BBC responds to HD picture quality furore
On August 5, Digital Spy forum members reported that BBC HD's bitrate had dropped from 16Mbps to approximately 9.7Mbps, a nearly 40% reduction. Quality issues including the appearance of compression artifacts, jitter on near-static scenes and problems with fades were reported by viewers.
In a post responding to the reports, the BBC's principal technologist for HD Andy Quested confirmed the introduction of new encoders and said that "there should be no significant change in quality", adding that "many programmes should look better". He acknowledged a "noise issue on some programmes and formats", citing material shot on Super16 and cameras using less than 50Mbps as examples of source material causing problems.
The reports of reduced picture quality continued both on DS and in a lengthy thread of comments on the BBC Internet Blog, where Quested kept viewers up to date about subsequent upgrades to the new encoders including how they dealt with mixes and fades.
In a subsequent comment, Quested said that the BBC had acted "to reduce the unsustainable bit rates for HD transmission", warning that in the absence of such action "it will have no future or will only be available to those willing to pay for and able to get super premium services".
He added that work was continuing and that "the pictures are overall better than before".
The discussion continued on both fora, with contributors pointing out that BBC HD is available on a commercial basis in Europe at a higher resolution and bitrate and that other channels - including Sky One HD - continue to use higher bitrates on a statistically multiplexed basis.
In a response post on the BBC Internet Blog, Nagler explained that the new encoders were installed t"o help us in handling the wide range of material which the BBC broadcasts in HD, and to help to improve the picture quality of some of our most challenging programmes" and that issues about the handling of some pictures were acknowledged "almost immediately".
"We have worked with our encoder supplier to address these issues in the long-term, and also put in place interim changes to minimise the difficulties," explained Nagler. "That the encoder change should generate problems for viewers watching BBC HD content is of course a matter for regret, but I do not believe that this was the result of errors in the preparation process."
The post added: "One of the central issues in selecting new encoders for BBC HD was to deliver pictures at the same or improved quality while allowing a reduction in the channel bit-rate. As MPEG 4 coders have evolved, the relationship between bit-rate and picture quality has also shifted. This is not an issue that is specific to BBC HD, or to the encoders that we have selected.
"The BBC has an absolute responsibility to use bandwidth efficiently - whether on digital terrestrial muxes or on satellite. Bandwidth is not unlimited, and on UK-footprint transponders the demand for capacity is very high. The current bit-rates were selected through a process which directly evaluated quality on the new and old encoders, using a wide range of programme material and both subjective and objective assessments.
"Bit-rate is not the only factor affecting picture quality and a higher bit-rate will not automatically deliver higher picture quality."
Nagler cautioned viewers not to confuse sharpness with resolution, and said that BBC HD would not encourage the use of electronic sharpening "since we prefer images to look more natural".
Andy Quested plans to post an update showing tests between the old and new encoders; meanwhile, the discussion continues.