BBC Pushes Dirac to the Forefront

Not only was the BBC's core operation at the Beijing Olympics fully HD and tapeless for the first time, its transmission was enabled by BBC-devised compression technology Dirac. The use of Dirac, under development by engineers from BBC Research & Innovation for several years, represented the first broadcast application of the technology.

"Dirac has now reached a degree of maturity," reports Tim Borer, who heads the BBC R&I Dirac team. "Dirac is a whole family of video codecs derived from the same technology. Dirac Pro is designed for professional links within and between studios; other versions of Dirac are aimed at contribution links and for end user distribution."

Host telecommunications from the venues was largely uncompressed, but for some venues the BBC was unable to secure HD links. To solve this problem it fitted Dirac Pro 270 encoders, manufactured by NuMedia Technology, to compresses the signals over SD SDI to Beijing's IBC. "Alternative compression schemes don't travel over standard SD links and have higher latency," he adds.

"Dirac's compression is at least comparable with H.264 (MPEG-4 AVC) but it has two significant advantages," says Borer. "These are that it is royalty free as well as being open source and cross platform."

BBC R&I has developed a special software version of the algorithm to compress the Super Hi-Vision (SHV) broadcast format proposed by NHK and being demonstrated by the Japanese broadcaster, EBU, RAI and the BBC at IBC. "NHK is looking to develop Dirac hardware for encoding SHV," says Borer. "The native bitrate from the SHV camera is 24 Gigabit a second and Dirac is one option for this since it provides a more graceful compression than MPEG. The wavelet compression on which Dirac is based fits well with SHV since wavelets scale well for high-resolutions.

The quality of Dirac's compression continues to improve above 40Mbps whereas H.264's quality begins to deteriorate at that point. "Often the BBC wants to use contribution links with bitrates as high as 80Mbps and currently the only option for that is MPEG-2. Dirac now provides another option."

There are concrete plans to implement Dirac Pro 270 within the central switching area of TV Centre.

By Adrian Pennington, IBC Daily