This article is about the storage and reproduction medium. For their content and production, seeFilm andFootage. For other uses, seeVideo (disambiguation).
Video developed from facsimile systems developed in the mid-19th century. Early mechanical video scanners, such as theNipkow disk, werepatented as early as 1884, however, it took several decades before practical video systems could be developed, many decades afterfilm. Film records using a sequence of miniature photographic images visible to the eye when the film is physically examined. Video, by contrast, encodes images electronically, turning the images into analog or digital electronic signals for transmission or recording.[3]
Video technology was first developed formechanical television systems, which were quickly replaced bycathode-ray tube (CRT)television systems. Video was originally exclusivelylive technology. Live video cameras used an electron beam, which would scan a photoconductive plate with the desired image and produce a voltage signal proportional to the brightness in each part of the image. The signal could then be sent to televisions, where another beam would receive and display the image.[4]Charles Ginsburg led anAmpex research team to develop one of the first practicalvideo tape recorders (VTR). In 1951, the first VTR captured live images fromtelevision cameras by writing the camera's electrical signal onto magneticvideotape.
Video recorders were sold for $50,000 in 1956, and videotapes cost US$300 per one-hour reel.[5] However, prices gradually dropped over the years; in 1971, Sony began sellingvideocassette recorder (VCR) decks and tapes into theconsumer market.[6]
Digital video is capable of higher quality and, eventually, a much lower cost than earlier analog technology. After the commercial introduction of theDVD in 1997 and later theBlu-ray Disc in 2006, sales of videotape and recording equipment plummeted. Advances incomputer technology allow even inexpensivepersonal computers andsmartphones to capture, store, edit, and transmit digital video, further reducing the cost ofvideo production and allowing programmers and broadcasters to move totapeless production. The advent ofdigital broadcasting and the subsequentdigital television transition are in the process of relegating analog video to the status of alegacy technology in most parts of the world. The development of high-resolution video cameras with improveddynamic range andcolor gamuts, along with the introduction of high-dynamic-rangedigital intermediate data formats with improvedcolor depth, has caused digital video technology to converge with film technology. Since 2013,[update] the use ofdigital cameras inHollywood has surpassed the use of film cameras.[7]
Frame rate, the number of still pictures per unit of time of video, ranges from six or eight frames per second (frame/s) for old mechanical cameras to 120 or more frames per second for new professional cameras.PAL standards (Europe, Asia, Australia, etc.) andSECAM (France, Russia, parts of Africa, etc.) specify 25 frame/s, whileNTSC standards (United States, Canada, Japan, etc.) specify 29.97 frame/s.[8] Film is shot at a slower frame rate of 24 frames per second, which slightly complicates the process of transferring a cinematic motion picture to video. The minimum frame rate to achieve a comfortable illusion of amoving image is about sixteen frames per second.[9]
Video can beinterlaced orprogressive. In progressive scan systems, each refresh period updates all scan lines in each frame in sequence. When displaying a natively progressive broadcast or recorded signal, the result is the optimum spatial resolution of both the stationary and moving parts of the image. Interlacing was invented as a way to reduce flicker in earlymechanical andCRT video displays without increasing the number of completeframes per second. Interlacing retains detail while requiring lowerbandwidth compared to progressive scanning.[10][11]
In interlaced video, the horizontalscan lines of each complete frame are treated as if numbered consecutively and captured as twofields: anodd field (upper field) consisting of the odd-numbered lines and aneven field (lower field) consisting of the even-numbered lines. Analog display devices reproduce each frame, effectively doubling the frame rate as far as perceptible overall flicker is concerned. When the image capture device acquires the fields one at a time, rather than dividing up a complete frame after it is captured, the frame rate for motion is effectively doubled as well, resulting in smoother, more lifelike reproduction of rapidly moving parts of the image when viewed on an interlaced CRT display.[10][11]
NTSC, PAL, and SECAM are interlaced formats. Abbreviated video resolution specifications often include ani to indicate interlacing. For example, PAL video format is often described as576i50, where576 indicates the total number of horizontal scan lines,i indicates interlacing, and50 indicates 50 fields (half-frames) per second.[11][12]
When displaying a natively interlaced signal on a progressive scan device, the overall spatial resolution is degraded by simpleline doubling—artifacts, such as flickering or "comb" effects in moving parts of the image that appear unless special signal processing eliminates them.[10][13] A procedure known asdeinterlacing can optimize the display of an interlaced video signal from an analog, DVD, or satellite source on a progressive scan device such as anLCD television, digitalvideo projector, or plasma panel. Deinterlacing cannot, however, producevideo quality that is equivalent to true progressive scan source material.[11][12][13]
Aspect ratio describes the proportional relationship between the width and height of video screens and video picture elements. All popular video formats arerectangular, and this can be described by a ratio between width and height. The ratio of width to height for a traditional television screen is 4:3, or about 1.33:1. High-definition televisions use an aspect ratio of 16:9, or about 1.78:1. The aspect ratio of a full 35 mm film frame with soundtrack (also known as theAcademy ratio) is 1.375:1.[14][15]
Pixels on computer monitors are usually square, but pixels used indigital video often have non-square aspect ratios, such as those used in the PAL and NTSC variants of theCCIR 601 digital video standard and the corresponding anamorphic widescreen formats. The720 by 480 pixel raster uses thin pixels on a 4:3 aspect ratio display and fat pixels on a 16:9 display.[14][15]
