BBC HD Using Variable-Bit-Rate Encoding To Improve Picture Quality

Jonathan Sutton

Following some persistent lobbying from a consumer campaign group, BBC has over the last few weeks taken some steps to improve the picture quality of its BBC HD channel.

When BBC HD introduced a new high-definition transmission encoder on 5 August 2009 which slashed broadcast bit-rate by a whooping 40% (from 16Mbit/s down to 9.7Mbit/s), some viewers who noticed a corresponding drop in image quality lodged complaints to the BBC Trust, and proceeded to form an aptly-named BBC HD Picture Quality Campaign Group.

Although the transmission bitrate of 9.7Mbits/s was lower compared to that of Sky HD and BBC Worldwide HD (distributed to European broadcasters) channels, in December 2009 Which? Consumers’ Association (UK’s product-testing and consumer campaign group) backed BBC HD’s decision by publishing a report of their independent testing. According to Which?, there was no significant deterioration in picture quality when they compared between BBC HD content packaged using the new encoders at reduced bit-rate, and archived BBC HD material prior to 5 August 2009.

In spite of this setback, the BBC HD PQ Campaign Group continued their appeals and lobbying, ultimately culminating in BBC HD opening its doors to a visit from representatives of the campaign group. These campaigners met Head of BBC HD, Danielle Nagler, and Head of Technology, Andy Quested, at the BBC Television Centre in London on 30 April 2010, where some technical briefings, blind testing, and intense discussions took place. (Source: BBC Internet Blog)

As a result of the feedback received from the visit, BBC HD recently switched to variable bit-rate (VBR) encoding, a process which cleverly maintains a similar average bit-rate despite being able to allocate more bandwidth for more demanding scenes such as fast-action sports, resulting in a higher quality-to-space ratio. An improvement in picture quality has been reported by discerning viewers on various forums, with HD programmes like Springwatch 2010 and live broadcast of FIFA World Cup 2010 football matches particularly impressive.

Having seen their efforts over the past 9 months bear the desired results, the BBC HD Picture Quality Campaign Group’s next target is to persuade BBC HD to increase its channel resolution from the current 1440 x 1080 to full HD 1920 x 1080.