On 10/10/2007, Duncan Barnes <[EMAIL PROTECTED]> wrote: > > On 10/10/2007, Martin Deutsch <[EMAIL PROTECTED]> wrote: > > Switching between SDI signals is easy; what Duncan's suggesting is a bit > > trickier. > > At present: the BBC and ITV, and Channel 4 have satellite multiplexes > which > > carry several channels which are the same for most of the day; they can > > differ when it comes to news, regional programmes and advertising. > Ideally, > > each mux could carry a high bitrate version of (eg) ITV1 during network > > programming, but during regional opts (local news, ad breaks, etc), > would > > carry each of the local streams, though probably at a lower bitrate, and > the > > viewer wouldn't notice a thing. > > I *think* that DVB can do this, but changing the SI tables (which tell > the > > receiver where to find the video and audio associated with a network) on > the > > fly isn't an exact enough science to be able to do it with the frame > > accuracy desired by the broadcasters > > > > I suspect someone - BBC R&D? - has already done research into this. > > > > - martin > > > > Yes that is what I was getting onto, wasn't sure about going into > broadcast acronyms too much was half my issue before. If it were > possible then it would free up an awful lot of bandwidth for a higher > quality ITV1 (for example) during non-regional times, I'd love it to > be possible, on Sky there are three multiplexes dedicated to ITV feeds > so there is quite a lot to be potentially saved. The switching your > talking about is half the issue, its also the fact that you can only > really cut from one stream to another roughly once every 12 frames due > to the nature of MPEG, hence why set-top boxes stop for a sec when you > change channel. This gives you a very tight window if you want to do a > seamless switch and would also be dependent on the set-top boxes > ability to traverse the SI table in that time.
The details of the satellite multiplexes can be seen here: http://www.lyngsat.com/astra2d.html I agree that it might take a bit of tweaking to get a perfect frame join as you switch into the national feed and then split out to the regional feed. I can't see why it's not been done before. It would be very handy if the main channel was, say in MPEG-4 HD quality and then the regional opt-outs were in MPEG-2, even if you had to carry an MPEG-2 version for "old boxes". I dunno how well all the decoders would cope though, without an old-fashioned "black frame". But it could be done on analogue - you could never see the join in the switch to regional programmes on the BBC even back as far as "Sixty Minutes", so it MUST be possible. If the MPEG-2 streams collapse, the "opportunistic data" on the interactive systems could claim all the spare bandwidth automatically.... Perhaps it is possible, I haven't seen anything from BBC R&D about it > (but then again the BBC management is reportedly flushing that away > like everything else of worth from the old corporation) and it would > be interesting to try it, assuming I can't find any reason not to, I > will try to scavenge enough equipment/rack space to run a little test > at some stage. It would save a fortune on transmission, but only if you could realistically use the space. ITV wishes to collapse it's regions anyway, and the only gain could be interactive services for ITV. I guess the BBC could use the spare space for either PROPER high definition versions of all the BBC channels, which would be nice. - > Sent via the backstage.bbc.co.uk discussion group. To unsubscribe, please > visit http://backstage.bbc.co.uk/archives/2005/01/mailing_list.html. > Unofficial > list archive: http://www.mail-archive.com/[email protected]/ > -- Please email me back if you need any more help. Brian Butterworth www.ukfree.tv
