The BBC is required to provide an audio description service on BBC HD. Typically
there are two to three programmes everyday which are audio described. In order to achieve this we have to use the HE-AAC/AAC audio codec present in Freeview HD receivers. The main audio is coded as AAC-LC and only the audio description is encoded as HE-AAC. There is a very common restriction in the silicon devices (which underpin all receivers) that only a single type of audio decoder can be working at any one time. So, the main audio and the audio description must use the same encoding family in order for them to be successfully combined together at the receiver. The current bitrates used for the AAC-LC audio are entirely commensurate with those that could have been used for a Dolby AC3 (Dolby Digital) solution.

In order to help manufacturers with their product development example streams with AAC-LC/HE-AAC audio description have been freely available to manufacturers since October 2009.

I've also been asked why we don't switch to using Dolby during the programmes which do not carry audio descriptions. Unfortunately the restriction I've just described around the ability only to have a single audio codec working at any one time means that changing audio codecs at programme junctions results in disruption to the decoded audio. This is because the receiver needs to detect the use of a different audio codec, load into its DSP the firmware for decoding that audio codec and then start decoding the audio. This disruption is acceptable when tuning between different services but not acceptable at programme junctions. Even if there are some decoders that can run different audio decoders concurrently it would not be possible to mandate the use of these specific solutions.

Changing the characteristics of the broadcast stream at programme junctions can be very difficult to achieve reliably in a broadcast head-end as many different systems need to be synchronised. It therefore needs very comprehensive monitoring to ensure the operation is reliable. Experience shows that some receivers can also have issues detecting such
changes unless the user tunes away and then reselects the service.

So the answer does lie in the need for the receiver to either transcode to a suitable format such as Dolby AC-3 or DTS or to output multi-channel linear PCM. Earlier versions of the UK D Book were only able to recommend the presence of transcoding. But now that transcoding solutions are becoming available the UK is no different from all other European standards. No doubt receiver manufacturers are working hard to incorporate these into their products.

However to help manufacturers develop products sometimes it is necessary to mandate certain features only after a suitable period has elapsed after publication in the D Book. So transcoding will become mandatory in all product launched after April 2011. In the meantime some manufacturers have already shown the capability of uploading this into existing deployed designs.

The use of an HDMI interface provides manufacturers with a choice of transcoding to Dolby AC-3 or DTS or delivering multi-channel linear PCM. The use of a SPDIF interface requires a transcoding operation as it cannot support the bitrates required for multi-channel linear PCM but can support bitstream formats.

Version 6.2.1 of the D Book requires receivers to support both the AAC (including
HE-AAC) and Dolby EAC3 (Dolby Digital+) formats. It is possible that the EAC3 format may also provide a future means for delivering receiver mix audio description.

These things are never straight forward but I hope that this answers the questions and gives you a little insight into why such decisions are made.

Phil Layton is Project Manager Digital Service Development BBC Research & Development.