Wasted DAB Multiplex Capacity

Number of Stereo Stations That Can Fit Into a DAB Multiplex

The DAB broadcasters often claim that they have to use low bit rates for their stereo stations in order to provide a wide range of stations, but in reality, this claim doesn't stand up to scrutiny. The following table shows how many stereo stations can be fit onto a DAB multiplex at different bit rates:

In the UK, 98% of all stereo stations on DAB use a bit rate of 128kbps MPEG Layer 2 (MP2), which has been officially classified as providing 'annoying' audio quality, and a bit rate of 160kbps 

DAB in the UK does need more spectrum, and an international frequency planning conference will start in May 2004, with a second session in 2005 or 2006 that is likely to allocate new spectrum for DAB in the UK. Once this happens then more DAB multiplexes can be started, this will be the only chance for DAB to be provided with decent audio quality, or stay as being a low audio quality medium for its lifetime, because once radio stations are started, then the broadcasters will not want to remove them.

Commercial Multiplexes

Since finding out that the BBC have deliberately been transmitting below the possible capacity that DAB allows at error protection level 3 I have calculated the situation for the commercial multiplexes and what I've found is that there is significant spare capacity being wasted. (these values are based on bit rates listed on the Wohnort UK DAB page)

National Multiplex: Digital 1

Updated 27/7/04 Data from Wohnort site

Note that there is a temporary station on Digital One called D1 Temp which just transmits bird-song. This is one of the "slots" that was left unfilled when the Bloomberg and ITN News stations withdrew from DAB in 2002, and remain unfilled to the present day (Feb 2004).

As you can see above, at present, all of the music stations could have their bit rates increased to a minimum of 160kbps and Classic FM could return to its previous bit rate of 192kbps.

So, because of the 864 CU limit that always applies no matter what, the Digital 1 multiplex is wasting 69 CU of capacity. This would allow Virgin and Classic FM to transmit at 192kbps and one of the 128kbps stations to transmit at 160kbps. Any CUs that are not transmitted are pure wastage because the DAB data rate is constant so on some of the OFDM subcarriers no bits will be transmitted at all.

Regional DAB Multiplexes

Updated 27/7/04 Data from Wohnort site (all MXR multiplexes without DNN and reduced bandwidth for data services)

Local DAB Multiplexes

 

