On the 5th of August BBC’s new transmission encoders were taken into operation. After some questions from viewers they have decided to respond on their Internet blog. The blogpost is actually a response from BBC’s head of BBC HD Danielle Nagler to an anonymous author. In my opinion this article gives a very interesting insight in the kitchen of world’s biggest high definition br0adcaster:

Hello Everyone

I thought that you might like to see the detailed response which I have sent to someone who contacted me regarding the picture quality issues which are under discussion here:

“…Your complaint refers to the introduction of the new transmission encoders for BBC HD which were introduced into operations on Wednesday 5th August. There was an extensive process of assessment in advance of the selection of new encoders for the BBC HD service, using both objective and subjective criteria. The encoders which were chosen then went through further testing in advance of operational use, not only for picture quality but for compatibility with the Sky and Freesat platforms and their ability to deliver other services such as subtitling and surround sound successfully.

The new encoders were intended to help us in handling the wide range of material which the BBC broadcasts in HD, and to help to improve the picture quality of some of our most challenging programmes. These may combine progressive and interlaced shooting or where the BBC has limited control over some aspects of the broadcast chain. I believe that the new encoders have achieved this in relation to programmes such as the series of BBC Proms broadcast, Gardeners’ World, Rick Stein’s Mediterranean Escapes and the recent Athletics World Championships, for which our coverage using the host broadcaster feed was as good as and sometimes better than other broadcasters covering the same event. However we of course continue to assess coder settings against the wide range of material which they have to handle to determine the best settings on an ongoing basis.

Following the introduction of the new encoders, there were some issues around the handling of some pictures – primarily mixes and fades – which we acknowledged through the BBC HD blog on picture quality almost immediately. We have worked with our encoder supplier to address these issues in the long-term, and also put in place interim changes to minimise the difficulties. That the encoder change should generate problems for viewers watching BBC HD content is of course a matter for regret, but I do not believe that this was the result of errors in the preparation process.

You have also highlighted the issue of the bit-rate at which BBC HD broadcasts, and the changes to this over time. I do not believe that the problems that arose following the introduction of the new encoders had anything to do with the broadcast bit-rate, even though they coincided, as you have rightly identified, with a reduction in bit-rate for the channel.

One of the central issues in selecting new encoders for BBC HD was to deliver pictures at the same or improved quality while allowing a reduction in the channel bit-rate. As MPEG 4 coders have evolved, the relationship between bit-rate and picture quality has also shifted. This is not an issue that is specific to BBC HD, or to the encoders that we have selected.

The BBC has an absolute responsibility to use bandwidth efficiently – whether on digital terrestrial muxes or on satellite. Bandwidth is not unlimited, and on UK-footprint transponders the demand for capacity is very high. The current bit-rates were selected through a process which directly evaluated quality on the new and old encoders, using a wide range of programme material and both subjective and objective assessments.

Bit-rate is not the only factor affecting picture quality and a higher bit-rate will not automatically deliver higher picture quality.

HD is still an evolving production technology. A variety of production techniques are – in my view quite rightly – deployed as experimentation continues to explore what HD can delivery creatively.

As in standard definition, it is also important that HD delivers a range of “looks” for producers, appropriate to the nature of the subject matter. I do not prescribe a single standard for HD work for the BBC. Decisions regarding frame rate and progressive versus interlaced styles are the responsibility of individual producers. These choices do not impact on quality provided that the camera is set up properly and the shutter speed set correctly, issues on which the BBC HD team provides ongoing advice and guidance. As the discussion on the BBC HD blog suggests, there is a range of views around these issues, and the degradation or quality they may bring to HD pictures. It is worth noting that 25 frames progressive mode in fact has more resolution than a 25 frame interlace image, and is used by the majority of drama, documentary and natural history programmes to great effect.

While very clear, sharp images have become closely associated with HD, it is important not to confuse “sharpness” with resolution. The use of electronic sharpening on standard definition pictures can make images clearer but does not increase the amount of information in the picture, one of the defining features of HD.

Electronic sharpening is not a characteristic which BBC HD encourages since we prefer images to look more natural, and to allow directors to offer contrasting focus in order to highlight the key features in a scene. Indeed, some of our dramas are now using the latest large image format cameras.These cameras use an image sensor about the same physical size as a 35mm film frame that gives the image a very shallow depth of field. This will put all but the key subject out of focus and allows a director to use focus as a story telling tool.

HD picture quality is not purely about a crispness of image, but about a richness of image which comes from the amount of detailed information included.

Within the BBC HD team we work consistently to explore new HD technology with a particular view to enhancing picture quality across the range of programme projects with which we are involved. Filming in certain environments or using small cameras remains challenging, and where it is not possible to deliver HD pictures to the standards we set, we limit the use of lower quality images to a maximum of 25% of an individual title.

Finally, you raise the issue of surround sound and the number of programmes broadcast which offer a 5.1 sound mix. Wherever possible we buy series and films with surround sound tracks, and try to ensure that sound is captured in this format for outside events. But 5.1 sound is not always available, and a genuine surround sound mix can add substantially to production costs in HD at a time when we are concentrating the resources available to increase the total volume of programmes made in HD. At present we do not routinely “up-mix” programmes from stereo to surround.

I can assure you that picture quality is a very important part of the work that we are doing in BBC HD. We clearly have different views on the picture quality that is delivered and the factors that contribute to it, but I do believe that we share a perspective that the quality of images is central to delivery of HD television.

I want to add that the BBC HD blog is a very important discussion and communications forum for us, but as you have clearly observed we don’t respond to every single comment raised on it. We do take on board all the views expressed, and look seriously at substantive issues that are highlighted, whether they attract one comment or many.

I am sorry that in this case you feel that your original contribution did not receive the attention which you believed that it should have done.”

Danielle Nagler | Danielle Nagler is Head of BBC HD, BBC Vision