For many years, I have had radios in my cars, which have been equipped with
the Radio Data System, or RDS. I recall when I had my first one, trying to
find out how this worked, but coming up against a brick wall. Since then, I
haven't taken that much notice of it, until I was stuck in traffic today.

I don't have a problem understanding the basic concept of traffic
announcements, station identifiers, music genres and content etc being
displayed. I also don't have any problem with understanding where and how
this data stream is transmitted by the station, and how the radio receives,
decodes, and displays the text. In fact now, there is plenty of stuff on the
'net about that. What I don't have an understanding of, and what I can't
seem to find any write-up on, is how the automatic transmitter hopping takes
place, and how it happens so seamlessly.

Today, stuck in that traffic, at one point, I was located at what must have
been a 'marginal' spot between two transmitters carrying the same
transmission. The radio was flicking back and forth between the two
transmitters, located several megs apart, but there was not so much as a
squeak when it did it, let alone a break in reception, or any sign of the
inevitable time difference that you pretty well must get when two
transmitters, geographically well separated from each other, are being fed
from the same studio source.

Presumably, each transmitter must carry information in its RDS stream about
the frequencies of other 'close by' transmitters carrying the same
programme. The radio must then retune itself to one of these 'alternatives'
based on some criteria such as analogue signal strength or bit error rate on
the RDS signal, to see if it can gain an improvement. But when it does this,
where is the music now coming from, and how does the seamless 'switch' to
the new (better) transmitter take place ? I am guessing that the demodulated
audio must have been digitised, and dropped in a bit bucket running some
distance behind the 'real' transmission, and then D-A'd back to audio. But
it would still have to be pretty cleverly done, to 'dovetail' the new
digitised data from the alternate transmitter, into the existing data, such
that you didn't hear the switch. I used to repair a lot of car radios a few
years back, and I don't recall ever having had a problem with this aspect of
the system.

So, anyone know properly how it's done, or know of any authoritive works on
the subject ?

Arfa