changed. In Table 1, a good display quality is indicated by the open circle ○, while a poor display quality is indicated by the cross ×.
• Table 1

  Frame frequency	50 Hz	60 Hz	75 Hz	90Hz	120 Hz
  PATENT DOCUMENT #1
  Improvement of viewing angle	○	○	○	○	○
  characteristic
  Image non-smoothness	×	×	×	×	×
  Flicker	○	○	○	○	○
  Reliability	○	○	○	○	○

  PATENT DOCUMENT #2
  Improvement of viewing angle	○	○	○	○	×
  characteristic
  Image non-smoothness	○	○	○	○	○
  Flicker	○	○	○	○	○
  Reliability	×	×	×	×	×

  EMBODIMENT 1 (seeFIG. 6)
  Improvement of viewing angle	○	○	○	○	×
  characteristic
  Image non-smoothness	○	○	○	○	○
  Flicker	×	○	○	○	○
  Reliability	○	○	○	○	○

  EMBODIMENT 3 (seeFIG. 28)
  Improvement of viewing angle	○	○	○	○	○
  characteristic
  Image non-smoothness	○	○	○	○	○
  Flicker	×	×	×	×	○
  Reliability	○	○	○	○	○

• According to Table 1, the liquid crystal display device of Patent Document No. 1 improves the viewing angle characteristic at every frame frequency but made the viewer find the image on the screen non-smooth at any frame frequency, which is a problem. Meanwhile, as for the liquid crystal display device disclosed in Patent Document No. 2, its reliability was too questionable to manufacture it on an industrial basis.
• On the other hand, the liquid crystal display devices of the first and third preferred embodiments of the present invention raised no reliability issues unlike the device of Patent Document No. 2, and therefore, can be manufactured on an industrial basis with no problem at all. Added to that, the liquid crystal display devices of the first and third preferred embodiments could also overcome the image non-smoothness problem with the device of Patent Document No. 1.
• Comparing the liquid crystal display devices of the first and third preferred embodiments to each other, however, it can be seen that the best selection should be made according to the frame frequency so that the improvement of the viewing angle characteristic and the reduction of the flicker are achieved at the same time. Specifically, as shown in Table 1, the liquid crystal display device of the first preferred embodiment achieved good display qualities at frame frequencies of equal to or more than 60 Hz and equal to less than 90 Hz. On the other hand, the liquid crystal display device of this preferred embodiment could present a flicker-free image as long as the frame frequency was equal to or higher than 120 Hz. The present inventors confirmed via experiments that if the frame frequency was equal to or higher than 120 Hz, the liquid crystal display device of this preferred embodiment could reduce the viewing angle dependence of the γ characteristic sufficiently effectively. Once the frame frequency exceeds that value, however, it is preferred that the response speed be increased by changing the liquid crystal materials or driving methods into more appropriate ones.
EMBODIMENT 4
• Hereinafter, a fourth preferred embodiment of a liquidcrystal display device100 according to the present invention will be described. The liquidcrystal display device 100 of this preferred embodiment is different from the counterparts described above in the brightness levels and polarities of subpixels and the order of change of the effective voltages in the four consecutive vertical scanning periods. In the following description, the repeated description is omitted for avoiding redundancy.
• It will be described with reference toFIG.31 how the brightness levels and polarities change in the subpixels and how the effective voltages applied to the liquid crystal layers of the first and second subpixels change in the liquidcrystal display device100 of this preferred embodiment.
• As shown in portion(a) ofFIG.31, the first, third and fifth periods are first polarity periods, while the second, fourth and sixth periods are second polarity periods in the liquidcrystal display device100 of this preferred embodiment. Looking at any series of four vertical scanning periods, it can be seen that two out of the four are first polarity periods and the rest is second polarity periods. For example, in the first through fourth periods shown in portion(a) ofFIG.31, the first and third periods are first polarity periods and the second and fourth periods are second polarity periods. In the liquidcrystal display device100 of this preferred embodiment, however, the first polarity periods include a period that satisfies |VLspa|>|VLspb| (e.g., the first period in this example) and a period that satisfies | VLspb|<|VLspb| I (e.g., the third period in this example). Also, in this liquidcrystal display device 100, the second polarity periods include a period that satisfies |VLspb|>| VLspb| (e.g., the fourth period in this example) and a period that satisfies |VLspa|<|VLspb| (e.g., the second period in this example).
• Portions(b) and (c) ofFIG.31 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. The effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels are the effective values of the differences between the voltages applied to the first and second subpixel electrodes and the voltageVc applied to the counter electrode. In this example, the voltageVc applied to the counter electrode is shown as being constant.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspb|>|VLspb|). For that reason, as shown in portion(a) ofFIG.31, the first period is a first polarity period and the first subpixel is brighter than the second subpixel.
• In the second period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspb|<|VLspb|). For that reason, as shown in portion(a) ofFIG.31, the second period is a second polarity period and the second subpixel is brighter than the first subpixel.
• In the third period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.31, the third period is a first polarity period and the second subpixel is brighter than the first subpixel.
• In the fourth period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspb|>|VLspb|). For that reason, as shown in portion(a) ofFIG.31, the fourth period is a second polarity period and the first subpixel is brighter than the second subpixel. From the fifth period on, the brightness levels and polarities of the first and second subpixels will vary in quite the same pattern as the first and second subpixels in the first through fourth periods.
• Thus, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (D, -), (D, +) and (B, -), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (B, -), (B, +) and (D, -) as shown in portion(a) ofFIG.31. In this manner, the liquid crystal display device of this preferred embodiment inverts the brightness levels of the subpixels every other vertical scanning period and also inverts their polarities every vertical scanning period. In this preferred embodiment, the frame frequency may be 120 Hz, for example.
• FIG.32 shows the brightness levels and polarities of the first and second subpixels and the first change of voltages on the storage capacitor lines at the vertical scanning period of the first and second subpixels. InFIG.32, the four consecutive frames are identified by n, n+1, n+2 and n+3, respectively.
• As shown inFIG.32, in frame n, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓". In the next frame n+1, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓".
• In the framen+2, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑". In the next framen+3, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑".