Area	Multiplex Operator	Actual Bit Rates of Services	CU	Total CU	Possible Bit Rates of Services	Possible Number of 160k+ Stations	Actual Number of 160k+ Stations
Aberdeen	Switch Digital	5 x 128k 1 x 112k 1 x 96k 2 x 64k	5 x 96CU 1 x 84CU 1 x 70CU 2 x 48CU	730	3 x 160k 4 x 128k 2 x 64k	3	0
Ayr	Score Digital	7 x 128k 1 x 64k	7 x 96CU 1 x 48CU	720	7 x 160k 1 x 64k	7	0
Bath & Wilts	Now Digital	6 x 128k 1 x 96k	6 x 96CU 1 x 70CU	646	7 x 160k	7	0
Birmingham	CE Digital	8 x 128k 2 x 64k	8 x 96CU 2 x 48CU	864	Full	0	0
Borders
Bournemouth	Now Digital	7 x 128k 1 x 96k 1 x 64k	7 x 96CU 1 x 70CU 1 x 48CU	790	4 x 160k 3 x 128k 1 x 80k 1 x 64k	4	0
Bradford/Hudd's'd	TWG	6 x 128k 2 x 64k	6 x 96CU 2 x 48CU	528	5 x 192k 1 x 128k 1 x 80k	5	0
Bristol & Bath	NowDigital	7 x 128k 1 x 96k 1 x 64k 1 x 96k (data)	7 x 96CU 1 x 70CU 1 x 48CU 1 x 70CU (data)	860	4 x 160k 3 x 128k 2 x 64k 1 x 20k (data)	4	0
Cambridge	Now Digital
Cardiff & Newport	Capital Digital	2 x 160k 4 x 128k	2 x 116CU 4 x 96CU	616	6 x 192k	6	2
Cardigan Bay
Carlisle
Central Lancashire	Emap	8 x 128k 2 x 64k	8 x 96CU 2 x 48CU	864	Full	0	0
Colwyn Bay
Cornwall	Now Digital
Coventry	NowDigital	6 x 128k 2 x 80k 1 x 64k	6 x 96CU 2 x 58CU 1 x 48CU	740	6 x 160k 2 x 80k 1 x 64k	6	0
Dumfries
Dundee/Perth	Score Digital	2 x 128k (PL2) 5 x 128k (PL3) 1 x 64k	2 x 116CU 5 x 96CU 1 x 48CU	760	7 x 160k 1 x 64k	7	0
East Lincolnshire
East Sussex
Edinburgh	Score Digital	7 x 128k 1 x 64k 1 x 32k (data)	7 x 96CU 1 x 48CU 1 x 24CU (data)	744	6 x 160k 1 x 128k 1 x 64k 1 x 32k (data)	6	0
Exeter/Torbay	Now Digital	5 x 128k 1 x 96k 1 x 64k	5 x 96CU 1 x 70CU 1 x 48CU	598	5 x 192k 1 x 160k 1 x 64k	6	0
Glasgow	Score Digital	8 x 128k 1 x 64k 1 x 64k (data)	8 x 96CU 1 x 48CU 1 x 48CU (data)	864	2 x 160k 6 x 128k 1 x 64k 1 x 8k (data)	2	0
Great Yarmouth
Heads of the Valleys
Humberside	Emap	7 x 128k 2 x 96k 1 x 64k	7 x 96CU 2 x 70CU 1 x 48CU	860	2 x 160k 5 x 128k 3 x 64k	2	0
Inverness	Score Digital	4 x 128k	4 x 96 CU	384	4 x 320k	4	0
Kent	Capital Digital	6 x 128k 1 x 32k (data)	6 x 96CU 1 x 24CU (data)	600	6 x 192k 1 x 24CU (data)	6	0
Kings Lynn
Leeds	Emap	7 x 128k 2 x 96k 1 x 64k	7 x 96CU 2 x 70CU 1 x 48CU	860	2 x 160k 5 x 128k 3 x 64k	2	0
Leicester	Now Digital	7 x 128k 2 x 96k	7 x 96CU 2 x 70CU	812	4 x 160k 3 x 128k 1 x 80k 1 x 64k	4	0
Liverpool	Emap	8 x 128k 2 x 64k	8 x 96CU 2 x 48CU	864	Full	0	0
London 1	CE Digital	1 x 160k 5 x 128k 1 x 96k 4 x 64k	1 x 116CU 5 x 96CU 1 x 70CU 4 x 48CU	858	2 x 160k 4 x 128k 5 x 64k	2	1
London 2	Switch Digital	6 x 128k 1 x 112k 1 x 96k 2 x 64k 1 x 48k	6 x 96CU 1 x 84CU 1 x 70CU 2 x 48CU 1 x 35CU	861	3 x 160k 3 x 128k 4 x 64k 1 x 48k	3	0
London 3	DRg	1 x 128k (PL3) 4 x 128k (PL4) 2 x 96k (PL4) 2 x 80k (PL3) 1 x 80k (PL4) 1 x 64k (PL3) 1 x 64k (PL4)	1 x 96CU 4 x 84CU 2 x 58CU 2 x 58CU 1 x 52CU 1 x 48CU 1 x 42CU	806	5 x 160k 5 x 80k 2 x 64k	5	0
Luton
Manchester	CE Digital	1 x 160k 7 x 128k	1 x 116CU 7 x 96CU	788	4 x 160k 4 x 128k	4	1
Montgomery
Morecambe Bay
Northern Ireland	Score Digital	8 x 128k 1 x 96k	8 x 96CU 1 x 70CU	838	2 x 160k 6 x 128k 1 x 64k	2	0
Norwich		7 x 128k  1 x 96k (PL4) 1 x 80k 1 x 64k	7 x 96CU 1 x 58CU 1 x 58CU 1 x 48CU	836	2 x 160k  5 x 128k 1 x 80k (PL4) 2 x 64k	2	0
Nottingham	Now Digital	7 x 128k 1 x 96k	7 x 96CU 1 x 70CU	742	7 x 160k 1 x 64k	7	0
Peterborough	Now Digital	5 x 128k (PL2) 1 x 128k (PL3) 1 x 96k (PL2) 1 x 64k (PL2)	5 x 116CU 1 x 96CU 1 x 84CU 1 x 58CU	818	7 x 160k 1 x 64k	7	0
Plymouth
Reading & Basingstoke	Now Digital
Scarborough
Southend/Chelmsford	Now Digital	6 x 128k 1 x 96k 1 x 64k	6 x 96CU 1 x 70CU 1 x 48CU	694	7 x 160k 1 x 64k	7	0
South Hampshire	Capital Digital	7 x 128k 2 x 96k	7 x 96CU 2 x 70CU	812	4 x 160k 3 x 128k 1 x 80k 1 x 64k	4	0
South Yorkshire	Emap	7 x 128k 1 x 96k 2 x 64k	7 x 96CU 1 x 70CU 2 x 48CU	838	2 x 160k 5 x 128k 3 x 64k	2	0
Stirling
Stoke-on-Trent	TWG Emap	5 x 128k	5 x 96CU	480	5 x 224k	5	0
Sussex Coast	Capital Digital	6 x 128k 1 x 96k	6 x 96CU 1 x 70CU	646	2 x 192k 5 x 160k	7	0
Swansea	TWG Emap	6 x 128k	6 x 96CU	576	6 x 190k	6	0
Swindon & West Wilts	Now Digital	7 x 128k 1 x 96k 1 x 80k 1 x 64k	7 x 96CU 1 x 70CU 1 x 58CU 1 x 48CU	848	2 x 160k 5 x 128k 3 x 64k	2	0
Teesside	Emap	8 x 128k 2 x 64k	8 x 96CU 2 x 48CU	864	Full	0	0
Tyne & Wear	Emap	7 x 128k 2 x 96k 1 x 64k	7 x 96CU 2 x 70CU 1 x 48CU	860	2 x 160k 5 x 128k 3 x 64k	2	0
West Cumbria
Weymouth & Dorchester
Wolverhampton, Shrewsbury & Telford	NowDigital	7 x 128k 2 x 80k 1 x 64k	7 x 96CU 2 x 58CU 1 x 48CU	836	2 x 160k 5 x 128k 3 x 64k	2	0

