Any benefit to keeping FM stereo pilot on w/ mono programming?

[Doctor_Technical]

So basically what you are saying then is IF you fool the box into thinking it's got the 19khz in but don't actually put the 19khz on the air, all is good then huh?  So use a junk stereo generator in the input of the box and just feed the RDS in like normal, right?

OKC, you don't even have to do that. I've used a lot of different RDS encoders* and have yet to find one that does not have the ability to auto-detect the presence of pilot and subsequently internally create the 57 KHz carrier when pilot is absent.

While I bring this up to encourage mono stations to use RDS (yes, OKC, just plug it in and have fun), I also want to note that it is in the NRSC-4 RDS standards -- and just good engineering practice -- to use a source of pilot to sync the RDS encoder when in stereo operation.

*from the manual of the unit my company (and several large radio companies with a "C" in their name) buys by the pallet-full:

"FAQ #8: If my FM station is Mono, is the Audemat-Aztec's encoder able to generate the 57 kHz subcarrier without the reference to the 19kHz stereo pilot?
» Answer: Yes of course! Audemat-Aztec's equipments switch automatically to an internal reference if no valid synchronization signal is present (no reference or reference out of 19 kHz +/- 2Hz range)."

 

[boiseengineer]

While an 8400 was out for repair, ran with a backup 8100. (Output 2 on the 8400 is set to output the 19 KHz.)

Forgot to T off the composite to the RDS encoder so the RDS wasn't synced to the 19 kHz. No problems on most radios except it totally destroyed the reception on a late model Clarion car stereo. A lot of L-R distortion that blasted you. Put the T in to the encoder for the 19 KHz reference and the radio worked again!

Any ideas?

 

[Doctor_Technical]

Any ideas?

Sure. Just install that Clarion in a 2009 Lexus IS F with Dynamic Radar Cruise Control, send the whole thing down to me and I'll be glad to take a look. (a hat-tip to Car Talk for that bit)

Seriously, I'd be curious how the output of the RDS encoder changed between the internal or external 19 kHz reference in terms of level and spectral purity. Maybe the internal source was noisy or off-freq? Perhaps it was somehow squirting out a large 19 or pure 57 kHz component -- I've mistakenly double-injected pilot and/or composite by having the encoder in loop-through vs side-chain, and the effects were similar to what you described.

RDS encoders are not all the same when it comes to quality of output; I've seen older units have some crud outside the normal RDS passband (+/- 2.4 kHz from 57 kHz).

And who knows what Clarion did or did not design in their radio. Just in looking at factory car radios I've seen some odd stuff. Good catch to fix it.

 

[boiseengineer]

The only car receiver I've not had "blending" on was the Pioneer Super Tuner. Had mono and AFC buttons on it. Should have removed it from that car before selling it.

They're Inovonics 711 or 712 (don't remember right now). Injected into the SUB (or is it MUX?) on the 606 C so the only thing changed was if it got it's 19 KHz reference.

Weird things happened when we first got them. Set up with HyperTerm then with an Inovonics 510. Surprised to find that all 4 of 'em were telling radios that there were a dozen or more AF - Alternate Frequencies to choose from! Most with frequencies like 95.4, 100.4, 85.3. And these didn't show up when queried in the terminal. Had to do a factory reset.

 

[maineengineer]

There are only psychological, listener-type benefits to leaving the stereo pilot on with mono programming. Almost unbelievably, I've found that listeners for the most part won't notice that the music is in mono if the stereo light is lit, and won't notice even drastically reduced frequency response if it is present, but will immediately call the radio station if the light suddenly goes out ("do you know you're not in stereo anymore?). For example, if you lose your composite stereo STL and temporarily set up a mono hop via Marti RPU, you can inject just enough 19 kHz. (even from a sine wave audio generator) along with it into the exciter to light the stereo light, and the phones simply will not ring. Don't ask me how I know.

 

[rrsounds]

So basically what you are saying then is IF you fool the box into thinking it's got the 19khz in but don't actually put the 19khz on the air, all is good then huh? So use a junk stereo generator in the input of the box and just feed the RDS in like normal, right?