• In the liquid crystal display device of this preferred embodiment as the liquid crystal display device of the third preferred embodiment, since the brightness levels of the subpixels are inverted every other vertical scanning period, the degree of non-smoothness of the image on the screen can be reduced. In the liquid crystal display device of this preferred embodiment as the liquid crystal display device of the third preferred embodiment, since the brightness levels of the first and second subpixels are inverted in each of the first and second polarity periods, as can be seen from portions(b) and(c) ofFIG.31, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be approximately equal to each other. Furthermore, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage. As a result, the residual image and other reliability-related problems can be overcome.
• If the polarities were inverted in portion(a) ofFIG.28, which has been referred to for the description of the liquid crystal display device of the third preferred embodiment, then the brightness levels and polarities of the subpixels in the second through fifth periods would correspond with those of the subpixels in the first through fourth periods shown in portion(a) ofFIG.31. Consequently, the liquid crystal display device of this preferred embodiment would achieve substantially the same effects as the counterpart of the third preferred embodiment described above.
• If the brightness levels and polarities of the subpixels1-a-A and1-a-B change as in the first through fourth periods shown in portion(a) ofFIG.31 when the liquid crystal display device of the third preferred embodiment is subjected to the dot inversion drive as already described with reference toFIGS.14 and15, then the brightness levels and polarities of the subpixels2-a-A and2-a-B will change as in the second through fifth periods shown in portion(a) ofFIG.28.
EMBODIMENT 5
• Hereinafter, a fifth preferred embodiment of a liquid crystal display device according to the present invention will be described. The liquidcrystal display device100 of this preferred embodiment is different from the counterparts described above in the brightness levels and polarities of subpixels and the order of change of the effective voltages in the four consecutive vertical scanning periods. In the following description, the repeated description is omitted for avoiding redundancy.
• It will be described with reference toFIG.33 how the brightness levels and polarities change in the subpixels and how the effective voltages applied to the liquid crystal layers of the first and second subpixels change in the liquidcrystal display device100 of this preferred embodiment.
• As shown in portion(a) ofFIG.33, the first, fourth and fifth periods are first polarity periods, while the second, third and sixth periods are second polarity periods in the liquidcrystal display device100 of this preferred embodiment. Looking at any series of four vertical scanning periods, it can be seen that two out of the four are first polarity periods and the rest is second polarity periods. For example, in the first through fourth periods shown in portion(a) ofFIG.33, the first and fourth periods are first polarity periods and the second and third periods are second polarity periods. In the liquidcrystal display device100 of this preferred embodiment, however, the first polarity periods include a period that satisfies |VLspa|>|VLspb| (e.g., the first period in this example) and a period that satisfies | VLspa|<|VLspb| (e.g., the fourth period in this example). Also, in this liquidcrystal display device100, the second polarity periods include a period that satisfies |VLspa|>| VLspb| (e.g., the second period in this example) and a period that satisfies VLspa|<|VLspb| (e.g., the third period in this example).
• Portions(b) and(c) ofFIG.33 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. The effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels are the effective values of the differences between the voltages applied to the first and second subpixel electrodes and the voltageVc applied to the counter electrode. In this example, the voltage Vc applied to the counter electrode is shown as being constant.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.33, the first period is a first polarity period and the first subpixel is brighter than the second subpixel.
• In the second period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.33, the second period is a second polarity period and the first subpixel is brighter than the second subpixel.
• In the third period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.33, the third period is a second polarity period and the second subpixel is brighter than the first subpixel.
• In the fourth period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.33, the fourth period is a first polarity period and the second subpixel is brighter than the first subpixel. From the fifth period on, the brightness levels and polarities of the first and second subpixels will vary in quite the same pattern as the first and second subpixels in the first through fourth periods. In the liquid crystal display device of this preferred embodiment, the frame frequency may be 120 Hz, for example.
• Thus, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (B, -), (D, -) and (D, +), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (D, -), (B, -) and (B, +) as shown in portion(a) ofFIG.33. In this manner, the liquid crystal display device of this preferred embodiment inverts the brightness levels of the subpixels every other vertical scanning period and also inverts their polarities every other vertical scanning period. But the timing of inversion of the polarities is shifted by one vertical scanning period from that of the brightness levels of the subpixels. In the liquid crystal display device of this preferred embodiment, since the brightness levels of the subpixels are inverted every other vertical scanning period unlike the liquid crystal display device disclosed in Patent Document No. 1, the degree of non-smoothness of the image on the screen can be reduced. Also, in the liquid crystal display device of this preferred embodiment, the brightness levels of the first and second subpixels are inverted in any of the first and second polarity periods unlike the liquid crystal display device disclosed in Patent Document No. 2. Thus, as can be seen from portions(b) and(c) ofFIG.33, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be approximately equal to each other. Furthermore, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage. As a result, the residual image and other reliability-related problems can be overcome.
• Next, it will be described with reference toFIG.34 how the voltages change over multiple vertical scanning periods.
• InFIG.34, Vg represents the voltage on the scan line, Vcsa and Vcsb represent the voltages on the first and second storage capacitor lines, respectively, and VLspa and VLspb represent the effective voltages applied to the respective liquid crystal layers of the first and second subpixels. In this example, the voltages on the first and second storage capacitor lines vary in regular cycles of 20H by increasing or decreasing every 10H through the display periodsAH. On the other hand, the voltages on the first and second storage capacitor lines increase or decrease every 18H during the first through fourth regulation periodsBH.
• FIG.35 shows the brightness levels and polarities of the first and second subpixels and the first change of voltages on the storage capacitor lines at the vertical scanning period of the first and second subpixels. InFIG.35, the four consecutive frames are identified by n, n+1, n+2 and n+3, respectively.
• As shown inFIG.35, in framen, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓", In the next framen+1, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑".
• In the framen+2, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓". In the next framen+3, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑".
• As described above, the effective voltages applied to the respective liquid crystal layers of the first and second subpixels change as the voltages on the first and second storage capacitor lines vary. As a result, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (B, -), (D, -) and (D, +), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (D, -), (B, -) and (B, +). Consequently, the liquid crystal display device of this preferred embodiment can minimize the deterioration of display quality with the viewing angle dependence of the γ characteristic reduced.
EMBODIMENT 6
• Hereinafter, a sixth preferred embodiment of a liquid crystal display device according to the present invention will be described. The liquidcrystal display device 100 of this preferred embodiment is different from the counterparts described above in the brightness levels and polarities of subpixels and the order of change of the effective voltages in the four consecutive vertical scanning periods. In the following description, the repeated description is omitted for avoiding redundancy.
