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Avideo 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.
There are complex relationships between thevideo quality, the amount of data used to represent the video (determined by thebit rate), the complexity of the encoding and decoding algorithms, sensitivity to data losses and errors, ease of editing, random access, and end-to-end delay (latency).
Historically, video was stored as an analog signal onmagnetic tape. Around the time when thecompact disc entered the market as a digital-format replacement for analog audio, it became feasible to also store and convey video in digital form. Because of the large amount of storage and bandwidth needed to record and convey raw video, a method was needed to reduce the amount of data used to represent the raw video. Since then,engineers andmathematicians have developed a number of solutions for achieving this goal that involve compressing the digital video data.
In 1974,discrete cosine transform (DCT) compression was introduced byNasir Ahmed, T. Natarajan andK. R. Rao.[1][2][3] During the late 1980s, a number of companies began experimenting with DCTlossy compression for video coding, leading to the development of theH.261 standard.[4] H.261 was the first practical video coding standard,[5] and was developed by a number of companies, includingHitachi,PictureTel,NTT,BT, andToshiba, among others.[6] Since H.261, DCT compression has been adopted by all the major video coding standards that followed.[4]
The most popularvideo coding standards used for codecs have been theMPEG standards.MPEG-1 was developed by theMotion Picture Experts Group (MPEG) in 1991, and it was designed to compressVHS-quality video. It was succeeded in 1994 byMPEG-2/H.262,[5] which was developed by a number of companies, primarilySony,Thomson andMitsubishi Electric.[7] MPEG-2 became the standard video format forDVD andSD digital television.[5] In 1999, it was followed byMPEG-4/H.263, which was a major leap forward for video compression technology.[5] It was developed by a number of companies, primarily Mitsubishi Electric, Hitachi andPanasonic.[8]
The most widely used video coding format, as of 2016, isH.264/MPEG-4 AVC. It was developed in 2003 by a number of organizations, primarily Panasonic,Godo Kaisha IP Bridge andLG Electronics.[9] H.264 is the main video encoding standard forBlu-ray Discs, and is widely used by streaming internet services such asYouTube,Netflix,Vimeo, andiTunes Store, web software such asAdobe Flash Player andMicrosoft Silverlight, and variousHDTV broadcasts over terrestrial and satellite television.
AVC has been succeeded byHEVC (H.265), developed in 2013. It is heavily patented, with the majority of patents belonging toSamsung Electronics,GE, NTT andJVC Kenwood.[10][11] The adoption of HEVC has been hampered by its complex licensing structure. HEVC is, in turn, succeeded byVersatile Video Coding (VVC).
There are also the open and freeVP8,VP9 andAV1 video coding formats, used by YouTube, all of which were developed with involvement fromGoogle.
Video codecs are used in DVD players,Internet video,video on demand,digital cable,digital terrestrial television,videotelephony and a variety of other applications. In particular, they are widely used in applications that record or transmit video, which may not be feasible with the high data volumes and bandwidths of uncompressed video. For example, they are used inoperating theaters to record surgical operations, inIP cameras in security systems, and inremotely operated underwater vehicles andunmanned aerial vehicles. Any video stream or file can be encoded using a wide variety of live video format options.[12]
Video codecs seek to represent a fundamentally analog data set in a digital format. Because of the design of analog video signals, which representluminance (luma) andcolor information (chrominance, chroma) separately, a common first step in image compression in codec design is to represent and store the image in aYCbCr color space. The conversion to YCbCr provides two benefits: first, it improves compressibility by providing decorrelation of the color signals; and second, it separates the luma signal, which is perceptually much more important, from the chroma signal, which is less perceptually important and which can be represented at lower resolution usingchroma subsampling to achieve more efficient data compression. It is common to represent the ratios of information stored in these different channels in the following way YCbCr. Different codecs use different chroma subsampling ratios as appropriate to their compression needs. Video compression schemes for Web and DVD make use of a 4:2:1 color sampling pattern, and theDV standard uses 4:1:1 sampling ratios. Professional video codecs designed to function at much higher bitrates and to record a greater amount of color information for post-production manipulation, sample in 4:2:2 and 4:4:4 ratios. Examples of these codecs include Panasonic's DVCPRO50 and DVCPROHD codecs (4:2:2), Sony's HDCAM-SR (4:4:4), Panasonic's HDD5 (4:2:2),Apple's Prores HQ 422 (4:2:2).[13]
It is also worth noting that video codecs can operate in RGB space as well. These codecs tend not to sample the red, green, and blue channels in different ratios, since there is less perceptual motivation for doing so—just the blue channel could be undersampled.
Some amount of spatial and temporaldownsampling may also be used to reduce the raw data rate before the basic encoding process. The most popular encoding transform is the 8x8 DCT. Codecs that make use of awavelet transform are also entering the market, especially in camera workflows that involve dealing withRAW image formatting in motion sequences. This process involves representing the video image as a set ofmacroblocks. For more information about this critical facet of video codec design, seeB-frames.[14]
The output of the transform is firstquantized, thenentropy encoding is applied to the quantized values. When a DCT has been used, the coefficients are typically scanned using azig-zag scan order, and the entropy coding typically combines a number of consecutive zero-valued quantized coefficients with the value of the next non-zero quantized coefficient into a single symbol and also has special ways of indicating when all of the remaining quantized coefficient values are equal to zero. The entropy coding method typically usesvariable-length coding tables. Some encoders compress the video in a multiple-step process calledn-pass encoding (e.g. 2-pass), which performs a slower but potentially higher quality compression.
The decoding process consists of performing, to the extent possible, an inversion of each stage of the encoding process.[citation needed] The one stage that cannot be exactly inverted is the quantization stage. There, a best-effort approximation of inversion is performed. This part of the process is often calledinverse quantization ordequantization, although quantization is an inherently non-invertible process.
Video codec designs are usually standardized or eventually become standardized—i.e., specified precisely in a published document. However, only the decoding process need be standardized to enable interoperability. The encoding process is typically not specified at all in a standard, and implementers are free to design their encoder however they want, as long as the video can be decoded in the specified manner. For this reason, the quality of the video produced by decoding the results of different encoders that use the same video codec standard can vary dramatically from one encoder implementation to another.
A variety of video compression formats can be implemented on PCs and in consumer electronics equipment. It is therefore possible for multiple codecs to be available in the same product, reducing the need to choose a single dominant video compression format to achieveinteroperability.
Standardvideo compression formats can be supported by multiple encoder and decoder implementations from multiple sources. For example, video encoded with a standardMPEG-4 Part 2 codec such asXvid can be decoded using any other standardMPEG-4 Part 2 codec such asFFmpeg MPEG-4 orDivX Pro Codec, because they all use the same video format.
Codecs have their qualities and drawbacks.Comparisons are frequently published. The trade-off between compression power, speed, and fidelity (includingartifacts) is usually considered the most important figure of technical merit.
Online video material is encoded by a variety of codecs, and this has led to the availability of codec packs — a pre-assembled set of commonly used codecs combined with an installer available as a software package for PCs, such asK-Lite Codec Pack,Perian andCombined Community Codec Pack.