My experience is that when the stereo pilot is present, a typical receiver's stereo circuitry is activated. If there is only noise in the spectral range of the where the subcarrier would be, then that noise will be 'decoded' to stereo.

Long story short: stereo pilot = potentially noisier received signal.

Kind Regards,
David

 

[Chuck]

So basically what you are saying then is IF you fool the box into thinking it's got the 19khz in but don't actually put the 19khz on the air, all is good then huh? So use a junk stereo generator in the input of the box and just feed the RDS in like normal, right?

My experience is that when the stereo pilot is present, a typical receiver's stereo circuitry is activated. If there is only noise in the spectral range of the where the subcarrier would be, then that noise will be 'decoded' to stereo.

Long story short: stereo pilot = potentially noisier received signal.

Kind Regards,
David

True, except most car radios blend to mono very early on. Most people are listening to mono, most of the time. That doesn't always help with boom boxes or home stereos though. Some blend automatically, others need to be manually switched to mono. Then, there are tons of people who listen on mono radios to begin with. Most cheap radios are mono. They will never know the difference.

As the operator of a "signal challenged" FM station, I've had more complaints when the stereo light didn't come on, than from people who thought the signal was noisy. YMMV

 

[techie2]

If I'm not mistaken, Nick, what you're asking is if you get the same perceived increase in reception distance by using the stereo / mono switch on your radio vs the station switching off the stereo pilot.

In my experience, it depends on the radio. The stereo setting next to my computer, which switched to mono, cleans the signal right up. Believe it or not, I've had at least one radio which, when you switched to mono, simply COMBINED THE (noisy) STEREO CHANNELS! I don't know who thought THAT was a helpful design... but it's NOT!

In my car, I have no stereo / mono switch, meaning I'm at the mercy of the station... meaning the 3 local mono programming FMs all spit, slur, and in some cases become too hissy to comfortably listen to... all so the "ST" light will come on...

Most modern car radios (and others as well) blend to mono when the signal goes below a pre-determined level. That tends to make the mono-stereo question a moot point. If you want to run RDS, you will need a 19KHz pilot. I've experimented both ways with our signal challenged LPFM. As far as I can tell, our listeners prefer seeing a stereo light. Those that are close enough really do get stereo. The rest get mono, but seem happy. YMMV

I didn't think about it before but it does make sense that when there's nothing on the L-R subcarrier all that will do is add noise. Guess that explains why when our LPFM is running high school sports the noise is more noticeable. I think it's a combination of mono content with the pilot on and the limited bandwidth on an analog POTS feed or especially the tinny cell phone audio playing havoc with the multiband processing in the Omnia. I don't notice it as much when we are doing a game on the Marti -- I suppose that's because of the greater audio bandwidth on the remote feed. Maybe I should scheme up something that hits a trigger script on the Omnia to go mono when we are on a sports remote.

 

[tshanks]

I've got an idea... how about auto-switching to mono and back? It'd work without having to tag the programming in automation or tag the jock (*staples reminder to forehead of jock, who still doesn't remember*). So what do we need to do this? I'm thinking a little microcontroller box that has L-R as an input, plus a demodulator that gives you L-R as an output (I've got that), and a contact closure output to the stereo generator (/ audio processor / transmitter controller) to tell it to switch.

If it can see 50 dB down and switch off after two minutes of L-R silence, on after ten seconds of L-R presence (counted using presence of L-R over threshold in each of the five 2-second periods)... Maybe this'll work.



OK, if I make this, who wants one?

Howmuch'llya give me? (ok, that's a businessey thing, so don't answer actual offers on-board, send a PM)

 

[littlejohn]

Call up Catel (the cable TV equipment provider) and buy a non-duplicate switcher. It will do what you need. I used one back in The Day when we had an FM - FM simulcast and wanted to be able to show Angelo (the FCC Engineer for Region 4) that we were in fact monitoring the air signal in real time off of both stations.