• It will be described with reference toFIG.36 how the brightness levels and polarities change in the subpixels and how the effective voltages applied to the liquid crystal layers of the first and second subpixels change in the liquidcrystal display device100 of this preferred embodiment.
• As shown in portion(a) ofFIG.36, the first, second, fifth and sixth periods are first polarity periods, while the third and fourth periods are second polarity periods in the liquidcrystal display device100 of this preferred embodiment. Looking at any series of four vertical scanning periods, it can be seen that two out of the four are first polarity periods and the rest is second polarity periods. For example, in the first through fourth periods shown in portion(a) ofFIG.36, the first and second periods are first polarity periods and the third and fourth periods are second polarity periods. In the liquidcrystal display device 100 of this preferred embodiment, however, the first polarity periods include a period that satisfies |VLspa|>|VLspb| (e.g., the first period in this example) and a period that satisfies | VLspa|<|VLspb|I (e.g., the second period in this example). Also, in this liquidcrystal display device 100, the second polarity periods include a period that satisfies |VLspa|| VLspb| (e.g., the fourth period in this example) and a period that satisfies |VLspa|<|VLspb| (e.g., the third period in this example).
• Portions(b) and (c) ofFIG.36 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. The effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels are the effective values of the differences between the voltages applied to the first and second subpixel electrodes and the voltage Vc applied to the counter electrode. In this example, the voltage Vc applied to the counter electrode is shown as being constant.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.36, the first period is a first polarity period and the first subpixel is brighter than the second subpixel.
• In the second period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.36, the second period is a first polarity period and the second subpixel is brighter than the first subpixel.
• In the third period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is smaller than that of the effective voltage applied to that of the second subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.36, the third period is a second polarity period and the second subpixel is brighter than the first subpixel.
• In the fourth period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.36, the fourth period is a second polarity period and the first subpixel is brighter than the second subpixel. From the fifth period on, the brightness levels and polarities of the first and second subpixels will vary in quite the same pattern as the first and second subpixels in the first through fourth periods.
• Thus, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (D, +), (D, -) and (B, -), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (B, +), (B, -) and (D, -) as shown in portion(a) ofFIG.36. In this manner, the liquid crystal display device of this preferred embodiment inverts the brightness levels of the subpixels every other vertical scanning period and also inverts their polarities every other vertical scanning period. But the timing of inversion of the polarities is shifted by one vertical scanning period from that of the brightness levels of the subpixels. In the liquid crystal display device of this preferred embodiment, since the brightness levels of the subpixels are inverted every other vertical scanning period as in the liquid crystal display device of the fifth preferred embodiment, the degree of non-smoothness of the image on the screen can be reduced. Also, in the liquid crystal display device of this preferred embodiment, the brightness levels of the first and second subpixels are inverted in any of the first and second polarity periods as in the liquid crystal display device of the fifth preferred embodiment. Thus, as can be seen from portions(b) and(c) ofFIG.36, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be approximately equal to each other. Furthermore, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage. As a result, the residual image and other reliability-related problems can be overcome.
• FIG.37 shows the brightness levels and polarities of the first and second subpixels and the first change of voltages on the storage capacitor lines at the vertical scanning period of the first and second subpixels. InFIG.37, the four consecutive frames are identified by n, n+1, n+2 and n+3, respectively.
• As shown inFIG.37, in framen, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓". In the next framen+1, the polarity of the first and second subpixels is positive "+", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑".
• In the framen+2, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is increase "↑", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is decrease "↓". In the next framen+3, the polarity of the first and second subpixels is negative "-", the first change of voltages on the storage capacitor line at the vertical scanning period of the first subpixel is decrease "↓", and the first change of voltages on the storage capacitor line at the vertical scanning period of the second subpixel is increase "↑".
• If the first and second subpixels shown in portion(a) ofFIG.36 were interchanged with each other, the brightness levels and polarities of the subpixels in the second through fifth periods would correspond with those of the subpixels in the first through fourth periods shown in portion(a) ofFIG.33, which has been referred to for the description of the fifth preferred embodiment. That is why if the display area of the first subpixel electrode is as large as that of the second subpixel electrode, then the liquid crystal display device of this preferred embodiment will achieve substantially the same effects as the counterpart of the fifth preferred embodiment described above.
• If the brightness levels and polarities of the subpixels1-a-A and1-a-B change as in the first through fourth periods shown in portion(a) ofFIG.36 when the liquid crystal display device of this sixth preferred embodiment is subjected to the dot inversion drive as already described with reference toFIGS.14 and15, then the brightness levels and polarities of the subpixels2-a-A and2-a-B will change as in the second through fifth periods shown in portion(a) ofFIG. 33.
EMBODIMENT 7
• Hereinafter, a seventh preferred embodiment of a liquid crystal display device according to the present invention will be described. The liquidcrystal display device100 of this preferred embodiment is different from the counterparts of the first through sixth preferred embodiments described above in the subpixels change their luminances by way of a moderate luminance. In the following description, the repeated description is omitted for avoiding redundancy.
• It will be described with reference toFIG.38 how the brightness levels and polarities change in the subpixels and how the effective voltages applied to the liquid crystal layers of the first and second subpixels change in the liquidcrystal display device100 of this preferred embodiment. As shown in portion(a) ofFIG.38, the first, third, and fifth periods are first polarity periods, while the second, fourth and sixth periods are second polarity periods in the liquidcrystal display device100 of this preferred embodiment. Looking at any series of four vertical scanning periods, it can be seen that two out of the four are first polarity periods and the rest is second polarity periods. For example, in the first through fourth periods, the first and third periods are first polarity periods and the second and fourth periods are second polarity periods. The first polarity periods include a period that satisfies |VLspa|>|VLspb| (e.g., the first period in this example) and a period that satisfies |VLspa|<|VLspb| (e.g., the third period in this example). On the other hand, in the second polarity periods, VLspa = VLspb (e.g., the second and fourth periods in this example).
• Portions(b) and(c) ofFIG.38 show the effective voltagesVLspa andVLspb that are applied to the respective liquid crystal layers of the first and second subpixels in the respective vertical scanning periods. The levels of these voltages are indicated by the bold lines. The effective voltagesVLspa andVLspb applied to the respective liquid crystal layers of the first and second subpixels are the effective values of the differences between the voltages applied to the first and second subpixel electrodes and the voltage Vc applied to the counter electrode. In this example, the voltage Vc applied to the counter electrode is shown as being constant.
