FM in Mono

[WayMikeC]

1) Is it true that an FM signal's strength is better in mono vs. stereo? If so, how much better?

2) Is it possible to make an FM music station sound GOOD in mono? I've noticed that when I drive into an area where we don't come in good, and the radio blends our audio into mono, that we sound pretty crappy.

 

[MattB]

> 1) Is it true that an FM signal's strength is better in
> mono vs. stereo? If so, how much better?
>
> 2) Is it possible to make an FM music station sound GOOD in
> mono? I've noticed that when I drive into an area where we
> don't come in good, and the radio blends our audio into
> mono, that we sound pretty crappy.
>

1. Could be around 30% better. Less bandwith since no need for the stero multiplex, so less bandwith with the same power = more distance. (without getting too technical).

2. Yes, definatley can sound good. If you are ever in Long Island, listen to WLNG 92.1 FM (you can also listen online at www.wlng.com). <P ID="signature">______________

</P>

 

[westlife]

> 1. Could be around 30% better. Less bandwith since no need
> for the stero multiplex, so less bandwith with the same
> power = more distance. (without getting too technical).

Even more importantly, on receivers with no stereo blending (such as many boom-boxes, walkmans, cheaper home stereo systems, etc.), there is up to a 23 dB improvement in signal-to-noise ratio between a weak FM stereo signal and a weak FM mono signal. That's a BIG difference! On radios with a manual mono/stereo switch, you can easily hear this for yourself by switching it to mono mode. Unfortunately many radios do not feature a manual mono/stereo switch, leaving you stuck with a noisy stereo signal, unless the station is broadcasting in mono to begin with.
<P ID="signature">______________

"This is the New York Emergency Broadcast System satellite channel. They took the crosstown bus."</P>

 

[SamBuca]

> 1) Is it true that an FM signal's strength is better in
> mono vs. stereo? If so, how much better?

I bet a decent radio could do FM mono down to like 0.003mV/m.

> 2) Is it possible to make an FM music station sound GOOD in
> mono? I've noticed that when I drive into an area where we
> don't come in good, and the radio blends our audio into
> mono, that we sound pretty crappy.

Once the radio loses the pilot, it doesn't know the signal needs to be demuxed for stereo...so it auto flips to mono. Of course your signal will sound crappy in that situation.<P ID="signature">______________
</P>

 

[tpt]

All true, but if you are considering car radio audience, I am not sure it makes that much difference. Most car radios blend to mono while still displaying the "stereo" light.

I can't hear stereo (only one working ear), so most of the time I can't hear the blend on stations where everything is in phase. Of course, if something is out of phase it leaps out at you.

Haven't seen any radios recently that will scan past mono signals, though, so mono or stereo won't make much difference.

 

[CW]

> Once the radio loses the pilot, it doesn't know the signal
> needs to be demuxed for stereo...so it auto flips to mono.
> Of course your signal will sound crappy in that situation.

Actually a lot of OEM car radios will blend to mono LONG before the pilot is lost...I have noticed that personally on my GM and Fords..the stereo indicator is still on but I am hearing the audio in mono...Aftermarket radios with a mono/stereo switch will allow you to hear the noise of the AM component of the stereo signal..Boomboxes/portables will usually stay in stereo longer than car radios...and you can REALLY hear the noise there on a weak stereo signal...
but a lot of them have a AM/FM/FM Stereo switch..
This will be one advantange that digital will have..it will be in stereo longer than analog...but when it starts to go, it will go! An analog signal would still be there in mono when the digital signal goes away.

 

[ssnake]

> 1. Could be around 30% better. Less bandwith since no need
> for the stero multiplex, so less bandwith with the same
> power = more distance. (without getting too technical).

The idea is that it should use the same bandwidth -- you recover the 9% modulation formerly used for the pilot, but 9% isn't too significant. One station I know has peak set to 94% due to ineptitude. Certainly it is better to keep the L-R signal small.

However, the idea that you *force* the receiver to mono does make a difference in fringe areas. With low signal strengths, the higher frequencies of the composite signal get even weaker (less energy there). So when you go to add the L-R to the L+R, it adds noise. If you could switch the receiver mono manually, you'd get the same benefit.

Note that if you send essentially mono audio out-of-phase -- L+R = 0%, L-R = 100% (of that signal) -- you have a *greatly* reduced coverage area. Any time it blends to mono, you get silence. Some folks don't pay enough attention to phasing...

 

[CW]

> > 1. Could be around 30% better. Less bandwith since no
> need
> > for the stero multiplex, so less bandwith with the same
> > power = more distance. (without getting too technical).
>
> The idea is that it should use the same bandwidth -- you
> recover the 9% modulation formerly used for the pilot, but
> 9% isn't too significant. One station I know has peak set
> to 94% due to ineptitude. Certainly it is better to keep
> the L-R signal small.

