Video compression
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Video compression refers to making a digital video signal use less data, without noticeably reducing the quality of the picture.
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.
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)
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)].
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 its 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.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.)
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.
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 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.
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.
See also
External links
- [Intro to Video Compression]
- [Video compression basics]
- [MPEG 1&2 video compression intro (pdf format)]
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