HIGH DEN8ITY CONPACT DISCThe present invention relates to a compact disc for reproduction only of recorded information, and more particularly, to a compact disc having a high track density.
The compact disc for reproduction only is currently on the market as a video compact disc (video-CD) on which sound, text and images are recorded. The video compact disc, compared to a magnetic recording video cassette tape, has a'shorter seek time, and does not contact the reproducing-medium which prevents deterioration of information. However, since the capacity of the video-CD is limited, i.e., approximately 650MB in the case of the 12cm disc, the disc cannot store enough image information for long reproduction.
Accordingly, in order to record enough image information for long reproduction, a high density disc is required.
Figure 1 shows the structure of a conventional compact disc and a recording format thereof.
The conventional compact disc has a substrate 1, a reflective film 2 and a protective layer 3. A multiplicity of pits 4, as a unit of information, are formed on the substrate 1. Here, the pits 4 are arranged in concentric circles or in a spiral with respect to a disc center on tracks having a groove and a land portion. When a laser beam 5 from an optical pickup (not shown) is focused onto the reflective film 2 through the substrate 1, a light spot 6 is formed on a recording surface (pit formation surface). When the light spot 6 moves on the pits 4, the reflectivity on the reflective film 2 changes. Accordingly, an electric signal is detected from the changed reflectivity of the reflected light so that it is possible to read both the information recorded on the compact disc and the position information for tracking and focusing of the light spot.
The pitch between the tracks, on which the pits 4 are arranged, is usually 1.6cm, with reference to the diameter of the light spot 6 so that the light spot 6 does not overlap the pits of adjacent tracks.
Meanwhile, the depth of the pit 4 is approximately 1/51/4 the wavelength of the light used.
In the conventional compact disc, in order to increase the recording density, there is known a method of narrowing the pitch between the tracks, thereby increasing the track density. However, in the case that only the pitch between tracks is narrowed in order to increase the track density, crosstalk occurs due to light interference caused by the pits of the adjacent track, generating noise in the reproduction signal.
Also, in the recording format of the conventional compact disc, since the pits of a track adjacent to a target track are arranged at random, the crosstalk cannot be suppressed. Accordingly, in order to increase the track density, it is necessary to change the recording format of the conventional compact disc.
Accordingly, it is an aim of preferred embodiments of the present invention to provide a high density compact disc having high track density for recording enough information for long reproduction.
Also, it is another aim to provide a high density compact disc having a high track density in which crosstalk does not occur during reproduction.
According to a first aspect of the present invention, there is provided a high density compact disc comprising a recording surface, on which pits representing information signals are arranged along a predetermined track or tracks, wherein the recording surface comprises grooves and lands alternately arranged along the tracks, and the pits are formed on both the grooves and the lands.
Preferably, the depth of the grooves is within the range of # #78 6n where X is the wavelength of light and v indicates the refractive index of a disc substrate.
Said grooves and lands may be arranged as concentric circles.
According to a second aspect of the invention, there is provided a high density compact disc comprising a recording surface, on which pits representing information signals are arranged along a predetermined track or tracks, wherein said recording surface comprises a spiral arrangement of a groove and a land and said pits are formed on both said groove and said land.
Preferably, the depth of said groove is within the range of 7# 6#7 68 where A is the wavelength of light and ii indicates the refractive index of a disc substrate.
For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which:Figure 1 is a partial perspective view showing both the structure and the recording format of a conventional compact disc;Figure 2 is a partial perspective view showing both the structure and the recording format of a high density compact disc according to an embodiment of the present invention;Figure 3 is a sectional view of the compact disc ofFigure 2;Figures 4A and 4B are schematic views for illustrating a method for optimizing the groove depth of the high density compact disc according to embodiments of the present invention; andFigure 5 is a graph showing the level of crosstalk with respect to the groove depth in a high density compact disc according to an embodiment of the present invention.
Referring to the drawings, a preferable embodiment for realizing a high density compact disc according to the present invention will be described in detail.
