Bit OT. Radio Data System or "RDS" ...

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:

More:

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

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?

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?

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:

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:

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.

Thanks Meat (and also Jeff). I'll follow up on these links tomorrow, and see if any enlighten me further ...

Arfa

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

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:

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:

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.

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

The feeds to transmitters are probably a combination of wired and microwave depending on the location.

Hi!

In the US, and concerning recent GM vehicles sold here in particular, the RDS implementation does support station switching of conventional analog broadcasts.

The radios offered by Ford here in the US also do this if they support RDS, as do ones offered by Chrysler in their vehicles.

RDS is, of course, pitifully underused here in the US. And while I have some radios that can receive it, little more than the ability to display the callsign of the station--and somtimes its format--is used. As such, I've never seen the automatic station switch take place--the only confirmation of its existence is in the vehicle owner's manual or the RDS spec itself.

I suspect the station hopping ability requires two tuners, and perhaps a sort of buffering circuit to assure that there is no delay in the audio output. How the switching criteria is met I have no idea--maybe some sort of "apparent" signal strength meter is observed?

The radio that's currently in my truck doesn't support RDS, and in a way I'm glad. RDS seemingly supports "emergency" announcements that will turn the radio back on and make it so that it is impossible to turn it off while the alert plays.

I'm sorry, but it's *MY* radio and it will be off if I want it to be!

William

William - see my further comments on how it's done (now that I've found the info on it courtesy of Meat !) at the bottom of the thread.

The traffic announcement facility can be switched on or off at will. I too don't like the station that I'm listening to being 'hijacked' by a traffic announcement, so I simply have that feature turned off. Shame you don't see more of the system's use your side of the pond. It really is quite good, and the frequency hopping behaviour is very useful, particularly when you want to stay tuned to a national station, over a long journey.

Arfa

My RDS radio has several selections as to what and how and when it interacts. But I gave up terrestrial radio and the hair-pulling aggravation of advertisements for Sirius sat radio back in 2006.

Sirius seems to have advertising:

or at least "advertising partners". I don't currently have (or need) a Sirius or XM subscription. Is there much advertising on Sirius?

Hi!

I'll certainly check that out. I'd like to know more about how the system works with regard to the automatic station switch.

GM and Chrysler don't let you shut those alerts off, at least not in the vehicles that I have.

Yeah, I would certainly welcome more of its use here in the US...although I'm sorry to say that I've kind of given up on radio as a whole. It's hard to find what I'd like to hear anyway...and ever since I hooked up an iPod set to shuffle play in the truck, I've found it hard to look back.

No commercials, no station fade, and any song I want to hear...

As it is, I found that nearly all the commercial RF modulators available for iPods are seriously deficient in some way or another, so I went for a better approach:

But that's another thread entirely...

William

Same here. I like classical and although there is a good local classical FM station KBOQ, I'm at the fringe area of reception. So, I listen to MP3's on my iPod Touch and other assorted MP3 players.

Some are better than others, but nothing beats a direct connection. If your radio supports it, controlling the iPod Touch from the radio display and controls is very convenient.

Bah. Who's afraid of topic drift? Your next upgrade will probably be a car computah. Instead of the radio in the dash, and LCD touchscreen display running a PC under the seat or in the trunk. The computah internet access, videos, TV, games, navigation, ODB monitor, rear view camera, etc. See:

for examples of what others have done while waiting for the manufacturers to do it all wrong. Some enterprising vendors have machined adapters and frames for installing LCD panels on DIN rail mounts. The TV/FM is usually handled by a USB dongle, which is usually installed at the base of the antenna, to minimize cable losses, and connected to the computah with a USB cable. A USB hub in the trunk also handles the backup camera, NAS storage, secondary Wi-Fi bridge, etc.