dave, as suggested on the BBC's boards, the response given in Radio Times, is highly equivocal. THey simply asserted that a wide variety of BBC channels would remain avaiulble using the currrent mp2 codec. They said nothing about audio quality of bitrates, I can well see them introducing AAC in the not tha distant future, in tandem with a reduced bitrate mp2 service.
Bryn
My readings of what they have said, and also what Ofcom have said, suggest that in the short term a move to DAB+ is unlikely, but I can always be wrong. As you say, there are strategies which may work to get a transfer to something like DAB+, but with limited spectrum available this would be difficult, and I'm not sure that they really would want to reduce the DAB bit rate too far during the process, as this could be considered intolerable. However, my suspicion is that quite of lot of the output of the BBC would work quite well on aac+ at around 40 kbps stereo, so assuming we have about 8-10 stations (R1,R2,R3,R4, R5, 5Extra, World Service, London, Asian -are there any more?Miss R6 and R7 first time round!!!) that'd "only" need about 320 kbps spare. It's not much, but when they have little space to play with, it would be tricky. Also, I personally don't think that aac+ at 40 kbps is good enough for R3, though the BBC may disagree and thousands of non complaining listeners might not notice. They could even go down to 32 kbps - for 256 kbps on 8 stations. It's arguably no worse than moderate quality FM.
I think there'd be a riot if they simply killed a few stations during a change-over, without an alternative form for reception. In my opinon they could probably lose World Service for a while, but if a channel like the Asian service went, even for a short time, there'd probably be cries of discrimination. Most of the channels could probably have a few kbps shaved off - R1-32 off, R2 32 off, R3 32 off, R4 32 off, lose World Service for a while - save 64, London, 32 off, R7 16 off - that's around 240 kbps - nearly enough to put on the low quality aac+ service in the transition period. They could also put out some of the aac+ channels in mono to start with, then switch over to stereo later, and switch the conventional DAB channels to mono at the same time. That'd give two parallel transmissions of most of the BBC output, but most would be in relatively poor quality sound. How long could such a transition last? 2-4 years maybe - perhaps longer! Will the audiences tolerate that?
Once they'd got a change-over under way, they could then kill off some/all of the MP2 channels, and if they were feeling kind they could then upgrade to AAC at (say) 128 kps for R3 - and at 96kbps for most of the rest - though I note that some listeners to R1,R2 and even R4 might say that the stations should all have the same quality. I also have reservations that 128 kbps AAC is really good enough for high quality sound, though arguably it's no worse than we have now.
I have no idea whether Ofcom would be receptive to allocating more spectrum on a short term basis in order to allow a change-over to take place more smoothly, though so far I'm not convinced that they would, or even that they've really thought much about it. As I said earlier, I have a feeling that things are a bit easier in some other countries to facilitate a switch.
In addition the BBC would also have to do some PR work to downplay the "success" of digital DAB (if it's so good, why "upgrade" it?) to get people to buy the new equipment. They might prefer to let commercial broadcasters spearhead the way forward, though I can't see any reason why the commercial broadcasters would be interested in doing that anyway, as they have little interest in quality, and my understanding is that Ofcom has been kinder to them as a group in terms of spectrum allocation.
Problems ahead I think.
