BBC's Quested talks HD programming styles
BBC principal technologist Andy Quested has outlined how certain techniques used by programme makers can affect perception of picture quality on BBC HD.
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 has been running a series of technical briefings on the BBC Internet Blog - including the audio issues affecting BBC HD, a history of HD encoders, current EBU recommendations and the BBC's HD testing processes.
Completing the series, Quested today discussed how certain production techniques can affect perceived picture quality on BBC HD rather than the bitrate.
Counter to various critical posts on the forums, Quested said that the bitrate on BBC HD has actually remained at a fixed level since August. He also claimed that the amount of up-conversion of non-HD content on the channel has remained below the maximum level permitted of 25%.
Quested acknowledged that certain viewers have complained about the depth of field/focus in the picture on BBC HD. However, he said that programmes such as Cranford and Wallander use feature film-style cameras with a shallow field depth to enable the director to maintain focus on the central characters.
"There have been many comments about HD being pin sharp and some people believe an HD image should be in focus from the nose of the person in close up to the trees on the horizon," he explained.
"I think many people would find that incredibly distracting and, as some of the posts lead me to believe, people would just be looking at the image quality not the programme. Focus is a very useful programme-making tool, and when used well it adds to the look and feel of a programme."
Other complaints have focused on the noise and gain levels in BBC HD's picture, particularly when the image has lots of dark areas. Quested said that programme makers often deliberately try to make pictures look "dirty" during shooting or post-production, which is a process also regularly used in filmmaking.
However, he said that the technique can cause problems on television if it is overdone because the encoder is stretched, which leads to "unpredictable quality changes". He also stressed that any programmes caught overdoing the technique "will usually fail a technical review".
Quested further acknowledged that many complaints have focused on BBC HD's motion portrayal, essentially that high action sequences are blurred because the picture is not being transmitted at a sufficient rate of frames-per-second.
He said that certain programme makers shoot using a camera's 25p option, often known as Film Motion, which delivers a low FPS rate but can generate greater detail.
"At the moment though we leave the decision about motion type to the producer and director of photography of the programme," he explained. "But there are times when we do comment on the inappropriate choice of 25i or 25p."
The blog finally addressed issues of softness and smear on BBC HD, where the trademark HD sharpness is perceived to be softened around the edges.
Quested said that certain programme makers "do not want extremely sharp images" and instead try to soften the picture with lens filtering or post-production effects. He cited the example of Criminal Justice, which used softening to "create a very distinctive look".
Again, though, Quested warned that if the technique is overdone then it can "significantly reduce the image resolution" and also increase the level of noise in the resulting picture.
"We do try to limit the amount of lens filtering programmes use but if extreme image softening is required, we encourage people to do it in post-production so that if the end result is unsatisfactory, at least we can ask for it to be removed," he said.
"Motion blur (or smearing) is another matter. It usually occurs when the camera shutter is not set appropriately. A programme shot at 25p should use a shutter speed around 1/50th sec (or 180˚). If the shutter is not turned on or is set too long, the images will smear. This doesn't look very nice and we do try and stop people doing it. If the producer wants to add motion blur, again it is always better to do it in post-production."
In his closing comments, Quested said: "There is no doubt this series will cause much comment and raise more questions about image quality and encoding generally. I will try and answer as many as I can but in the end we may have to admit we will never be able to satisfy all of you when it comes to what is or isn't high definition."