• In the first period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the first subpixel is greater than that of the effective voltage applied to that of the second subpixel (| VLspa|>|VLspb|). For that reason, as shown in portion(a) ofFIG.38, the first period is a first polarity period and the first subpixel is brighter than the second subpixel.
• In the second period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the effective voltage applied to the liquid crystal layer of the first subpixel is equal to the one applied to that of the second subpixel (VLspa=VLspb). For that reason, as shown in portion(a) ofFIG.38, the second period is a second polarity period and the first subpixel is as bright as the second subpixel.
• In the third period, the voltages applied to the first and second subpixel electrodes are higher than the voltage applied to the counter electrode, and the absolute value of the effective voltage applied to the liquid crystal layer of the second subpixel is greater than that of the effective voltage applied to that of the first subpixel (| VLspa|<|VLspb|). For that reason, as shown in portion(a) ofFIG.38, the third period is a first polarity period and the second subpixel is brighter than the first subpixel.
• In the fourth period, the voltages applied to the first and second subpixel electrodes are lower than the voltage applied to the counter electrode, and the effective voltage applied to the liquid crystal layer of the first subpixel is equal to the one applied to that of the second subpixel (VLspa = VLspb) : For that reason, as shown in portion(a) ofFIG.38, the fourth period is a second polarity period and the first subpixel is as bright as the second subpixel. From the fifth period on, the brightness levels and polarities of the first and second subpixels will vary in quite the same pattern as the first and second subpixels in the first through fourth periods.
• Thus, the (brightness, polarity) combination of the first subpixel changes in the order of (B, +), (M(oderate), - ), (D, +) and (M, -), while the (brightness, polarity) combination of the second subpixel changes in the order of (D, +), (M, -), (B, +) and (M, -) as shown in portion(a) ofFIG.38, where "M" means that the brightness (or luminance) of the first subpixel is equal to that of the second subpixel. In this manner, the liquid crystal display device of this preferred embodiment changes the luminances of the subpixels in three steps by way of a moderate luminance every vertical scanning period and also inverts the polarities every vertical scanning period.
• In the liquid crystal display device of this preferred embodiment, since the brightness levels of the subpixels are inverted, the degree of non-smoothness of the image on the screen can be reduced. Also, as can be seen from portions(b) and(c) ofFIG.38, in the liquid crystal display device of this preferred embodiment, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be approximately equal to each other. Furthermore, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage. As a result, the residual image and other reliability-related problems can be overcome.
• Next, it will be described with reference toFIGS.39A,39B and40 how the effective voltages applied to the respective liquid crystal layers of subpixels vary in the liquid crystal display device of this preferred embodiment. In the following description, a series of four frames (corresponding to four vertical scanning periods) will be identified herein by n, n+1, n+2 and n+3, respectively.
• FIG.39A illustrates the brightness levels and polarities of respective subpixels that have changed in frame n, whileFIG.39B illustrates the brightness levels and polarities of respective subpixels that have changed inframen+1. The liquid crystal display device of this preferred embodiment has a pixel arrangement such as the one shown inFIGS.39A and39B, which is the same as the one that has been described for the liquid crystal display device of the first preferred embodiment with reference toFIG.14. Thus, the repeated description is omitted in order to avoid complicating the description excessively. The liquid crystal display device of this preferred embodiment includes twelve storage capacitor trunks. InFIGS.39A and39B, the storage capacitor lines that are connected to the twelve storage capacitor trunks are identified herein by CS1, CS2, CS3, ... and CS12, respectively.
• As an example, it will be described how the brightness levels and polarities of subpixels that are included in pixels1-a,1-b,2-a and2-b change. In the framen, the pixels1-a and2-b have the first polarity (+), while the pixels 1-b and 2-a have the second polarity (-) as shown inFIG.39A. Also, each of the subpixels1-a-A,1-b-B,2-a-A and2-b-A is brighter than the other subpixel of the pixel. Next, in the framen+1, the luminances of the respective subpixels change into a moderate one and the polarities of the respective subpixels are inverted compared to the ones during the frame n as shown inFIG.39B. Subsequently, in the framen+2, the polarities of the respective subpixels are inverted compared to the ones during the framen+1 to be the same as the ones shown inFIG.39A, while the brightness levels of the respective subpixels are inverted compared to the ones shown inFIG.39A. Thereafter, in the framen+3, the luminances of the respective subpixels change into a moderate one and the polarities of the respective subpixels are inverted to be the same as the ones shown inFIG.39B.
• Next, it will be described how the liquid crystal display device of this preferred embodiment satisfies the three conditions described above to minimize a flicker.
• Just like the liquid crystal display device of the first preferred embodiment that has already been described with reference toFIG.15, the liquid crystal display device of this preferred embodiment regulates the voltages on the respective signal lines and the voltage applied to the counter electrode appropriately, thereby equalizing the effective voltages applied to the liquid crystal layer in respective electric field directions as closely as possible and satisfying the first condition. In addition, in the liquid crystal display device of this preferred embodiment, pixels with mutually different polarities are arranged adjacent to each other as shown inFIGS.39A and39B, thereby satisfying the second condition as well. Furthermore, in the liquid crystal display device of this preferred embodiment, subpixels, each of which is brighter than the other subpixel of the same pixel, are arranged as randomly as possible, e.g., such that the "B" and "D" signs are arranged on a subpixel-by-subpixel basis in a checkered pattern as shown inFIG.39A, thereby satisfying the third condition, too.
• The following Table 2 summarizes how the display qualities of the liquid crystal display devices of the first, third and the present preferred embodiments were affected when the frame frequencies were changed. In Table 2, a good display quality is indicated by the open circle ○, while a poor display quality is indicated by the cross ×. As shown in Table 2, the liquid crystal display device of this preferred embodiment achieved good display qualities at frame frequencies of 90 Hz or more.
