Natural history programmes shot in HD have attracted praise

The quality of pictures broadcast on the BBC High Definition (HD) channel has been criticised by viewers who complain of fuzzy and grainy images.

The head of the channel, Danielle Nagler, says that despite a series of tests, no technical fault can be found.

Not all HD programmes have the "bright, crisp look which for some is synonymous with HD", she wrote on the BBC's internet blog.

Camera and production techniques all affect the overall quality, she added.

The BBC replaced the encoders which process HD pictures in August 2009.

The replacement encoders work at a bitrate of 9.7Mbps (megabits per second), while their predecessors worked at 16Mbps, the standard for other broadcasters.

The BBC HD website describes HD as "exceptionally clear, crisp pictures with vivid colours and up to five times more detail than standard definition."

Some disgruntled viewers think the new encoders have affected this viewing experience.

"We did extensive testing on the new encoders which showed that they could produce pictures at the same or even better quality than the old encoders at the higher bitrate," a spokesperson told BBC News.

In her blog post, Ms Nagler said that the services was created to be at its best for "typical viewing set ups" and that user groups with standard equipment were happy with the service.

She told the BBC that "there is not a major issue with picture quality" and added the corporation had only received one formal complaint.

The majority of those leaving comments on her blog post disagreed.

"Even my wife can see a reduction in picture quality and she's got cataracts," wrote one.