Re: In praise of 'steam radio'

On 3 Feb 2009, at 13:38, David Farbey wrote:

> What I'd like to know (and I apologise for still being UK-centric,  
> and for perhaps drifting a little off-topic) is why the broadcast I  
> receive on my super-duper ultra-sleek new DAB radio is a couple of  
> seconds behind the broadcast I receive on my clapped-out old FM  
> transistor radio?


The signals should arrive at roughly the same time as FM (although  
there's a bit more equipment in between studio and transmitter with  
DAB, so they'll be transmitted a bit later than FM, by which I mean  
microseconds).

The problem is because they're using a digital encoding with  
compression (like MP3 but a bit older and dumber). So, your radio set  
receives the signal, and then has to decode it. The processing  
required to do the decoding is quite a bit of work, so they need to  
buffer a tiny bit of audio in case they run into a particularly tricky  
bit which might take longer to decode than the audio lasts... so, it  
takes a second or two to start spitting uncompressed analogue audio  
out the other end. (The chips they use to do it are quite cheap,  
therefore quite puny.)

The problem is more pronounced with 'Pause Plus' and similar radios  
which let you 'pause' live radio. What they do is store the digital  
signal in RAM, and they typically have a 15 second (or thereabouts)  
long buffer which they have to fill first, so you get a really delayed  
signal with them.

HTH,

Matt

-- 
   Matt Patterson | Design & Code
   <[email protected]> | http://www.reprocessed.org/


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