No, it would use less bandwidth(BW). Carson's rule says BW = 2x dev + 2x highest mod freq....if the mod freq is lower, the B/W is too......thus the 19kHz pilot AND the 38 kHz DSB L-R is gone....the max mod freq is lower, thus the BW is lower...Remember, the L-R also takes % of mod...so with that gone, you can increase the L+R more than 10%..more like 15% on average. Remember 100% modulation is 75kHz dev, NOT bandwidth..if the freq of mod changes, your dev can remain the same but the BW will change.

> However, the idea that you *force* the receiver to mono does
> make a difference in fringe areas. With low signal
> strengths, the higher frequencies of the composite signal
> get even weaker (less energy there). So when you go to add
> the L-R to the L+R, it adds noise. If you could switch the
> receiver mono manually, you'd get the same benefit.

L-R is also an AM subcarrier component...that adds noise! A TRUE mono station will have higher L+R audio....and better coverage than a stereo station even with the rcvr in the mono mode...audio is lower and your S/N is much lower.
Typically, a TRUE mono signal will have 23db SN better at fringe coverage vs a stereo signal.

> Note that if you send essentially mono audio out-of-phase --
> L+R = 0%, L-R = 100% (of that signal) -- you have a
> *greatly* reduced coverage area. Any time it blends to
> mono, you get silence. Some folks don't pay enough
> attention to phasing...

How true!! I have heard a few CC stations where they played a song in stereo but I was in the mono area (radio blended to mono) and sure enough, out of phase...it was funky to hear the audio pop in and out as I got into stereo coverage band then blended back to mono on the fringe!

 

[99-5jamz]

> > 1) Is it true that an FM signal's strength is better in
> > mono vs. stereo? If so, how much better?
> >
> > 2) Is it possible to make an FM music station sound GOOD
> in
> > mono? I've noticed that when I drive into an area where
> we
> > don't come in good, and the radio blends our audio into
> > mono, that we sound pretty crappy.
> >
>
> 1. Could be around 30% better. Less bandwith since no need
> for the stero multiplex, so less bandwith with the same
> power = more distance. (without getting too technical).
>
> 2. Yes, definatley can sound good. If you are ever in Long
> Island, listen to WLNG 92.1 FM (you can also listen online
> at www.wlng.com).
>

Another case. WTOP 107.7 Right outside of Washington DC is a FM Mono and I can Pick them up almost past Downtown Richmond. And If I hit the right spot in my back yard I can pick them up I stay in Northern NC Right on I-95 about 3 hours from the Washington Metro area. <P ID="signature">______________
[email protected]</P>

 

[semoochie]

> 1) Is it true that an FM signal's strength is better in
> mono vs. stereo? If so, how much better?
>
> 2) Is it possible to make an FM music station sound GOOD in
> mono? I've noticed that when I drive into an area where we
> don't come in good, and the radio blends our audio into
> mono, that we sound pretty crappy.
>
I once heard that an FM stereo signal was 18 times noisier than one in mono however the largest difference would be between a stereo broadcast and stereo receiver versus a monaural broadcast and monaural receiver.

 

[oldrover]

Translate this for an engineering idiot: let's say I'm looking at a radio-locator coverage map of an FM. How much additional coverage could I expect in switching to mono? Would it extend to the 50dbu level shown on their maps?

 

[littlejohn]

The bandwidth, if the radio is properly adjusted in each case will be the same. CW's reasoning is correct, but once we dump the pilot and the 38KHz signal, we are no longer modulating the carrier 100% e.g. we are no longer forcing it to swing 75KHz up or down on peaks. So, we turn the modulation up to 100%, creating a 75KHz swing for the mono signal alone. Whether we swing the frequency at a 38KHz rate or at a 15KHz rate, if we swing it 75KHz, we'll occupy 150KHz of bandwidth.
The 50dBu contour won't move... we've not increased the transmitter RF output, so the RF footprint is the same whether we broadcast in stereo or not. Subjectively, there will be a fairly substantial increase in coverage mono to stereo. What happens is, the noise floor reduces. Now everyone has a point beyond which the sound is too noisy, resulting in a change of station or turning the radio off. Each person's noise tolerance is different, hence this point is different for each person. BUT, in mono, that point uis substantially further from the transmitting antenna than it is in stereo. As someone pointed out above, it's quantifiable as a function of signal to noise, but remember, every person's threshold is different.
Bottom line is, turn the stereo off, you can hear it further away.

