Ignore what the display says, ie BBC R4, that's irrelevant as far as what decisions the receiver makes. What's important is the RDS PI code for FM, a four digit hexadecimal code, and the SID for DAB.
For Radio 4 the FM RDS PI is C204, on DAB the SID is C224.
As you can see they don't match.
Classic FM's do, they are C2A1 on both systems.
In my experience (limited) DAB radios do switch between the two formats on Classic.
I'm sure that a DAB service can point towards an alternative FM service with extra codes (similar to EON on RDS) but whether the broadcasters or manufacturers support that I don't know.
PI/SID codes can be obtained from this spreadsheet:-
I have got a Ford dab radio which kept going from FM to dab and back again which was very annoying, I thought the radio was faulty so I took it back to the supplier who altered a setting , now it is fine. I cannot see the point of it switching. Also radio 2 switched so the BBC must use this daft system.
Ford and VW DAB radios successfully service link from DAB to FM on the BBC national stations.
When the BBC say "ODA 147 (open data application) within the FM RDS" this is really for service linking the other way around, from FM to DAB. In this case the BBC transmit RDS data on FM which identifies the DAB station.
For DAB to FM service linking, of more relevance is ETSI EN 300 401 which can be found here.
8.1.11.3 The SId field shall be used to signal the RDS PI code (see EN 62106 [5]) or a "dummy" code using the same format.
8.1.15 The Service linking information feature provides service linking information for use when services carry the same primary service component (hard link) or when the primary service components are related (soft link). The feature is encoded in extension 6 of FIG type 0 (FIG 0/6).
So a broadcaster may put the PI code in the SId but they don't have to since the service linking information goes in FIG 0/6.
I would guess that the BMW radio is not extracting the service linking information from FIG 0/6.
Despite the slight delay between FM and DAB I can still see the appeal for services switching if one of the two is suddenly dropping out on longer journeys.
Comments
Thanks for this. I did wonder why it works on commercial but not the BBC!
It certainly works ok with Classic, Smooth and GEM.
If I try to tune to any of these on FM, they swap to DAB within about 2 seconds!
Come on BBC, you're letting the side down. You should be leading.
When the BBC say "ODA 147 (open data application) within the FM RDS" this is really for service linking the other way around, from FM to DAB. In this case the BBC transmit RDS data on FM which identifies the DAB station.
For DAB to FM service linking, of more relevance is ETSI EN 300 401 which can be found here.
8.1.11.3 The SId field shall be used to signal the RDS PI code (see EN 62106 [5]) or a "dummy" code using the same format.
8.1.15 The Service linking information feature provides service linking information for use when services carry the same primary service component (hard link) or when the primary service components are related (soft link). The feature is encoded in extension 6 of FIG type 0 (FIG 0/6).
So a broadcaster may put the PI code in the SId but they don't have to since the service linking information goes in FIG 0/6.
I would guess that the BMW radio is not extracting the service linking information from FIG 0/6.
Despite the slight delay between FM and DAB I can still see the appeal for services switching if one of the two is suddenly dropping out on longer journeys.