Updated 27/7/04 Data from Wohnort site

So, out of a total of 183 stations that could be using 160kbps or above (including 3 BBC stations and the Digital One stations), we currently have only 6 stations using 160kbps or above!!

The Possible Mux Configuration column shows that there is lots of unused capacity on most of the local commercial multiplexes, yet they just won't increase the bit rates to allow us to hear DAB at decent audio quality. If this isn't Rip-Off Britain then I don't know what is?

I think that the multiplex operators are conditioning our ears to 128kbps so that this becomes the de facto standard for DAB music stations and so that they can cram as many stations into each multiplex as possible and unless we complain then I'm sure we will end up with dire audio quality because the regulator (the Radio Authority) aren't bothered about audio quality. They have now merged with the Radiocommunications Agency (plus other agencies) to form Ofcom, and I wouldn't be surprised if it turned out to be another totally toothless watchdog being ordered about by the commercial operators.

Most Frequently-Occurring Local & Regional DAB Stations

Updated 12/9/04 Data from Wohnort site

Bit Rate versus Capacity Units (CU)

DAB multiplex capacity is measured in capacity units (CU), not in bit rate. DAB multiplexes always have a capacity of 864 CU. The relationship between how many capacity units a certain bit rate requires depends on the protection level (PL), and the protection level shows how much protection (or redundancy) is added to the data to allow bits received in error to be corrected at the receiver. Strangely, the higher the protection level the lower the protection for the data! That is, protection level 4 (PL4) provides a lower level of protection than protection level 3 (PL3).

However, the error correction is not perfect, and when too many errors are made the typical "bubbling mud" sound can be heard.

The following table shows the relationship between bit rate and all of the possible protection levels PL2, PL3 and PL4, which are in use in the UK. Virtually all DAB services in the UK use PL3. The only exceptions to this are a few services on the Dundee, DRG London, Norwich and Peterborough local DAB multiplexes (see Local DAB Multiplexes table at the bottom of this page for details).

 

Bit Rate kbps	PL2 CU	PL3 CU	PL4 CU
32	29	24	21
48	42	35	29
56	52	42	35
64	58	48	42
80	70	58	52
96	84	70	58
112	104	84	70
128	116	96	84
160	140	116	104
192	168	140	116
224	208	168	140
256	232	192	168
320	280	-	208
384	-	280	-