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Encyclopedia > Video compression

Video compression refers to making a digital video signal use less data, without noticeably reducing the quality of the picture. Digital video is a type of video recording system that works by using a digital, rather than analog, representation of the video signal. ... Data is the plural of datum. ...


Digital video always requires high data rates - the better the picture, the more data is needed. This means powerful hardware, and lots of bandwidth when video is transmitted. However much of the data in video is either redundant or easily predicted - for example, successive frames in a movie rarely change much from one to the next - this makes data compression work well with video. Video compression can make video files far smaller with little or no loss in quality. For example, DVDs use a system called MPEG-2 that makes the movie 15 to 30 times smaller. Without data compression, either the picture would look 30 times worse, or one would need 30 disks per movie. Bandwidth is a measure of frequency range, measured in hertz, of a function of a frequency variable. ... In computer science and information theory, data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an unencoded representation would use through use of specific encoding schemes. ... MPEG-2 (1994) is the designation for a group of coding standards for digital audio and video, agreed upon by MPEG (Moving Pictures Experts Group), and published as the ISO/IEC 13818 international standard. ...

Contents


Theory

Video is basically a three-dimensional array of color pixels. Two dimensions serve as spatial (horizontal and vertical) directions of the moving pictures, and one dimension represents the time domain. A frame is a set of all pixels that correspond to a single point in time. Basically, a frame is the same as a still picture. (These are sometimes made up of fields. See interlace) Color is an important part of the visual arts. ... This example shows an image with a portion greatly enlarged. ... Time-domain is a term used to describe the analysis of mathematical functions, or real-life signals, with respect to time. ... Look up Frame in Wiktionary, the free dictionary. ... Interlace is a method of reducing the bandwidth required for video transmission without having to reduce resolution or refresh rate which was invented by RCA engineer Randall Ballard in the late 1920s. ...


Video data contains spatial and temporal redundancy. Similarities can thus be encoded by merely registering differences within a frame (spatial) and/or between frames (temporal). Spatial encoding is performed by taking advantage of the fact that the human eye is unable to distinguish small differences in colour as easily as it can changes in brightness and so very similar areas of colour can be "averaged out" in a similar way to jpeg images (JPEG image compression FAQ, part 1/2). With temporal compression only the changes from one frame to the next are encoded as often a large number of the pixels will be the same on a series of frames (About video compression). Redundancy, in general terms, refers to the quality or state of being redundant, that is: exceeding what is necessary or normal, containing an excess. ...


Lossless compression

If the inverse of the process, decompression, produces an exact replica of the original data then the compression is lossless. While lossless compression of video is possible, any lossless compression system will sometimes result in a file (or portions of) that is as large and/or has the same data rate as the uncompressed original. As a result, all hardware in a lossless system would have to be able to handle uncompressed video as well. This kills off much of the benefit of compressing the data at all. For example, digital videotape can't vary it's data rate easily so dealing with short bits of maximum-data-rate video would be more complicated then something that was fixed at that rate all the time. Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. ...


Intraframe vs interframe compression

One of the most powerful techniques for compressing video is interframe compression. This works by comparing each frame in the video with the previous one. If the frame contain areas where nothing has moved, no new data needs to be sent - the system simply issues a command that copies that part of the previous frame into the next one. This can also work if objects move in a simple manner - parts of the frame can be shifted, rotated, lightened or darkened during the copy so that less new data needs to be transmitted. Interframe compression is best for finished programs that will simply be played back by the viewer. But it can cause problems if it is used for editing.


Since Interframe compression copies data from one frame to another, if the original frame is edited out, successive frames cannot be reconstructed. Formats such as DV avoid this problem by compressing each frame separately as if they were all unrelated still images. This is called intraframe compression. Another difference between intraframe and interframe compression is that with intraframe systems, each frame uses the same amount of data. In interframe systems, certain frames called "I frames" aren't allowed to copy data from other frames, and so require more data then other frames nearby. (The "I" stands for independent.) A MiniDV tape For other uses, see DV (disambiguation). ...


It is possible to build a computer-based video editor that spots problems caused when I frames are edited out while other frames need them. This has allowed newer formats like HDV to be used for editing. However, this process demands a lot more computing power then editing intraframe compressed video with the same picture quality. High Definition Video (HDV) is a video format designed to record compressed HDTV video on standard DV media (DV or MiniDV cassette tape). ...