Referring to Figures 2 and 3, reference numeral 10 denotes a transparent substrate having a predetermined refractive index, which transmits laser light. The substrate 10 has a recording surface formed on the upper surface thereof. The recording surface is formed with a plurality of grooves 11 and lands 12 which are adjacent to each other along tracks in concentric circles or in a spiral with respect to a disc center. Information is recorded on a series of pits 13 formed in the grooves 11 and the lands 12, each pit representing a unit information signal. A reflective film 14, such as a metal lamina for reflecting the laser light, is coated on the recording surface of the substrate 10, i.e., on the grooves 11 and the lands 12. A protective layer 15 is formed on the reflective film 14.
If the interval between the adjacent grooves 11 or the adjacent lands 12 is the same as the pitch between the tracks of the conventional compact disc, the track density obtained is twice as dense as that of the conventional format. In this state, when a laser beam is projected and focused onto a target groove from the optical pickup, a light spot formed on the recording surface partially hits two lands 12 at both sides of the target groove 11 (see Figure 4A and 4B).
Here, crosstalk due to pits 13 in the land 12 may occur. However, according to the present invention, the level of crosstalk can be reduced by optimizing the recording format by adjusting the depth d of the groove 11, based on conditions to be described below.
Crosstalk can be illustrated by interference in light reflected from the groove region and light reflected from the land region. For example, as shown in Figures 4A and 4B, if the pits 13 are arranged only in the grooves 11 by predetermined size and in intervals of q, and no information is recorded on the land region 12, and the light spot 16 moves along a center of the land 12 region, the intensity of the reflected light changes according to the pit 13 formed on the adjacent groove 11 and at a period of the interval q. That is, the amplitude of the change in the intensity can be defined as the crosstalk.
In the case that the light spots 16 are located at positions shown in Figures 4A and 4B, respectively, the phase differences between the reflected light in the groove 11 region and the land 12 region can be expressed as: 4sA d,4#4= d +s respectively, wherein X is the wavelength of the laser beam, and d is the depth of the groove 11.
The conditions for identifying the degree of interference in the phase differences are as follows.
cos (4As d) = cos ( 4As d +s)x88 Also, in the case that pits 13 are arranged only in the lands 12 and the light spot 16 is focused on the center of a groove 11, the conditions for identifying the degree of interference in the phase differences are as follows, where the result is the same as that of the above case.
cos ( 4As d) = cos ( 4A d - it)I I8 That is, when the depth d of the groove 11 is approximately 1/8 the wavelength X of the light, crosstalk is at a minimum.
The above result is a simplified example of the recording formats of the high density compact disc, where the refractive index of the above-mentioned substrate is not considered.
The following are actual conditions which may be applied. These are data for measuring the level of crosstalk according to the depth of the groove by a computer simulator, based on the theory of diffraction by Hopkins.
wavelength A of laser: 680nmaperture of an object lens: 0.6width of groove and land: 0.7tjm track pitch: 1 4pm interval of pits: 1.1 mpit depth: A/4x ('7= 1.51, the refractive index ofsubstrate)pit length: 0.55pmpit width: 0.Spm The levels of crosstalk were measured, changing only the groove depth data, and is shown as a graph in Figure 5. On the graph, the unit along the horizontal axis shows the groove depth in nm, and the unit along the vertical axis shows the level of crosstalk in dB.
According to the graph, when the groove depth is approximately 70nm, i.e., within A/7,7 - A/6Q, the crosstalk is reduced significantly. Here, the level of crosstalk changes depending on the form of the pits of an adjacent track, however, the level of crosstalk is small enough that the variations are insignificant.
As described above, according to the present invention, tracks are divided into grooves and lands, where pits representing information signals may be formed at both the grooves and lands, to thereby increase the track density by two times, and further a high density compact disc reproducible for a long time without noise due to crosstalk is provided.
Accordingly, the present invention is effective in enhancing the practicality of the compact disc as a medium for preserving large amounts of sound, character and image information.
Also, in the high density compact disc according to the present invention, tracking signals can be simply detected from changes in the light intensity due to the depth of both grooves and lands. Thus, the tracking control by a push-pull method, which could not be adopted for the conventional compact disc, is possible, and a light pickup optical system therefor can be simplified.
The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.
All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.
Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.