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Dirac is a prototype algorithm for the encoding and decoding (see codec) of raw video and sound. It was presented by the BBC in January 2004 as the basis of a new file format for the transmission of video over the Internet.

The codec is still not finished as a product, and thus regarded as still being under development. "The immediate aim is to be able to decode standard digital TV definition (720 x 576i pixels per frame at 25 frames per second) in real time; the current version can decode a quarter-standard resolution at 20 frames per second, which is enough for Internet streaming." [1]

Similar to common video codecs such as the the ISO/IEC Moving Picture Experts Group (MPEG)'s MPEG-4 Part 2 or Microsoft's WMV 7, it can compress any size of picture from low-resolution QCIF (176x144 pixels) to HDTV (1920x1080) and beyond. However, it promises significant savings in bandwidth and improvements in quality over these codecs, by some claims even superior to those promised by the latest generation of codecs such as H.264/AVC (which is technically identical to MPEG's MPEG-4 Part 10) or SMPTE's VC-1 (which is based on Microsoft's WMV 9), by employing wavelet compression, replacing or augmenting the discrete cosine transforms used in older codecs with Wavelet transforms. Dirac's implementors make the preliminary claim of "a two-fold reduction in bit rate over MPEG-2 for high definition video"[2], an estimate which would put the design in about the same class of compression capability as the latest standardization efforts of H.264/AVC and VC-1. MPEG-2 is the previous generation video codec used in the standard DVD format today.

Dirac is currently the most active project attempting to apply wavelets to video compression. The other one of note is the Foundation's Tarkin, developed by the creators of the Vorbis audio codec, which offered impressive performance but saw only limited adoption. Tarkin showed much promise, but was put on hold so its creators could focus their efforts on Theora, a less experimental video codec based on On2 Technologies' VP3.

Wavelet compression is not merely experimental. It has already proven its viability in the JPEG 2000 compression standard for photographic images. However, it has not been shown to provide substantially superior compression capability than other techniques based on more traditional block-based coding. For example, H.264 is often as good or better in compression capability for still images, despite the wavelet-based design of JPEG-2000. The primary benefits of JPEG-2000 relative to other codecs are functionality aspects, such as scalability features, rather than compression capability.

The codec is named in honour of the British scientist Paul Dirac.


The experimental and unfinished Dirac codec, written in C++, was released under an Open Source licence at SourceForge on 11 March 2004.

While the BBC owns some patents on Dirac, they have irrevocably given royalty-free license of the Dirac related patents to all of humanity, enabling the public to utilize Dirac for any imaginable purpose. The BBC releases Dirac under the Mozilla Public License which also allows GPL and LGPL programs to use it. This may accelerate its adoption and lower entry costs into the emerging industry of Internet television.

See also

External links

Last updated: 10-15-2005 10:03:45
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