[cma-l] FW: MiniDAB

[Alex Gray, Two Lochs Radio]

No Glyn (and Ian), no ‘old thinking’ going on, quite the reverse, so maybe you were as you say missing my points! 

 

I’m familiar with the workings of DAB multiplexes and SFNs, but I apologise for any confusion I caused you by my very careless use of the word ‘channels’ (in the sense of programme channels, not spectrum channels), when I should have written ‘programme services’.

 

I too was envisaging an SFN with perhaps 3 or 4 relays for covering a large city.  As you say, a single DAB+ multiplex (ie one using AAC+) could provide 30 services, but that would imply services averaging 32kbps each, which I would regard as adequate quality for general service, which is why I suggested 24 channels. I think 48kbps should be the minimum target except perhaps for mono or speech-only services, but that is to an extent a subjective matter of course, and it also depends on the quality/price of the encoders used.

 

You mention the common misconception that an SFN requires all transmitters to contain the same MUX content. Certainly that is the simplest and most conventional arrangement, and what would in almost all circumstances be the practice, but in appropriate situations some regional variation is possible within careful design (which Ofcom has the people to do) . Practical experiments carried out some years ago in Germany showed that it is a possible even within an SFN to have some local variation in programmes service, subject to careful design and suitable topography. 

 

That is where my comment about interference arose – if alternative audio services are put in the same time slot on different relays there are significant regions of mutual digital interference where there is no usable reception of that slot. But that was all just a possible extra worth investigating as a side issue in areas of particularly high demand, not something to get hung up on. 

 

The most obvious starting point would be a dedicated multiplex with 24 (or your 30) slots for community-based services across a wide area.

 

Alex

 

From: cma-l-bounces at mailman.commedia.org.uk <mailto:cma-l-bounces at mailman.commedia.org.uk>  [mailto:cma-l-bounces at mailman.commedia.org.uk] On Behalf Of Associated Broadcast Consultants
Sent: 09 March 2016 14:25
To: The Community Media Association Discussion List <cma-l at mailman.commedia.org.uk <mailto:cma-l at mailman.commedia.org.uk> >
Subject: Re: [cma-l] MiniDAB

 

I don't understand these comments about number of channels and interference.  I wonder if there is some old analogue/FM thinking going on?

 

DAB has the capability to transmit on the same channel without causing interference (in fact delivering a significant gain in the overlap area).  It's called SFN - but it requires all the transmitters to carry the same Mux content.

 

So for large metropolitan areas you could have one mux covering the centre, and three or four relays in the suburbs - all on the same channel.  Use AAC+ coding and it could easily deliver 30 services with pretty good audio quality.   The alternative is not worth considering - 5 transmitters on separate channels would deliver far too much capacity (150+ programme services), and 5 separate muxes on same channel would deliver far too much interference rendering it unusable.

 

Unless I'm missing your points...?

 

Glyn

 



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