Can it be the case that the same DAB radio station is broadcast on two or more different ensembles? Or is each radio station totally unique within all ensemble collections available for a region? Can it happen an ensemble contains a DAB station in southern UK but not in northern UK (e.g. regional differences)?
Peter kindly provides two examples for this question:
“DAB station MYDAB found on ensemble 11B but also on 11C.”
“DAB station MYDABsouth found on ensemble 11B in northern UK but on ensemble 11C in southern UK?”
Peter also mentions how it was normal for FM radio stations to have alternative frequencies. The AF feature in car radios would ensure that the listener would continue to hear the station as the first frequency was lost. He asks if DAB has a similar feature.
Ensembles and multiplexes
Peter raises some interesting questions. It’s worth explaining briefly how DAB transmissions and ensembles work.
Digital radio offers more choice thanks to the use of multiplexing. This is the process of combing multiple data streams – in this case radio stations – into a single channel. The same principle applies to digital television where there are now dozens of terrestrial services instead of just four or five.
A group of radio stations is known as an ensemble. The terms multiplex and ensemble are sometimes used interchangeably.
In the UK we have three national multiplexes (or ensembles): BBC National DAB, Digital One and Sound Digital. The first carries only BBC services and the latter two carry only commercial radio stations. The line-up of stations on these multiplexes is the same across the UK.
Local digital radio multiplexes have been established for areas like London, Manchester, Norfolk and Northern Ireland. These multiplexes tend to carry local radio stations, such as BBC Radio Norfolk or Cool FM. Local multiplexes can also carry non-local radio stations like Gold. An ensemble always has the same line-up of radio stations.
In most of the country your DAB radio will find national and local multiplexes, enabling you to find BBC Radio 4 alongside your local radio stations.
One station on multiple ensembles
Can it be the case that the same DAB radio station is broadcast on two or more different ensembles?
Yes, a single radio station can be broadcast on multiple ensembles. A radio station like Magic Soul is offered on nine local ensembles across the UK. There is no difference between these stations in each area and they all have the same service identifier (or SId) of C3D9. This also applies to Absolute Classic Rock with its SId of C7C9.
Or is each radio station totally unique within all ensemble collections available for a region?
This is true for some local radio stations on multiple ensembles. Capital North East is available on both the Teesside and Tyne & Wear multiplexes. However, the stations each have a unique service identifier. This could be because the station broadcasts different adverts or news bulletins in those two areas.
Can it happen an ensemble contains a DAB station in southern UK but not in northern UK (e.g. regional differences)?
Due to the single frequency network (see below) it’s not possible for ensembles to have different line-ups in regional areas. In the example you mentioned blocks 11B and 11C. These frequency blocks are re-used around the UK. For example, block 11B is used in Ayr, Bournemouth, Bradford and Huddersfield, Bristol, Cornwall, Inverness, Leicestershire, London, Tayside and Wolverhampton Shropshire.
As there is sufficient distance between these locations the unique ensembles in those areas won’t interfere with eachother, even though they all use the same frequency.
Single frequency network vs. AF
Some FM radio stations cover a fairly large area and require multiple transmitters. These transmitters can’t be on the same frequency where they might overlap, so a radio station like BBC Essex would use two frequencies. In the north of the county you might tune in to 103.5 FM and in the south you would listen on 95.3 FM.
If you were to tune in to BBC Essex on 103.5 FM, the car radio would understand from the RDS data that an alternative frequency is available. As long as the AF feature on the radio is enabled, the car radio would automatically switch to the alternative frequency when the original signal becomes too weak. If you were to drive from Colchester to Basildon this would negate the need to scan or manually change to the other frequency.
DAB digital radio uses a single frequency network – or SFN. This means that multiple transmitters use the same frequency, as long as they are carrying the same stations. The Essex DAB ensemble which carries BBC Essex uses 11 transmitters on the frequency 229.072 MHz. If you were to make the same journey in the county the DAB radio would not need to retune to each of the transmitters, due to the single frequency network.
In 2014 a test area was established to trial service following. An example given was a station which is available on local multiplexes in Swindon, Berkshire and London. The trial looked at how a receiver could retune to the ensemble with the strongest signal so that the station could be heard uninterrupted on a journey through those areas.