The popularity of viewing video on mobile phones has led to the growth ofvertical video.Mary Meeker, a partner at Silicon Valley venture capital firmKleiner Perkins Caufield & Byers, highlighted the growth of vertical video viewing in her 2015 Internet Trends Report – growing from 5% of video viewing in 2010 to 29% in 2015. Vertical video ads likeSnapchat's are watched in their entirety nine times more frequently than landscape video ads.[16]
Thecolor model uses the video color representation and maps encoded color values to visible colors reproduced by the system. There are several such representations in common use: typically,YIQ is used in NTSC television,YUV is used in PAL television,YDbDr is used by SECAM television, andYCbCr is used for digital video.[17][18]
The number of distinct colors a pixel can represent depends on thecolor depth expressed in the number of bits per pixel. A common way to reduce the amount of data required in digital video is bychroma subsampling (e.g., 4:4:4, 4:2:2, etc.). Because the human eye is less sensitive to details in color than brightness, the luminance data for all pixels is maintained, while the chrominance data is averaged for a number of pixels in a block, and the same value is used for all of them. For example, this results in a 50% reduction in chrominance data using 2-pixel blocks (4:2:2) or 75% using 4-pixel blocks (4:2:0). This process does not reduce the number of possible color values that can be displayed, but it reduces the number of distinct points at which the color changes.[12][17][18]
Video quality can be measured with formal metrics likepeak signal-to-noise ratio (PSNR) or throughsubjective video quality assessment using expert observation. Many subjective video quality methods are described in theITU-T recommendationBT.500. One of the standardized methods is theDouble Stimulus Impairment Scale (DSIS). In DSIS, each expert views anunimpaired reference video, followed by animpaired version of the same video. The expert then rates theimpaired video using a scale ranging from "impairments are imperceptible" to "impairments are very annoying."
Uncompressed video delivers maximum quality, but at a very highdata rate. A variety of methods are used to compress video streams, with the most effective ones using agroup of pictures (GOP) to reduce spatial and temporalredundancy. Broadly speaking, spatial redundancy is reduced by registering differences between parts of a single frame; this task is known asintraframe compression and is closely related toimage compression. Likewise, temporal redundancy can be reduced by registering differences between frames; this task is known asinterframe compression, includingmotion compensation and other techniques. The most common modern compression standards areMPEG-2, used forDVD, Blu-ray, andsatellite television, andMPEG-4, used forAVCHD, mobile phones (3GP), and the Internet.[19][20]
Two channels: a right channel for the right eye and a left channel for the left eye. Both channels may be viewed simultaneously by usinglight-polarizing filters 90 degrees off-axis from each other on two video projectors. These separately polarized channels are viewed wearing eyeglasses with matching polarization filters.
Anaglyph 3D, where one channel is overlaid with two color-coded layers. This left and right layer technique is occasionally used for network broadcasts or recent anaglyph releases of 3D movies on DVD. Simple red/cyan plastic glasses provide the means to view the images discretely to form a stereoscopic view of the content.
One channel with alternating left and right frames for the corresponding eye, usingLCD shutter glasses that synchronize to the video to alternately block the image for each eye, so the appropriate eye sees the correct frame. This method is most common in computervirtual reality applications, such as in aCave Automatic Virtual Environment, but reduces effective video framerate by a factor of two.
Many analog and digitalrecording formats are in use, anddigital video clips can also be stored on acomputer file system as files, which have their own formats. In addition to the physical format used by thedata storage device or transmission medium, the stream of ones and zeros that is sent must be in a particular digitalvideo coding format, for which a number is available.
Analog video is a video signal represented by one or moreanalog signals. Analog color video signals includeluminance (Y) andchrominance (C). When combined into one channel, as is the case among others withNTSC,PAL, andSECAM, it is calledcomposite video. Analog video may be carried in separate channels, as in two-channelS-Video (YC) and multi-channelcomponent video formats.
Analog video is used in both consumer and professionaltelevision production applications.
Video can be transmitted or transported in a variety of ways including wirelessterrestrial television as an analog or digital signal, coaxial cable in a closed-circuit system as an analog signal. Broadcast or studio cameras use a single or dual coaxial cable system usingserial digital interface (SDI). SeeList of video connectors for information about physical connectors and related signal standards.
An analog video format consists of more information than the visible content of the frame. Preceding and following the image are lines and pixels containing metadata and synchronization information. This surrounding margin is known as ablanking interval orblanking region; the horizontal and verticalfront porch and back porch are the building blocks of the blanking interval.
Early television was almost exclusively a live medium, with some programs recorded to film for historical purposes usingKinescope. The analogvideo tape recorder was commercially introduced in 1951. The following list is in rough chronological order. All formats listed were sold to and used by broadcasters, video producers, or consumers; or were important historically.[23][24]
A video codec issoftware orhardware thatcompresses anddecompressesdigital video. In the context of video compression,codec is aportmanteau ofencoder anddecoder, while a device that only compresses is typically called anencoder, and one that only decompresses is adecoder. The compressed data format usually conforms to a standardvideo coding format. The compression is typicallylossy, meaning that the compressed video lacks some information present in the original video. A consequence of this is that decompressed video has lower quality than the original, uncompressed video because there is insufficient information to accurately reconstruct the original video.[28]
^abcBrown, Blain (2013).Cinematography: Theory and Practice: Image Making for Cinematographers and Directors.Taylor & Francis. pp. 159–166.ISBN9781136047381.
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