• Table 2

  Frame frequency	50 Hz	60 Hz	75 Hz	90Hz	120 Hz
  EMBODIMENT 1 (seeFIG. 6)
  Improvement of viewing angle	○	○	○	○	×
  characteristic
  Image non-smoothness	○	○	○	○	○
  Flicker	×	○	○	○	○
  Reliability	○	○	○	○	○

  EMBODIMENT 3 (seeFIG. 28)
  Improvement of viewing angle	○	○	○	○	○
  characteristic
  Image non-smoothness	○	○	○	○	○
  Flicker	×	×	×	×	○
  Reliability	○	○	○	○	○

  EMBODIMENT 7 (seeFIG. 38)
  Improvement of viewing angle	○	○	○	○	○
  characteristic
  Image non-smoothness	○	○	○	○	○
  Flicker	×	×	×	O	O
  Reliability	○	○	○	○	○

• Hereinafter, the changes in the voltages on the signal lines, the voltages on the first and second storage capacitor trunks, the voltages on the scan line, and the effective voltages applied to the respective liquid crystal layers of subpixels1-a-A and1-a-B that are enclosed with the dashed lines inFIGS.39A and39B in the liquid crystal display device of this preferred embodiment will be described with reference toFIG.40. InFIG.40, Vsa and Vsb represent the voltages on the signal linesSa andSb, Vcs1 and Vcs2 represent the voltages on the first and second storage capacitor trunkscs2 andcs2, Vg1 represents the voltages on the scan lineG1, and VLsp1-a-A and VLsp1-b-B represent the effective voltages applied to the liquid crystal layer of the subpixels1-a-A and1-a-B, respectively.
• FIG.40 shows the waveforms of the respective voltages in the four frames of n through n+3. As described with reference toFIGS.38,39A and39B, the subpixels1-a-A and1-a-B have their polarities inverted in the order of (+, - , +, -) while having their luminances changed in the patterns (B, M, D, M) and (D, M, B, M), respectively. In each frame, the write operation is started when the voltageVg1 on the scan lineG1 goes VgH (high level). One vertical scanning period V-Total of the input video signal has a duration of 801H. The voltageVcs1 on the first storage capacitor trunkcs1 has such a waveform that completes one cycle of its level change in the order of the first, second, third and second levelsVL1,VL2,VL3 andVL2 every 6H period. And the voltagesVcs1 andVcs2 have phases that are different from each other by 180 degrees.
• InFIG.40, the interval between the point in time when the voltageVg1 on the scan lineG1 goes VgL (i.e., low level) and the point in time when the voltagesVcs1 andVcs2 on the storage capacitor lines change for the first time is 3H. The display period of the voltageVcs1 on the first storage capacitor trunkcs1 (i.e., the first waveform period) has a cycle of 24H and each period in which its amplitude continues to be constant at the first, second or third level has a length of 6H. That is why 3H is a half of the period in which the voltageVcs on the storage capacitor line has constant amplitude (i.e., a quarter of one cycle of each display period).
• In the framesn andn+2, while the scan lineG1 is selected, the voltageVsa on the signal lineSa is higher than the voltage at the counter electrode. On the other hand, in the framesn+1 andn+3, while the scan lineG1 is selected, the voltageVsa on the signal lineSa is lower than the voltage at the counter electrode.
• Hereinafter, it will be described with reference toFIG.40 how the brightness levels and polarities of these subpixels1-a-A and1-a-B of the pixel1-a change from the frame n through theframen+3.
• In the frame n, when the voltageVcs1 on the first storage capacitor trunk is maintained at the first level after having decreased from the second level, the scan lineG1 is selected (i.e., the voltageVg on the scan line goes VgH). When the scan lineG1 is selected, voltages higher than the one at the counter electrode are applied to the subpixel electrodes of the subpixels1-a-A and1-a-B. After the voltageVg1 on the scan lineG1 has fallen to VgL again, the voltageVcs1 on the first storage capacitor trunk will vary periodically. In the case that the voltageVg1 on the scan lineG1 goes down from VgH to VgL again, the voltageVcs1 on the first storage capacitor trunk is VL1, while the voltageVcs2 on the second storage capacitor trunk is VL3. Since the average voltageVL2 of the voltagesVcs1 andVcs2 on the first and second storage capacitor trunks is higher than VL1 but lower than VL3, the absolute value of the effective voltage applied to the liquid crystal layer of the subpixel1-a-A becomes greater than that of the effective voltage applied to that of the subpixel1-a-B. As a result, the subpixel1-a-A looks brighter than the subpixel1-a-B.
• Next, in the framen+1, when the voltageVcs1 on the first storage capacitor trunk is maintained at the second level after having decreased from the third level, the scan lineG1 is selected (i.e., the voltageVg on the scan line goes VgH). When the scan lineG1 is selected, voltages lower than the one at the counter electrode are applied to the subpixel electrodes of the subpixels1-a-A and1-a-B. After the voltageVg1 on the scan lineG1 has fallen to VgL again, the voltageVcs1 on the first storage capacitor trunk will vary periodically. In the case that the voltageVg1 on the scan lineG1 goes down to VgL again, the voltagesVcs1 andVcs2 on the first and second storage capacitor trunks are equal to the average voltageVL2 of the voltagesVcs1 andVcs2 on the first and second storage capacitor trunks. That is why the absolute value of the effective voltage applied to the liquid crystal layer of the subpixel1-a-A becomes equal to that of the effective voltage applied to that of the subpixel1-a-B. As a result, the subpixel1-a-A looks as bright as the subpixel1-a-B.
• . Next, in the framen+2, when the voltageVcs1 on the first storage capacitor trunk goes up from the second level to the third level, the scan lineG1 is selected (i.e., the voltageVg on the scan line goes VgH). When the scan lineG1 is selected, voltages higher than the one at the counter electrode are applied to the subpixel electrodes of the subpixels1-a-A and1-a-B. When the voltageVg1 on the scan lineG1 goes down from VgH to VgL again, the voltageVcs1 on the first storage capacitor trunk is VL3, while the voltageVcs2 on the second storage capacitor trunk is VL1. That is why the absolute value of the effective voltage applied to the liquid crystal layer of the subpixel1-a-A becomes smaller than that of the effective voltage applied to that of the subpixel1-a-B. As a result, the subpixel1-a-A looks darker than the subpixel1-a-B.
