Science Questions

Who decides DAB bit rate?

Sun, 16th Dec 2012

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Question

@NateLanxon asked:

DAB broadcasts have different bit rates. Some sound better than others. Who decides what these should be?

Answer

John -   In the early days when the Eureka 147 project was launched, the plan was that they would have high quality services of 256 kilobits and maybe a compromise will be 192kb.  The difficulty with that is commercially, that didn’t make it all that attractive because there weren’t really very many more channels than we could get on the FM dial.  To make DAB attractive, they wanted more stations particularly in the commercial sector.  So, the way to do that with a finite amount of bandwidth is to reduce the bit rate of each station until you could squeeze more channels in.

That's an advantage of DAB really that you can dynamically control the bandwidth used by stations.  So for example, when radio 5 Live has a special going out of 5 Live Sports Extra, they will change the bit rate of 5 Live.  Likewise Radio 4 FM carried on DAB will have a certain bit rate normally, but when the morning service goes out on the Radio 4 long wave version on DAB or Today in Parliament is broadcast then the bit rate is squeezed down, and it goes into mono so that you can maximise the choice for the bandwidth available.

Yes, quality does vary station to station.  Some stations are using as little as 64 kilobits which does not produce very pleasant effects.  It restricts you to mono, the old mpeg coding, it doesn’t perform well at these bit rates.  However, there's something else I might talk to you about and that's DAB plus which is a means by which you can use even lower bit rates and get reasonable sound quality, but the UK isn’t adopting that.

Chris -   Why not?

John -   The problem is backwards compatibility.  If you have a radio that you bought before 2008, the chances are, it would not be able to decode DAB plus.  So all these people who have gone out in good faith and bought their DAB radios would find they wouldn’t be able to get their DAB plus stations.  And of course, the longer we leave it, the more difficult it becomes because ever more people have bought sets that might not be compatible.  Other countries are looking at that and introducing DAB plus, but that's because they're not so far ahead in getting DAB launched in the first place.  I can give you a parallel of freeview HD and freeview.  If you have a free view box, you cannot decode free view HD.  However, a free view HD box will decode basic freeview.

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