Current forms

Today, nearly all video compression methods in common use (e.g., those in standards approved by the ITU-T or ISO) apply a discrete cosine transform (DCT) for spatial redundancy reduction. Other methods, such as fractal compression, matching pursuits, and the use of a discrete wavelet transform (DWT) have been the subject of some research, but are typically not used in practical products (except for the use of wavelet coding as still-image coders without motion compensation). Interest in fractal compression seems to be waning, due to recent theoretical analysis showing a comparative lack of effectiveness to such methods. The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. ... The International Organization for Standardization (ISO) is an international standard-setting body composed of representatives from national standards bodies. ... 2-D DCT compared to the DFT The discrete cosine transform (DCT) is a Fourier-related transform similar to the discrete Fourier transform (DFT), but using only real numbers. ... Fractal compression is a lossy compression method used to compress images using fractals. ... In numerical analysis and functional analysis, the discrete wavelet transform (DWT) refers to wavelet transforms for which the wavelets are discretely sampled. ...


The use of most video compression techniques (e.g., DCT or DWT based techniques) involves quantization. The quantization can either be scalar quantization or vector quantization; however, nearly all practical designs use scalar quantization because of its greater simplicity. Quantization of x using Q(x) = floor(Lx)/L. In digital signal processing, quantization is the process of approximating a continuous signal by a set of discrete symbols or integer values; that is, converting an analog signal to a digital one via analog-to-digital conversion. ... In data compression, vector quantization is a quantization technique often used in lossy data compression in which the basic idea is to code or replace with a key, values from a multidimensional vector space into values from a discrete subspace of lower dimension. ...


In broadcast engineering, digital television (DVB, ATSC and ISDB ) is made practical by video compression. TV stations can broadcast not only HDTV, but multiple virtual channels on the same physical channel as well. It also conserves precious bandwidth on the radio spectrum. Nearly all digital video broadcast today uses the MPEG-2 standard video compression format, although H.264/MPEG-4 AVC and VC-1 are emerging contenders in that domain. Broadcast engineering is the field of electrical engineering, and now to some extent radio and television broadcasting. ... It has been suggested that Digital terrestrial television be merged into this article or section. ... Official DVB logo, found on compliant devices DVB, short for Digital Video Broadcasting, is a suite of internationally accepted, open standards for digital television maintained by the DVB Project, an industry consortium with more than 270 members, and published by a Joint Technical Committee (JTC) of European Telecommunications Standards Institute... The Advanced Television Systems Committee (ATSC) is the group that helped to develop the new digital television standard for the United States, also adopted by Canada, Mexico, and South Korea and being considered by other countries. ... Integrated Services Digital Broadcasting (ISDB) is the digital television (DTV) and digital audio broadcasting (DAB) format. ... A television station is a type of radio station that broadcasts both audio and video to television receivers in a particular area. ... individually-donated time and energy direct government payments or operation indirect government payments, such as radio and television licenses grants from foundations or business entities selling advertising or sponsorship public subscription or membership fees charged to all owners of TV sets or radios, regardless of whether they intend to receive... Projection screen in a home theater, displaying a high-definition television image. ... Bandwidth is a measure of frequency range, measured in hertz, of a function of a frequency variable. ... Radio frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ... MPEG-2 (1994) is the designation for a group of coding standards for digital audio and video, agreed upon by MPEG (Moving Pictures Experts Group), and published as the ISO/IEC 13818 international standard. ... H.264, MPEG-4 Part 10, or AVC, for Advanced Video Coding, is a digital video codec standard which is noted for achieving very high data compression. ... It has been suggested that this article or section be merged with WMV HD. (Discuss) VC-1 is the informal name of the draft SMPTE standard 421M describing a video codec based on Windows Media Video version 9. ...


See also

Image compression is the application of data compression on digital images. ... Look up Video in Wiktionary, the free dictionary. ... A video codec is a device or software module that enables the use of data compression for digital video. ... Video quality is a characteristic of video passed through a video processing system. ... Subjective video quality is a subjective characteristic of video quality. ...

External links


  Results from FactBites:
 
Video compression - Wikipedia, the free encyclopedia (914 words)
With temporal compression only the changes from one frame to the next are encoded as often a large number of the pixels will be the same on a series of frames (About video compression).
While lossless compression of video is possible, any lossless compression system will sometimes result in a file (or portions of) that is as large and/or has the same data rate as the uncompressed original.
Other methods, such as fractal compression, matching pursuits, and the use of a discrete wavelet transform (DWT) have been the subject of some research, but are typically not used in practical products (except for the use of wavelet coding as still-image coders without motion compensation).
  More results at FactBites »

 
 

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