 

[oldrover]

Good info. Here's what I'm looking at: taking a class A FM to news/talk/local sports and going mono... IF that means that I can sell into towns in the station's 50dbu contour. If the signal is strong enough to pick up consistently, a little noise is fine.

 

[littlejohn]

Run it mono. It's worth the increase in coverage.

 

[Josh_Cols]

In Celina, Ohio there is a station WCSM 96.7 (www.wcsmradio.com) that is mono, and their signal seems to get out well for being a 3kw class A. WCSM is a full service station, that also plays a fair amount of AC music, with a primary rural audience, their demo is alot of farmers who listen in their pickup trucks and tractors, and alot of local business listens thru their mono PA's, so they probably can't justify updating their vintage equipment to support stereo. WCSM's cross town competition WKKI (www.wkki.net) is stereo, and more focused on music and not as full service as WCSM, they don't seem to come in as clear 30 miles away as WCSM does.

 

[oldrover]

There's a 3KW FM near me that broadcasts in mono... sports. It reaches very clearly into the 40dbu coverage area, giving the sales team a lot of extra ammo. Before their format change, they were schmutzy even in the projected 50dbu coverage area. That's why I'm interested in this.

 

[zach_morton]

It's not that it really "gets out further"...it just removes the multipath interference and other noise that the stereo pilot introduces to the signal that becomes apparent the further you get from the antenna. Power is power...stereo or mono, but it is cleaner at distance than stereo.

 

[CW]

It's not that it really "gets out further"...it just removes the multipath interference and other noise that the stereo pilot introduces to the signal that becomes apparent the further you get from the antenna. Power is power...stereo or mono, but it is cleaner at distance than stereo.

Zach?? You no longer in the mountains??
(and did you hear Margaret is no longer at the RN in Dallas?)

 

[Turntable]

Ok, How many times do we have to say this?

Simple, again... (again???)

By switching off your stereo gen (thereby eliminating the 19Khz pilot) you will be able to increase deviation and eliminate both multipath distortion and mono blending (because to the average receiver you will always be mono). The drawback is all those people who get frustrated when they don't see "ST" lit up on their crappy radios... (how many of those are there anyway)

The answer is, YES! A RESOUNDING YES!!!!!! you will gain more effective coverage in mono-- somewhere on the order of 20-25 % depending on the terrain surrounding your TX Plant.

Why don't you just try it and find out??

"You don't say..."
" (He didn't say...)"

Har Dee Har Har...

-A

 

[rrsounds]

Ok, How many times do we have to say this?

Simple, again... (again???)

By switching off your stereo gen (thereby eliminating the 19Khz pilot) you will be able to increase deviation and eliminate both multipath distortion and mono blending (because to the average receiver you will always be mono). The drawback is all those people who get frustrated when they don't see "ST" lit up on their crappy radios... (how many of those are there anyway)

The answer is, YES! A RESOUNDING YES!!!!!! you will gain more effective coverage in mono-- somewhere on the order of 20-25 % depending on the terrain surrounding your TX Plant.

Why don't you just try it and find out??

"You don't say..."
" (He didn't say...)"

Har Dee Har Har...

-A

Because the stereo subcarrier is interleaved with the L+R, you do not get a significant modulation advantage by going mono (just the 8-10% from having no pilot).

As Alan and others have said, stereo increases the noise in the fringe-area reception versus mono-only. But it's not because of what you're transmitting, it's because the receiver is picking up something that you are NOT transmitting: noise. The stereo pilot is telling the receiver to decode whatever is in the signal's composite spectrum from just above 19 kHz to just below 57 kHz, and in low signal areas that spectrum will include a LOT of noise.

In actuality, since the stereo system uses a suppressed carrier, when there is no L-R going in to the stereo generator **there is no subcarrier or sidebands!** Zero. Zip. Zilch. Nada.
For all intents, the composite signal is then identical to a mono-only signal, with the obvious addition of the pilot. But you will still see peaks up to 100% with this mono signal, proving that because of interleaving, stereo does not significantly add to the peak level or compromise the mono signal's peak level.

Next time you have access to a spectrum analyzer, you can see this in action. Looking at the stereo composite spectrum, whenever an announcer speaks (a mono signal with no background music), the content in the subcarrier region will disappear entirely. At least it will if everything is adjusted properly!!

As for the station contemplating mono-only operation... You could always rig up something to detect L-R in your outgoing program signal to ramp on or off the stereo pilot as appropriate, keeping the clients happy should they require stereo for their commercials, but maintaining decent S/N during the bulk of the programming.

Keep in mind, however, that if you are using RBDS, you may need to keep the pilot on at all times for sync reference.

Kind Regards and Happy Holidays!

David Reaves
TransLanTech Sound, LLC
Home of the Award-Winning "Ariane Sequel Digital Audio Leveler"