BBC's Quested talks HD encoder history
Monday, December 7 2009, 11:35 GMT
By Andrew Laughlin,
BBC principal technologist Andy Quested has discussed the history of HD encoders as part of a lengthy response to criticism of the BBC HD channel.
After new encoders were introduced by the BBC on August 5, the bitrate on BBC HD dropped from 16Mbs to 9.7Mbs, a reduction of nearly 40%. Viewers subsequently reported various problems with picture quality and sound on the channel.
In response, Quested will release a daily technical breakdown of issues affecting BBC HD until December 11. The series started last Friday with audio problems on the channel and continues today with a history of HD encoders.
When BBC HD began testing in 2006, Quested said that real-time H264 coding was relatively new, meaning that "early versions of encoders were not that much more efficient than the existing MPEG-2 HD encoders".
The original encoders operated at 19Mbps, but Quested felt that the bitrate "struggled to maintain the quality we wanted for a mixed genre channel".
In 2007, the corporation started testing new firmware to bring a "substantial" software upgrade to help the encoder "cope with the wider range of programmes and programme styles" on the BBC HD channel. The new version went live in time for the 2007 Wimbledon event.
"Direct comparisons between the 2006 and 2007 tournaments were very encouraging, especially when matches went on late into the evening and camera gain (and therefore camera noise level) was higher than I would really like to see," Quested said.
However, complaints about the channel's new bitrate first arose during the Planet Earth series in 2006, particularly around the famed helicopter tracking shot sequence over Angel Falls.
"The Angel Falls sequence was an early use of the helicopter set-up that gave the series such iconic shots but after seeing the rushes we were worried about what looked like interference on the tapes," Quested explained.
"When the sequence first went out, the original encoder and software didn't resolve the noise but when the clip was used later, the noise was clearly visible."
Quested posted a screen grab from the programme, including a blown-up section which clearly shows the horizontal lines of noise from the original tape.
However, he cautioned that "getting identical stills from an MPEG signal to use for quality comparison is not straightforward". Perfecting the comparison process primarily involves matching the individual frame types - I-Frames, P-Frames and B-Frames.
A selection of pictures can be formed from sequences of I, P and B frames, which takes into account "shot changes and motion within a sequence".
He added: "Although still frames are a useful tool for picture analysis, quality comparison using stills is best done with reference to the original source and using the appropriate I-Frames.
"Looking through the stills that have been posted it has been really interesting to see [the] quality of some of them, including the odd one or two that suggested the new encoder is better!"
Quested will tomorrow discuss the BBC's engagement with European Broadcasting Union recommendations for HD broadcasting.