• Next, in the framen+3, after the voltageVcs1 on the first storage capacitor trunk goes up from the first level to the second level, the scan lineG1 is selected (i.e., the voltageVg on the scan line goes VgH). When the scan lineG1 is selected, voltages lower than the one at the counter electrode are applied to the subpixel electrodes of the subpixels1-a-A and1-a-B. When the voltageVg1 on the scan lineG1 goes down from VgH to VgL again, the voltagesVcs1 andVcs2 on the first and second storage capacitor trunks are equal to VL2. That is why the absolute value of the effective voltage applied to the liquid crystal layer of the subpixel1a-A becomes equal to that of the effective voltage applied to that of the subpixel1-a-B. As a result, the subpixel1-a-A looks as bright as the subpixel1-a-B.
• As can be seen from the description that has just been given with reference toFIG.40, the (brightness, polarity) combination of the subpixel1-a-A changes in the order of (B, +), (M, -), (D, +) and (M, -), while the (brightness, polarity) combination of the subpixel1-a-B changes in the order of (D, +), (M, -), (B, +) and (M, -). Also, although not shown, the (brightness, polarity) combination of the subpixel2-a-A changes in the order of (B, -), (M, +), (D, -) and (M, +). In this manner, the liquid crystal display device of this preferred embodiment not only changes the brightness levels of each subpixel in the order of bright, moderate, dark and moderate every vertical scanning period but also inverts the polarity every vertical scanning period, thereby reducing the degree of non-smoothness of the image on the screen. Also, in the liquid crystal display device of this preferred embodiment, each set of first and second polarity periods has a period in which the first subpixel is brighter than the second subpixel as in the liquid crystal display device of the first preferred embodiment. Thus, as can be seen from portions(b) and(c) ofFIG.38, the average of the effective voltagesVLspa and that of the effective voltagesVLspb over multiple vertical scanning periods (e.g., the first through fourth periods) can be equal to each other. Furthermore, the averages of the effective voltagesVLspa andVLspb can be both controlled to zero by adjusting the counter voltage. As a result, the residual image and other reliability-related problems can be overcome.
• In the liquid crystal display device of the first through seventh preferred embodiments of the present invention described above, each pixel is supposed to consist of two subpixels. However, the present invention is in no way limited to those specific preferred embodiments. Each pixel may also consist of three or more subpixels. The greater the number of subpixels per pixel, the more significantly the non-uniformity in γ characteristic can be reduced. For example, if the pixel division number is increased from two to four, the degree of the non-uniformity produced by a variation in display grayscale can be reduced and the display qualities can be further improved. However, the greater the division number, the lower the (frontal) transmittance will be in the case of white display. Particularly if the division number is increased from two to four, the transmittance in the white display will decrease significantly. Such a significant decrease is caused partly because each subpixel has a much smaller display area in that case. Thus, the division number needs to be appropriately adjusted according to the intended application of the liquid crystal display device so as to strike an adequate balance between the degree of reduction in the viewing angle dependence of the γ characteristic and the magnitude of decrease in the transmittance in the white display. It should be noted that the reduction in the viewing angle dependence of the γ characteristic is most noticeable if a non-divided pixel is divided into two subpixels (i.e., when each pixel consists of two subpixels). Considering the inevitable decreases in transmittance in the white display and in mass-productivity when each pixel is divided into a greater number of subpixels, each pixel preferably consists of two subpixels, after all.
• Optionally, a configuration for supplying the voltagesVcs to respective storage capacitor lines independently of each other may also be adopted as already described with reference toFIGS.13 and14. In that case, each voltageVcs will have an increased number of waveform options in the display period and the regulation period, which is beneficial. Nevertheless, the voltageVcs should change its levels at least once after the voltage on the scan line has gone low during one vertical scanning period. For example, in a liquid crystal display device that includes twice as many storage capacitor lines as scan lines and that has a configuration for supplying voltagesVcs to those storage capacitor lines independently of each other, if the voltageVcs needs to change its levels only once after the voltage on each scan line has gone low, then the interval between the point in time when the voltage on the scan line goes low and the point in time when the voltageVcs changes its levels or the interval between the point in time when the voltageVcs changes its levels and the point in time when the voltage on the scan line goes high next time is preferably defined equally for every display line.
• Meanwhile, if a configuration in which a number of storage capacitor lines are provided for each storage capacitor trunk is adopted, then the voltagesVcs on those multiple storage capacitor lines connected to a single storage capacitor trunk can have their oscillation amplitudes exactly matched with each other, which is advantageous. Naturally, the circuit configuration can also be simpler than a situation where a lot of voltages should be supplied independently of each other.
• Furthermore, the liquid crystal display device according to any of the first through seventh preferred embodiments of the present invention described above is supposed to adopt the multi-picture element driving method disclosed in Patent Document No. 1, i.e., make the luminances of two subpixels that form one pixel different from each other by applying a rectangular wave voltage to a CS bus line. However, the present invention is in no way limited to those specific preferred embodiments.
• The present invention has the following two important points, and embodiments embodied these points are in no way limited to the above described embodiments.
• The first point of the present invention is to switch the luminance levels of multiple subpixels that form a single pixel one after another, thereby averaging the luminance levels of those subpixels over a predetermined period of time and optimizing the variation in the luminance level of each subpixel with time such that the difference in luminance level between the subpixels becomes substantially equal to zero.
• The second point of the present invention is to invert the polarities of respective subpixels such that the averages of the voltages applied to those subpixels over a certain period of time becomes substantially equal to each other among them, thereby optimizing the variation in the effective voltage applied to the liquid crystal layer (or the variation in luminance). It should be noted that to ensure reliability, the difference in average effective voltage between the subpixels is preferably 1 V or less.
• Examples of liquid crystal display devices that embody these two important points include a device in which subpixels that form each pixel have the same number of sets of four frames with the pixel polarity-subpixel brightness combinations (B, +), (B, -), (D, +) and (D, -) (where B and D stand for "bright" and "dark", respectively) within a certain period and another device in which subpixels that form each pixel have the same number of sets of four frames with the pixel polarity-subpixel brightness combinations (B, +), (D, +), (M, -) and (M, -) or (B, -), (D, -), (M, -) and (M, -) (where M stands for "moderate") within a certain period.
• To embody these points, the polarities and luminances of subpixels may be controlled on a frame-by-frame basis unlike the liquid crystal display device according to any of the first through seventh preferred embodiments of the present invention described above. For example, in an alternative liquid crystal display device, a TFT provided for each subpixel may drive it with data signals and scan signals applied independently to respective subpixels.
