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From: Arfa Daily on 12 Feb 2010 21:18
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


From: Jeff Liebermann on 12 Feb 2010 23:21
On Sat, 13 Feb 2010 02:18:56 -0000, "Arfa Daily"
<arfa.daily(a)ntlworld.com> wrote:

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

I have no idea how it handles multiple RDS data streams. My guess(tm)
is one at a time based on either the strongest signal, or the lowest
BER (bit error rate). If it really did switch back and forth between
stations, it's probably because the two data streams were time synced
to an atomic or GPS based time standard and are therefore effectively
simulcasting the exact same data. However, I'm guessing.

Here's the UK version of the specification:
<http://www.rds.org.uk/download/R05_036_1.pdf>
More:
<http://www.nprlabs.org/research/pad/RDSPlus_Description.pdf>

--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: Arfa Daily on 13 Feb 2010 08:55

"Jeff Liebermann" <jeffl(a)cruzio.com> wrote in message
news:ls9cn51a3p37nhguubpnq0icelhgk1h8ja(a)4ax.com...
> On Sat, 13 Feb 2010 02:18:56 -0000, "Arfa Daily"
> <arfa.daily(a)ntlworld.com> wrote:
>
>>So, anyone know properly how it's done, or know of any authoritive works
>>on
>>the subject ?
>
> I have no idea how it handles multiple RDS data streams. My guess(tm)
> is one at a time based on either the strongest signal, or the lowest
> BER (bit error rate). If it really did switch back and forth between
> stations, it's probably because the two data streams were time synced
> to an atomic or GPS based time standard and are therefore effectively
> simulcasting the exact same data. However, I'm guessing.
>
> Here's the UK version of the specification:
> <http://www.rds.org.uk/download/R05_036_1.pdf>
> More:
> <http://www.nprlabs.org/research/pad/RDSPlus_Description.pdf>
>
> --
> Jeff Liebermann jeffl(a)cruzio.com
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

Some interesting content in those documents Jeff, but still no mention of
the RDS having control of this frequency-hopping behaviour, or how the radio
carries it out 'seamlessly'. Bear in mind that we are not talking digital
radio here, but standard local area analogue FM transmissions, with the
digital RDS data piggy-backed on the transmission, in a bit of unused
channel width.

This is not any kind of new feature of the system, either. I first came
across it probably twenty years ago when I was a field engineer for a
company, and driving from one end of the country, to the other. I always
used to listen to UK BBC Radio 2. I could drive perhaps 300 miles
up-country, passing through probably eight or ten transmitter service areas,
and not once would I have to retune the radio in order to keep listening to
that same station. If you kept half an eye on the radio's display though,
the frequency readout at the corner of the RDS info display, would keep
changing throughout the journey. Curiously clever ...

Arfa


From: Jeff Liebermann on 13 Feb 2010 16:07
On Sat, 13 Feb 2010 09:53:36 -0500, Meat Plow wrote:

>On Fri, 12 Feb 2010 20:21:38 -0800, Jeff Liebermann
><jeffl(a)cruzio.com>wrote:
>
>>
>>On Sat, 13 Feb 2010 02:18:56 -0000, "Arfa Daily"
>><arfa.daily(a)ntlworld.com> wrote:
>>
>>>So, anyone know properly how it's done, or know of any authoritive works on
>>>the subject ?
>>
>>I have no idea how it handles multiple RDS data streams
>
>Then why did you answer?

Half an answer is better than no answer.
Besides, I was curious and did the necessary Googling.
Why did you complain?

--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: Michael A. Terrell on 13 Feb 2010 16:19

Arfa Daily wrote:
>
> "Jeff Liebermann" <jeffl(a)cruzio.com> wrote in message
> news:ls9cn51a3p37nhguubpnq0icelhgk1h8ja(a)4ax.com...
> > On Sat, 13 Feb 2010 02:18:56 -0000, "Arfa Daily"
> > <arfa.daily(a)ntlworld.com> wrote:
> >
> >>So, anyone know properly how it's done, or know of any authoritive works
> >>on
> >>the subject ?
> >
> > I have no idea how it handles multiple RDS data streams. My guess(tm)
> > is one at a time based on either the strongest signal, or the lowest
> > BER (bit error rate). If it really did switch back and forth between
> > stations, it's probably because the two data streams were time synced
> > to an atomic or GPS based time standard and are therefore effectively
> > simulcasting the exact same data. However, I'm guessing.
> >
> > Here's the UK version of the specification:
> > <http://www.rds.org.uk/download/R05_036_1.pdf>
> > More:
> > <http://www.nprlabs.org/research/pad/RDSPlus_Description.pdf>
> >
> > --
> > Jeff Liebermann jeffl(a)cruzio.com
> > 150 Felker St #D http://www.LearnByDestroying.com
> > Santa Cruz CA 95060 http://802.11junk.com
> > Skype: JeffLiebermann AE6KS 831-336-2558
>
> Some interesting content in those documents Jeff, but still no mention of
> the RDS having control of this frequency-hopping behaviour, or how the radio
> carries it out 'seamlessly'. Bear in mind that we are not talking digital
> radio here, but standard local area analogue FM transmissions, with the
> digital RDS data piggy-backed on the transmission, in a bit of unused
> channel width.
>
> This is not any kind of new feature of the system, either. I first came
> across it probably twenty years ago when I was a field engineer for a
> company, and driving from one end of the country, to the other. I always
> used to listen to UK BBC Radio 2. I could drive perhaps 300 miles
> up-country, passing through probably eight or ten transmitter service areas,
> and not once would I have to retune the radio in order to keep listening to
> that same station. If you kept half an eye on the radio's display though,
> the frequency readout at the corner of the RDS info display, would keep
> changing throughout the journey. Curiously clever ...


Telemetry systems often use two (or more) receivers and a combiner to
select the best signal(s). It wouldn't be hard to do the same in your
radio. Then the RDS selects the best signal between the outputs. The
distance to the transmitters isn't enough to cause a time delay you
could detect in simple audio feeds.

The speed of light is 299,792,458 m/s or 299,792.458 km/s. Do you
have any idea how far you were from either transmitter?


--
Greed is the root of all eBay.
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