H.264 is a high compression digital video codec standard written by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG) as the product of a collective effort known as the Joint Video Team (JVT). This standard is identical to ISO MPEG-4 part 10, also known as AVC, for Advanced Video Coding. The final drafting work on the first version of the standard was completed in May of 2003.
H.264 is a name related to the ITU-T line of H.26x video standards, while AVC relates to its ISO/IEC MPEG roots. It is usual to call the standard as H.264/AVC, or AVC/H.264 to emphasize the common heritage. The name H.26L, also related to its ITU-T history, is far less common, but still used. Occassionally, it has also been referred to as "the JVT codec", in reference to the organization that developed it.
The intent of H.264/AVC project has been to create a standard that would lead to good video quality at bit rates that are substantially lower (e.g., half or less) that what previous standards would need (e.g., relative to MPEG-2, H.263, or MPEG-4 part 2), and to do so without so much of an increase in complexity as to make the design impractically expensive to implement. An additional goal was to do this in a flexible way that would allow the standard to be applied to a very wide variety of applications (e.g., for both low and high bit rates and low and high resolution video) and to work well on a very wide variety of networks and systems (e.g., for broadcast, DVD storage, RTP/IP packet networks, and ITU-T multimedia telephony systems).
Since the completion of the original version of the standard in May of 2003, the JVT has done one round of "corrigendum" errata corrections and has developed a set of enhanced-functionality extensions called the "Fidelity Range Extensions" (FRExt). An additional round of corrigendum work is now nearing completion and should be finished in early 2005.
H.264/AVC contains several new features that allow it to compress video much more effectively than older codecs. CABAC (Context-Adaptive Binary Arithmetic Coding) is a clever technique that can be used in H.264 to losslessly compress syntax elements in the video stream. H.264 also implements an in-loop deblocking filter which helps prevent the ringing and blocking artifacts common to other DCT-based image compression techniques. H.264/AVC defines a way to perform motion compensation using previously-encoded pictures as references in a much more flexible way than in past standards -- allowing up to 32 reference pictures to be used in some cases (unlike in prior standards, where the limit was typically 1 or 2). This particular feature usually allows modest improvements in bitrate and quality in most scenes. But (for example) in certain types of scenes with rapid repetitive flashing, it allows a massive reduction in bitrate. These ideas, along with many other new ideas, help H.264 to perform significantly better than any prior standard can. H.264 can usually perform radically better than MPEG-2 -- obtaining the same quality at half of the bitrate or less.
The JVT recently completed the development of some extensions to the original standard that are known as the Fidelity Range Extensions (FRExt). These extensions support higher-fidelity video coding by supporting increased sample accuracy (including 10-bit and 12-bit coding) and higher-resolution color information (including sampling structures known as YUV 4:2:2 and YUV 4:4:4). Several other features are also included in the Fidelity Range Extensions project. The drafting work on the Fidelity Range Extensions was completed in September of 2004.
H.264/AVC is already widely used for videoconferencing, including its support in products of the two main companies in that market (Polycom and Tandberg). It has also been preliminarily adopted as a mandatory part of both of the major rival formats for future enhanced DVD uses, which are known as the HD-DVD and Blu-Ray Disc formats. The Digital Video Broadcast (DVB) organization in Europe has recently approved the use of H.264/AVC for European broadcast television. A number of broadcasters in Japan and Korea have announced future support for the codec, and it is under consideration for other broadcast use -- for example, it is under consideration in the United States' Advanced Television Systems Committee (ATSC) standards body. In the wireless world, it has been adoped as part of design release 6 of the 3rd-Generation Partnership Project (3GPP).
As of the time of this writing (late 2004), four companies are producing sample custom chips capable of decoding H.264/AVC video (specifically, Broadcom, Conexant, Sigma Designs, and ST Micro). Such chips will allow widespread deployment of low-cost devices capable of playing H.264/AVC video at standard-definition and high-definition television resolutions.
Like many ISO/IEC video standards, H.264/AVC has a reference implementation that can be freely downloaded. Its main concern is to give examples of H.264/AVC features, instead of being a useful application per se.
A tweaked variant of this codec is implemented in the form of the Sorenson codec, as was found by an FFmpeg developer working on reverse-engineering the Sorenson codec.
Like many other versions of MPEG, H.264/AVC implementors and users have to pay royalties for the use of it.
Applications
Apple Computer is working on integrating H.264 into Tiger, the next version of Mac OS X, version 10.4. Apple has already incorporated H.264/AVC directly into QuickTime.
The PlayStation Portable console will feature hardware decoding of video files in the H.264 format.
The HD-DVD format planned for product deployment in late 2005 by the DVD Forum includes H.264/AVC as a mandatory player feature.
The Blu-Ray Disc format planned for product deployment in late 2005 by the Blu-Ray disc Association (BDA) includes H.264/AVC as a mandatory player feature.
The Digital Video Broadcast (DVB) standards body in Europe approved the use of H.264/AVC for broadcast television in Europe in late 2004.
The Advanced Television Systems Committee (ATSC) standards body in the United States is in final consideration work on potential use of H.264/AVC for U.S. broadcast television.
The Third Generation Partnership Project (3GPP) has approved the inclusion of H.264/AVC as an optional feature in release 6 of its mobile telephony services specifications.
The ITU-T has adopted H.264/AVC in its H.32x suite of multimedia telephony systems specifications.
Ahead Software has integrated a H.264 encoder into it's Nero Recode package.