The BBC has come under fire for the perceived poor picture quality of its high definition television service. It follows the introduction of new video encoders and a 40% reduction in data rate allocated to the satellite service in preparation for the launch of a high-definition channel on terrestrial television. Never mind the quality, feel the bandwidth. After months of complaints from viewers in its online forums, the debate has spread to critical coverage in the national newspapers.

When BBC HD first launched as a trial service in June 2006 it had a bit rate of around 19Mbps using first generation MPEG-4 H.264 AVC encoders.

New encoders were introduced in August 2009 and the data rate for the BBC HD channel was reduced to 9.7Mbps. That is lower than most high-definition channels on Sky, which average at over 11Mbps, although sports channels are typically 15-17Mbps.

While some gain in encoding efficiency can be expected from improvements in encoder technology, many suspect that the reduction in bitrate may have more to do with the launch of the channel on Freeview.

The available spectrum for terrestrial transmissions is much more limited than on satellite. Rather than draw attention to the difference, the BBC has apparently reduced the bandwidth for its single high-definition channel across the board in order to provide a platform neutral service. As a result, its offering now compares unfavourably to other high-definition channels on satellite.

The result has not gone unnoticed by viewers. Landmark documentaries, such as the David Attenborough series Life, featuring nature sequences that took weeks to capture in incredibly adverse conditions, beautifully shot and packed with detail and complex motion, have been marred by blurred and contoured reproduction as a direct result of the lower bitrate.

Danielle Nagler, the Head of BBC HD, has so far failed to placate critics with her responses to comments on the BBC web site. She states that “The reduction in bitrate is not specifically related to Freeview HD”.

On the BBC programme Points of View she said “There’s no evidence that reducing the bitrate has an impact on picture quality or that there is an absolute relationship between bitrate and picture quality.”

In her most recent posting on the BBC web site she says “There will be no ‘closing down’ of this debate” but adds “I feel that it is now time to draw a line under my further contribution here to the debate here.”

Andy Quested, who is principal technologist for HD at the BBC, has attempted to inform the discussion, without addressing some of the specific comments that have been raised by viewers, many of whom demonstrate remarkable knowledge of the subject compared to some of the responses from executives.

It has been suggested that high-definition can be used to deliver a range of ‘looks’. These may range from the characteristically crisp to a softer filmic appearance. Nonetheless, there is no defence for images that simply break down during transitions or complex motion.

Emma Scott, the managing director of Freesat, has called it a “a bit of a geek issue” but even if only a minority of viewers have noticed, and the BBC HD service is only available to a minority of viewers, the criticism has now spilled into the pages of national newspapers.

While the issues are involved are complex, if viewers notice a difference or deterioration then there is clearly a problem. Viewers that not unreasonably expect the BBC to establish the gold standard for technical specifications are instead effectively being told that nanny knows best and they are simply imagining things.

Broadcasters are under strategic and commercial pressures to offer more channels across more platforms and technical quality is often compromised. As a result, digital television and radio services are often delivered at lower technical quality than many engineers and experts would advise. Many would argue that this is the case on digital terrestrial television and radio, which falls far short of the quality of which the technology is capable.

In the past, professionals generally had access to equipment that was far superior to that available in the average living room and required expensive test equipment for measurement.

Now consumers can connect their own cameras directly to displays of astounding capability, and compare the output of high-definition broadcasts with that of Blu-ray disc players. As a result they are not readily fobbed off by platitudes from executives.

Licence payers are concerned that they may be being short-changed by the BBC. They may have invested in a Full HD 1920×1080 progressive display and become accustomed to watching Blu-ray movies encoded at 15Mpgs to 30Mbps or more. They are therefore surprised to see that the BBC HD channel is 1440×1080 interlaced at under 10Mbps and is consequently compromised in quality.

In the case of the BBC HD channel, in order to reduce the bitrate the horizontal resolution is reduced by a quarter prior to transmission and is subsequently interpolated by the display. The result is an inevitable loss of resolution and the introduction of aliasing artefacts.

The licence for the service states that “BBC HD should deliver a very high quality technical service to viewers, by adhering to, or seeking to exceed, industry standards for picture resolution.”

Strangely, the BBC continues to broadcast high-definition to the rest of Europe at around 16Mbps in Full HD 1920×1080.

Ironically, the real winner in this is Sky, which offers its subscribers the highest available broadcast quality across over thirty high-definition channels with which the free BBC HD service must stand technical comparison.