• Alternatively, the liquid crystal display device according to the present invention may also be designed such that a TFT provided for each subpixel controls the luminance with a data signal that has been applied independently on a subpixel-by-subpixel basis but that those TFTs are driven through a common scan line as shown inFIG.25. In that case, the luminances and polarities of respective subpixels can be controlled with independent data signals applied to those subpixels.
• Still alternatively, the liquid crystal display device according to the present invention may also be designed such that a TFT provided for each subpixel controls its luminance with a data signal applied in common for respective subpixels but that the TFTs are driven through respectively different scan lines. In that case, by further subdividing one frame period, defining luminances and polarities for respective subpixels with the same data signal applied thereto, and setting the scan periods or timings for the respective subpixels (i.e., by performing time sharing within one frame), the luminances and polarities of the respective subpixels can be controlled.
• It should be noted that the disclosure of Japanese Patent Application No.2006-228476, upon which the present application claims the benefit of priority, and the disclosure of its related Japanese Patent Application No.2006-228475 are hereby incorporated by reference.
INDUSTRIAL APPLICABILITY
• The present invention provides a big-screen or high-definition liquid crystal display device that realizes very high display qualities with the viewing angle dependence of the r characteristic reduced significantly. The liquid crystal display device of the present invention can be used effectively as a TV monitor of a big screen size of 30 inches or more.
• Further embodiments are:
1. 1. A liquid crystal display device comprising a plurality of pixels, each including a first subpixel and a second subpixel,
   wherein each of the first and second subpixels includes:
   • a counter electrode; a subpixel electrode; and a liquid crystal layer interposed between the counter electrode and the subpixel electrode, and
   wherein the subpixel electrodes of the first and second subpixels are provided separately from each other as first and second subpixel electrodes, respectively, while the first and second subpixels share the same counter electrode with each other, and
   wherein when a predetermined grayscale tone is displayed through four or more consecutive even number of vertical scanning periods, the first and second subpixels have mutually different luminances in at least two of the even number of vertical scanning periods, first polarity periods that are included in the even number of vertical scanning periods and that maintain a first polarity are as long as second polarity periods that are also included in the even number of vertical scanning periods and that maintain a second polarity for each of the first and second subpixels, and in each of the first and second polarity periods, the difference between the average of effective voltages applied to the liquid crystal layer of the first subpixel and that of effective voltages applied to the liquid crystal layer of the second subpixel is substantially equal to zero.
2. 2. The liquid crystal display device ofitem 1, wherein if the effective voltages applied to the respective liquid crystal layers of the first and second subpixels of each said pixel are represented by VLspa and VLspb, respectively, then two of the four consecutive vertical scanning periods are the first polarity periods and the other two vertical scanning periods are the second polarity periods, and
   wherein in at least one the first polarity periods and the second polarity periods, one of the two vertical scanning periods thereof satisfies |VLspa|>|VLspb| and the other vertical scanning period satisfies |VLspa|<|VLspb|.
3. 3. The liquid crystal display device ofitem 1, wherein if the effective voltages applied to the respective liquid crystal layers of the first and second subpixels of each said pixel are represented by VLspa and VLspb, respectively, then two of the four consecutive vertical scanning periods are the first polarity periods and the other two vertical scanning periods are the second polarity periods, and
   wherein in at least one of the first polarity periods and the second polarity periods, the |VLspa| and |VLspb| values of one of the two vertical scanning periods thereof are equal to those of the other vertical scanning period.
4. 4. The liquid crystal display device ofitem 2 or 3, wherein of the four vertical scanning periods, the number of vertical scanning periods that satisfy |VLspa|>|VLspb| is equal to that of vertical scanning periods that satisfy | VLspa|<|VLspb|.
5. 5. The liquid crystal display device of any ofitems 1 to 4, wherein the plurality of the pixels are arranged in column and row directions so as to form a matrix pattern, and
   wherein in each of the plurality of the pixels the first and second subpixels are arranged in the column direction.
6. 6. The liquid crystal display device of any ofitems 1 to 5, wherein in each of the plurality of the pixels, voltages applied to the first and second subpixel electrodes change as voltages on their associated storage capacitor lines vary.
7. 7. The liquid crystal display device ofitem 6, wherein in each of the plurality of the pixels, a voltage on a storage capacitor line associated with the first subpixel electrode and a voltage on a storage capacitor line associated with the second subpixel electrode change mutually differently.
8. 8. The liquid crystal display device of any ofitems 5 to 7, wherein a voltage applied to the second subpixel electrode of a particular one of the plurality of the pixels and a voltage applied to the first subpixel electrode of another pixel that is adjacent to the particular pixel in the column direction change as the voltage on their common storage capacitor line varies.
9. 9. The liquid crystal display device of any ofitems 5 to 7, wherein a voltage applied to the second subpixel electrode of a particular one of the plurality of the pixels and a voltage applied to the first subpixel electrode of another pixel that is adjacent to the particular pixel in the column direction change as voltages on their associated storage capacitor lines vary.
10. 10. The liquid crystal display device of any ofitems 1 to 9, wherein in each of the plurality of the pixels, the first and second subpixel electrodes are connected to the same signal line by way of their associated switching element.
11. 11. The liquid crystal display device of any ofitems 1 to 5, wherein in each of the plurality of the pixels, the first and second subpixel electrodes are respectively connected to first and second signal lines by way of first and second switching elements, respectively.
12. 12. The liquid crystal display device of any ofitems 1 to 11, wherein in each of the first and second polarity periods, one of the two vertical scanning periods satisfies | VLspa|>|VLspb| and the other vertical scanning period satisfies |VLspa|<|VLspb|.
13. 13. The liquid crystal display device of any ofitems 1 to 12, wherein in each of the plurality of the pixels, |VLspa | and |VLspb| switch their magnitudes every vertical scanning period and the polarities of the first and second subpixels are inverted every other vertical scanning period.
14. 14. The liquid crystal display device of any ofitems 1 to 13, wherein the frame frequency is 60 Hz.
15. 15. The liquid crystal display device of any ofitems 1 to 12, wherein in each of the plurality of the pixels, |VLspa | and |VLspb| switch their magnitudes every other vertical scanning period and the polarities of the first and second subpixels are inverted every vertical scanning period.
16. 16. The liquid crystal display device of item 15, wherein the frame frequency is 120 Hz.
