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What is H.264?

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H.264 is a standard for video compression. Also known as MPEG-4 AVC (for Advanced Video Coding), it was jointly written by ITU - VCEG (International Telecom Union - Video Coding Experts Group) and ISO/IEC MPEG (Moving Pictures Experts Group) in 2003. The two groups maintain the codec together so as to have identical technical content. Hence, the codec is most often referred to as H.264/AVC.

H.264/AVC is a major video compression scheme and is widely used in a number of applications as it provides a far more efficient codec than any other algorithm. Its main advantage is that it can encode video with 3 times fewer bits than any other comparable format. This enables it to fit more television programs into a given channel bandwidth, deliver quality video over limited bandwidth channels (like 3G mobiles), and fit a high-definition video onto a standard DVD.

Other features of H.264/AVC include: flexibility – thus allowing up to 16 reference frames, which gives better quality scenes; variable block sizes in the range of 4x4 and 16x16 with supported luma and chroma predictions; ability to use multiple motion vectors per macro-block; six-trap filtering for sharper sub-pixel motion-compensation; precise description of displacements of moving areas; lossless macro-block coding; flexible inter-laced scan video coding; exactly-specified decoding; user definable motion estimation settings; CAVLC (Context-Adaptive Variable Length Coding) to encode transform coefficients; and CABAC (Context-Adaptive Binary Arithmetic Coding) to compress data more efficiently.

H.264/AVC has several different profiles, or capabilities, that target different applications. These are BP (Baseline Profile) for lower cost applications, used mainly in video-conferencing and mobile applications; MP (Main Profile) for broadcast and storage applications; XP (Extended Profile) for streaming video profile; HiP (High Profile) for high-definition television applications; Hi10P (High 10 Profile) improves HiP with the support of up to 10 bits per sample of decoded picture; Hi422P (High 4:2:2 Profile), with added support on Hi10P, primarily used for professional applications that use interlaced video; and Hi444PP (High 4:4:4 Predictive Profile), a built up on Hi422P, with better color coding. Besides these there are some more “all-intra” profiles that are used for professional applications like camera and editing systems.

Among the applications that support H.264/AVC are Windows Media Player, QuickTime, Real Video, iTune, and iPhone. It is also used on the internet by YouTube, MySpace, and Google Video.


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