BBC principal technologist Andy Quested has detailed European Broadcasting Union recommendations to answer criticism of BBC HD's picture quality.

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 is releasing a daily technical breakdown of issues affecting BBC HD, which started last Friday with audio problems on the channel, continued yesterday with a history of HD encoders and today covers EBU guidelines.

Writing on theBBC Internet Blog, Quested cited a recurring complaint about the frame size on BBC HD that "1440 horizontal pixels is not HD".

He pointed to EBU documents which stipulate that 1440x1080 is "an acceptable standard for high definition transmission". The guidelines also claim that there is "arguably no point in broadcasting material derived from this format at more than 1440 samples per line".

Quested further said that HD-ready "does not actually specify the horizontal resolution for an HD display". Rather, the picture should just have a minimum vertical resolution of 720 pixels and correctly display 16:9 images.

Some viewers have claimed that the BBC is failing to meet EBU transmission standards with the current 9.7Mbps bitrate on BBC HD.

Indeed, EBU documents recommend that the 1080i HD TV format should run at a minimum bitrate of 12Mbps to 14Mbps. However, Quested said that the documents were issued in May 2008, which was "long before" the BBC started testing on the new encoders.

"Tests have demonstrated that at transmission bitrates, H264 encoders should deliver approximately a 2:1 efficiency over MPEG-2 encoders. Depending on the manufacturer and their current stage of development good HD at 8-10Mbps is achievable now," he said.

"The BBC has made contributions to most of the documents mentioned above and many others besides. We are also involved in testing and trials for high definition production, contribution and transmission compression and were one of the earliest activists in the latest round of HD activity in the EBU."

Quested also pointed to EBU guidelineTech 3334, which says: "With the expected future developments in video coding, it is assumed that HD fixed bitrate requirements will be reduced to 8Mbps to 10Mbps per programme. There will also be advances in the transmission system such as DVB-T2."

He further noted that several encoder manufacturers recently showed off early versions of HD coding which can work at an average of just 4Mbps to 6Mbps.

"One final point worth noting here is the lifecycle of the hardware. As I mentioned earlier, the old encoders had reached the end of their life but the new encoders are at the beginning," he concluded.

"They exploit more of the AVC toolset than the old encoders even though we are on version 1 of the firmware. We will continue to add improvements as and when they are available."

Quested will tomorrow address the key sticking point of picture quality analysis on BBC HD, including how the BBC sets up its encoder testing.