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 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 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.
More recently, the JVT has been working on a corrigendum (a list of errata corrections) to the original standard. The drafting work on the corrigendum should be completed in May of 2004.
The intents of H.264/AVC project were to create a standard that would lead to fast implementations, using low bitrates. That is: implementations that would demand little from the decoder hardware and from the network bandwidth.
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. In previous video standards, motion compensation is handled by allowing blocks in a frame to refer only to the frame before it. H.264/AVC allows frames to be predicted from other frames that are arbitrarily far in the past. This 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 MPEG-4 ASP can. H.264 can usually perform radically better than MPEG-2 at a fraction of the bitrate.
In addition, the JVT is nearing completion of the development of some extensions to the original standard that are known as the Fidelity Range Extensions. These extensions will 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 should be completed in the Summer of 2004.
H.264 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 the future DVD specification known as HD-DVD, now under development by the DVD Forum. 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) and in Europe's Digital Video Broadcast (DVB) standards bodies. In the wireless world, it is under consideration for adoption by the 3rd-Generation Partnership Project (3GPP).
Like many ISO 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.
External link
- H.264/MPEG-4 Part 10 Tutorials
- H.264/AVC Software Coordination
- ftp.imtc-files.org/jvt-experts FTP site for reference implementation history as well as expert meetings
- MPEG LA Announces Terms of Joint H.264/MPEG-4 AVC Patent License
- A fast GPL H.264 encoder library with support for most H.264 features