17. 17. The liquid crystal display device of any ofitems 1 to 12, wherein in each of the plurality of the pixels, |VLspa | and |VLspb| switch their magnitudes every other vertical scanning period and the polarities of the first and second subpixels are inverted every other vertical scanning period, and
    wherein |VLspa| and |VLspb| switch their magnitudes non-synchronously with the inversion of the polarities of the first and second subpixels.
18. 18. The liquid crystal display device of any ofitems 1 to 11, wherein in either the first polarity periods or the second polarity periods, one of the two vertical scanning periods satisfies |VLspa|<|VLspb| and the other vertical scanning period satisfies |VLspa|<|VLspb|, and
    wherein in the other polarity periods, VLspa is equal to VLspb in each of the two vertical scanning periods.
19. 19. The liquid crystal display device of item 18, wherein voltages on storage capacitor lines associated with the first and second subpixel electrodes change between a first level, a second level that is higher than the first level, and a third level that is higher than the second level.
20. 20. The liquid crystal display device of any ofitems 1 to 19, wherein the first and second subpixel electrodes have the same display area.

Claims (11)

1. A liquid crystal display device (100) comprising a display section (110) and a driver (120) which is adapted to generate a drive signal based on an input video signal to drive the display section (110),
   the display section (110) comprises a plurality of pixels, each including first subpixel (10a) and a second subpixel (10b),
   wherein each of the first and second subpixels (10a, 10b) includes: a counter electrode (17); a subpixel electrode; and a liquid crystal layer interposed between the counter electrode (17) and the subpixel electrode, and
   wherein the subpixels electrodes of the first and second subpixels (10a, 10b) are provided separately from each other as first and second subpixel electrodes (18a, 18b), respectively, while the first and second subpixels (10a, 10b) share the same counter electrode (17) with each other, and
   wherein when the input video signal indicates a predetermined grayscale tone through at least two of four or more consecutive even number of vertical scanning periods, the driver (120) drives the display section (110) such that the first and second subpixels (10a, 10b) have mutually different luminances in two of the even number of vertical scanning periods, first polarity periods that are included in the even number of vertical scanning periods and that maintain a first polarity are as long as second polarity periods that are also included in the even number of vertical scanning periods and that maintain a second polarity for each of the first and second subpixel, the first polarity indicating a direction of an electric field applied on the liquid crystal display and the second polarity indicating a direction, being opposite to that of the first polarity, of an electric field applied on the liquid crystal display, and in each of the first and second polarity periods, the difference between the average of effective voltages applied to the liquid crystal layer of the first subpixel (10a) and that of effective voltages applied to the liquid crystal layer of the second subpixel (10b) is substantially equal to zero, and
   wherein in either of the first polarity periods or the second polarity periods, one of the two vertical scanning periods satisfies |VLspa|>|VLspb| and the other vertical scanning periods satisfies |VLspa|<|VLspb|, and
   wherein in the other polarity periods, VLspa is equal to VLspb in each of the two vertical scanning periods.
2. The liquid crystal display device (100) of claim 1, wherein if the effective voltages applied to the respective liquid crystal layers of the first and second subpixels (10a, 10b) of each said pixel are represented by VLspa and VLspb, respectively, then two of the four consecutive vertical scanning periods are the first polarity periods and the other two vertical scanning periods are the second polarity periods, and
   wherein in at least one the first polarity periods and the second polarity periods, one of the two vertical scanning periods thereof satisfies |VLspa|>|VLspb| and the other vertical scanning period satisfies |VLspa|<|VLspa|.
3. The liquid crystal display device (100) of claim 1, wherein if the effective voltages applied to the respective liquid crystal layers of the first and second subpixels (10a, 10b) of each said pixel are represented by VLspa and VLspb, respectively, then two of the four consecutive vertical scanning periods are the first polarity periods and the other two vertical scanning periods are the second polarity periods, and
   wherein in at least one of the first polarity periods and the second polarity periods, the |VLspa| and |VLspb| values of one of the two vertical scanning periods thereof are equal to those of the other vertical scanning period.
4. The liquid crystal display device (100) of claim 2 or 3, wherein of the four vertical scanning periods, the number of vertical scanning periods that satisfy |VLspa| >|VLspb| is equal to that of vertical scanning periods that satisfy |VLspa|< |VLspb|.
5. The liquid crystal display device (100) of any of claims 1 to 4, wherein the plurality of the pixels are arranged in column and row directions so as to form a matrix pattern, and
   wherein in each of the plurality of the pixels the first and second subpixels (10a, 10b) are arranged in the column direction.
6. The liquid crystal display device (100) of any of claims 1 to 5, wherein in each of the plurality of the pixels, the first and second subpixel electrodes are respectively connected to storage capacitor lines (24a, 24b) each connected to a storage capacitor of the liquid crystal display device(100), and voltages applied to the first and second subpixel electrodes change as voltages on the respectively connected storage capacitor lines (24a, 24b) vary.
7. The liquid crystal display device (100) of claim 6, wherein in each of the plurality of the pixels, a voltage on a storage capacitor line (24a) associated with the first subpixel electrode and a voltage on a storage capacitor line (24b) associated with the second subpixel electrode change mutually differently.
8. The liquid crystal display device (100) of any of claim 6 or 7, wherein the second subpixel electrode of a particular one of the plurality of the pixels and the first subpixel electrode of another pixel that is adjacent to the particular pixel in the column direction are connected to their common storage capacitor line (24a, 24b), and a voltage applied to the second subpixel electrode of the particular pixel and a voltage applied to the first subpixel electrode of the other pixel change as the voltage on the common storage capacitor line (24a, 24b) varies.
9. The liquid crystal display device (100) of any of claims 1 to 5, wherein in each of the plurality of the pixels, the first and second subpixel electrodes are respectively connected to first and second signal lines by way of first and second switching elements, respectively.
10. The liquid crystal display device (100) of any of claims 1 to 9, wherein in each of the first and second polarity periods, one of the two vertical scanning periods satisfies |VLspa|>|VLspb| and the other vertical scanning period satisfies |VLspa|< |VLspb|.
11. The liquid crystal display device (100) of claim 1, wherein voltages on storage capacitor lines (24a, 24b) associated with the first and second subpixel electrodes change between a first level, a second level that is higher than the first level, and a third level that is higher than the second level.

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