Service linking

Now that an FM/DAB+ receiver is legally standard in every new EU car, listeners can listen to their favorite local/regional broadcaster even in a large part of the respective region. Switching from DAB+ to FM even happens automatically.

But sometimes it doesn’t … ?

A few weeks ago, reports reached us that on a regional DAB MUX in Belgium and on some models of cars, there are problems with automatic switching between FM and DAB+: the car radio switches to a radio broadcaster on VRT’s DAB MUX, usually Radio 1, Radio2 or Studio Brussel. In some car models, it was even impossible to listen to the local/regional broadcaster via FM if the FM frequency was set to a “preset”.

Because we wanted to know the ins and outs of it and, above all, we were looking for a solution to the problem, we compared the FIC data of the small scale DAB+ pilot project in Flanders (Belgium) with the FIC data of the MUX of VRT (Broadcast Partners) and those of Norkring. The cause was soon found.

Apparently, some manufacturers’ car radio software does something wrong when a different code is used for the “SId” than the “PI code” (FM-RDS) used for the FM transmitter. But also adding the FM frequency(s) in the FIC data is something we don’t see in some pilot projects.

Theoretically, you could use different SId and PI and rectify this by explicitly linking both by transmitting additional information via DAB+.

Unfortunately, some car radios do not respond to this explicit link info. So overriding the implicit linking (SId simply equal to the PI) does not work. The phenomenon appears to be related to the price range of the radio (ditto car).

If one wants to make it work on ALL car radios, one should undoubtedly opt for the implicit linking: choose the same code for the DAB SId as the PI code for the FM-RDS.

By the way, using explicit linking is only used if the two stations (DAB+ and FM) broadcast different program schemes for several hours a day. Then, by switching off explicit linking, one can decouple both stations (see ETSI document). This is a situation that never occurs in the local/regional radio world.

Implicit linking is thus the best way to link your DAB+ broadcaster to your FM-RDS broadcaster, see also ETSI TS 103 176:

Our suspicion is that some radios work with limited FIC functionality and do not interpret certain instructions.

We therefore recommend checking (or having checked) whether the “PI code” is used as “SId” and that also the FM frequency(s) are added in the FIC data. The sysop or operator of the MUX should be able to answer that. In this way, you can ensure that the listener can continue to listen to his/her favorite broadcaster without any problem regardless of which car he/she is driving.

Important to remember:

  • Those using an SId code different from their FM PI code are guaranteed to have switching problems with certain brands of cars
  • Those who do not forward alternative FM frequencies via their DAB+ MUX will possibly have problems switching between DAB+ and FM

A check of both parameters is therefore recommended to achieve maximum listener ship.

An example of what is broadcast in the FIC data can be found here.

The term “Service linking” is used for the action itself. For the system of switching, the term “service following” is used. The system is described in ETSI TS 103 176, chapter A.3.4 (Receiver Behaviour).

Finally, a word about audio volumes.
The audio level of many FM stations is often FAR too high. Achieving a similar volume with DAB+ is technically IMPOSSIBLE without causing substantial distortion of the digital audio signal. This is because one has a finite number of bits at one’s disposal that CANNOT cope with these FM audio levels. This distortion will result in particularly poor audio quality in a digital system such as DAB+.

So quality starts with lowering the FM audio level so that DAB+ can work qualitatively. Many countries have an agreement on this between “Broadcasters”. You can read how to do it in “Headroom in digital audio“.