The BBC's high-definition (HD) television service has lost its pin-sharp pictures, viewers are claiming.
Hundreds of complaints have been posted online saying that broadcasts became fuzzy and grainy after the Corporation lowered the bitrate of its HD encoding technology from 16 megabytes to 9.7MB.
The BBC has been accused of stifling the criticism by closing user forums on its websites that contained negative comments from viewers. Forums that have been re-opened have been bombarded with fresh responses filling page after page. Viewers who claim they have been "fobbed off" have contacted the BBC Trust, demanding that it investigates the problem.
Protests began in August after the BBC changed its encoding system, leading to claims that its HD service, available on the Freesat platform, was little better than normal television. The Corporation's main commercial rival Sky transmits HD programmes at a bitrate of between 1MB and 15MB.
Viewers watching the BBC HD channel, which screens top shows such as Gavin & Stacey, said they had seen a dip in picture quality.
"The BBC didn't tell anyone and now people are finding out and are up in arms about it," said Paul Shakeshaft, of Alton, Hampshire. "Before the quality used to be as good as Blu-ray. You would watch it and think it was fantastic but now it is quite soft and grainy. The BBC are saying the bitrate has gone down by 40 per cent and the quality is better. It's not – it's worse.
"There are hundreds of people complaining all the time. The BBC will open a forum, get hundreds of complaints, respond to them, shut down the forum, and then it will start all over again." According to one theory, the BBC changed encoders so that it could offer more programmes on HD BBC1, which is due to be launched next year on Freeview, where bandwidth is limited. But Nick Caley, a spokesman for the BBC, rejected the idea, saying: "The reason the [encoders] were replaced was because they had come to the end of their lives. We did extensive testing which showed [the new encoders] could produce pictures at the same or even better quality than the old encoders at the higher bitrate."
Denying any stifling of dissent, he added: "We have actively debated the issue of BBC HD picture quality, via our blog, with those viewers who feel the bitrate change has affected picture quality. Only this week the head of technology posted a blog per day addressing the issues raised."
Danielle Nagler, the BBC's head of HD, admitted there had been "some issues" with picture quality on certain shows but she did not believe this "had anything" to do with the lower bitrate.
One blogger responded: "Sorry but the post seems to be the the usual BBC HD fob-off. When the channel started out the picture quality was stunning. Then the channel lowered the bandwidth and the quality went down."