Bit OT. Radio Data System or "RDS" ... [Repair]

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From: Jeff Liebermann on 13 Feb 2010 16:30

On Sat, 13 Feb 2010 13:55:13 -0000, "Arfa Daily"
<arfa.daily(a)ntlworld.com> wrote:

>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'.

Sorry. I thought it might be buried under there, somewhere.

I don't understand how it works completely, but this looks like a
useful clue:
<http://news.bbc.co.uk/2/hi/science/nature/592972.stm>
By simulating the RDS traffic information data, the RDS data will keep
the radio on the pirates channel, rather than return it to whatever
channel was previously used.
When a local traffic announcement is being broadcast, a
signal is sent and the car radio switches to it. When it ends,
RDS takes the listener back to the station or tape or CD they
were originally listening to.

They appear to have used the published RDS standards to
make devices that constantly transmit the signal to switch,
thereby grabbing listeners.

Clever. If RDS is turned on in the radio, then it will search for a
station that is transmitting RDS and stay there until the traffic info
is finished. My guess(tm) is that if it can't hear an RDS data
stream, or gets a high error rate, it may go looking for a better RDS
signal, which explains the frequency switching.

More:
<http://www.newscientist.com/article/mg16522200.800-do-not-adjust-your-set----we-have-assumed-radio-control.html>

We have some RDS data stations in the US, but I have no experience
using them. Besides, my car radio is ancient and feature deficient.

--
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 20:32

From: Arfa Daily on 13 Feb 2010 20:51

"Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote in message
news:4B771744.8E703CD3(a)earthlink.net...
>
> 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?
>
>
> --

Thanks for your insights Michael. Understood and agreed on speed of radio
waves etc, but in cases like this, that isn't the be all and end all of
delay paths in the transmissions, especially in these days of digital signal
manipulation. I'm sure that the original studio signal passes through many
different phone switches and trunk distribution systems on its way to the
various different transmitter sites from which it is being 'simultaneously'
broadcast. I'm also sure that the number of switches and networks involved
in each path varies enormously, and that many, if not most of these will
involve digitisation, with the attendant processing overheads. These can add
up to significant delays in the audio. As a 'for instance', in the town
where I live, there are three readily received TV transmitters, all around
about the same distance from the town. In my lounge, I tend to use the
transmitter that is to the south of the town, but in my kitchen, I use the
one that is to the east. If I have both TV sets receiving the same station,
and stand in my hallway where I can hear them both, there is a very
significant difference - probably as much as 1 second - in the audio. As
both transmitters are around 30 miles from me, path length is obviously not
a contributing factor in this.

Also understood and agreed on the principles of telemetry receivers etc. I
agree that two receivers within the car radio would be one possible way for
it to work, but I am not sure that this would be financially practical in a
bog standard car radio, would it ? I might see if I can lay hands on a
schematic set for a Ford (Sony) radio, and see if there is any evidence of
dual receivers.

Arfa

From: Jeff Liebermann on 13 Feb 2010 21:16

On Sun, 14 Feb 2010 01:51:33 -0000, "Arfa Daily"
<arfa.daily(a)ntlworld.com> wrote:

>If I have both TV sets receiving the same station,
>and stand in my hallway where I can hear them both, there is a very
>significant difference - probably as much as 1 second - in the audio. As
>both transmitters are around 30 miles from me, path length is obviously not
>a contributing factor in this.

Nice testing. I'm not surprised that the TV signals vary signigantly
in delays. However, the RDS data signals might be synchronized to a
common clock such as GPS. There's no requirement that it be synced
with the TV for FM audio.

Simulcast is commonly done with paging systems and has been around for
at least 15 years:
<http://www.google.com/patents/about?id=eR0dAAAAEBAJ&dq=5010317>
I don't know how the transmitters are arranged in the UK, but if they
can be GPS synced, then it's possible to have more than one
transmitting exactly the same data at exactly the same time on exactly
the same frequency. It can also be done with voice. We have a local
ham radio system of about 6 xmiters doing exactly that. Equalizing
the delays without GPS is tricky, but not impossible:
<http://www.wb6ece.org>
I have no idea if the UK RDS system uses simulcast, but if it does,
then the audio delay is not a good indication of the RDS data delays.

--
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 14 Feb 2010 12:26

<Meat Plow> wrote in message news:3h3453.9ah.17.4(a)news.alt.net...
> 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 ?
>>
>>Arfa
>
> http://www.rds.org.uk/rds98/rds98.htm
>
> http://www.radio-now.co.uk/faq1.htm
>
> http://www.trafficmaster.co.uk/__assets/asset194.pdf
>
> http://lmgtfy.com/?q=how+does+RDS+work

OK. First two of those links explain a feature of the system called "AF"
which is Alternative Frequency. Seems that with modern radios, the RDS is
integrated into a chip, along with a PLL controlled FM receiver, where the
PLL has a very fast lockup time of just a few mS. Data regarding frequencies
of other nearby transmitters carrying the same programme material, is
embedded into the RDS data stream. When the system determines that the
quality of the received FM transmission has dropped below a predetermined
level (it doesn't say whether this is by analogue signal strength, or BER of
the RDS data stream), the receiver PLL is rapidly shifted to one of the
alternative frequencies, to check if that betters the situation. Whilst this
is taking place, the receiver audio is momentarily muted.

So, it would seem that the key to not being able to hear the frequency
switch taking place, is a very quick-locking PLL, that allows the audio
muting period to be kept too short to hear.

Even given that, the broadcast network must be totally free from adjacent
area delays in the audio, for you not to be able to hear any 'jump' in the
content. I wonder if the feeds to the transmitters are now all digital over
the phone system, allowing for easy 'tweaking out' of any delays betewwen
feeds to adjacent transmitters ?

Thanks Meat. Your links have found me the answer to something that has
bugged me off and on, for years ! :-)

Arfa

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