FM in Mono

[Ted Russell]

And one more thing to consider:

If you are operating a small community station that puts out 1kw or less, the effective percieved clarity on the fringes will have less of an effective area then when this is done on a big signal station with 6kw or 10kw or more. In other words, the bigger your signal is to begin with, the more fringe listenability you have to gain by operating in mono. So the big 25kw commercial FM across town will be able to get a few more listenable miles out of their signal, but the local High School putting out 125 watts is only going to get a couple of extra blocks, maybe 1/8 mile or so.

Most modern car radios have built in blending circuitry that blend the audio to mono well before the stereo noise is going to be perceived. Also a lot will even automatically reduce bandwidth on the extreme fringes to reduce noise as well.

 

[PhilJSmith67]

Assuming the station doesn't have their L/R phasing screwed up, the mono FM signal has all of the fidelity of both the L+R channels.

If phasing is jacked, even the stereo signal will sound "wrong" -- I've heard this on stations before, where you turn up the volume but hear little to no bass, even though the entire car is vibrating -- move the balance to left OR right and the "uncancelled" bass punches through. I'm not sure how stations, big and small, manage to screw that up. It's aurally noticeable, and on a scope showing phase relationship, the line is either flipped the wrong way, or is an oval. Duh.

As far as receivers go, they're all over the place in the way they respond to a missing pilot. I have an older Pioneer stereo that when the Tuning Mode is set to "auto scan" it SKIPS anything without a solid pilot, regardless of signal strength. When WBEZ Chicago used to air NPR talk shows in mono, WBEZ was mysteriously "missing!" I have another stereo that sounds excellent in mono or stereo, except for the fact that weaker stations maintain full separation but are very noisy in stereo.

My Blaupunkt in the car (DigiCeiver) has a DSP that is apparently programmed to blend back to mono. It sounds good with or without a stereo carrier, with a strong or weak signal.

I used to have a Zenith Apollo home stereo that had poor audio quality, but much improved selectivity, when the stereo pilot was weak or missing. That receiver narrowed the IF bandwidth according to the pilot's strength, or when the switch was set to FM-Mono.

 

[zach_morton]

Actually, I've seen several instances where the audio plant is wired entirely wrong. Usually comes from engineers who became engineers right out of high school and thought they could wire their stereo's at home qualified them. I found it here...nobody's "fault"...just wired the building too quick.

And yes CW...moved from the mountains. Got tired of all the damn snow!

 

[zach_morton]

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[DudeFan]

One other thing you can do to clear up the perceived noise and smooth the receiver blend to mono is to adjust the stereo separation, if your audio processor or stereo generator will allow it.

The Omnia's will let you do it and the effect is pretty noticeable on the fringe.

 

[Tom Wells]

I've added phase-reverse switches to speaker outputs on amps before, so I could simply flip a switch to listen to a station which had gotten the phase wrong somehow. I'm sure I've heard at least one LP "greatest hits of the ?'s" where the dub to disc had somhow been out-of-phase on one song...how THAT happened, I can't guess.

 

[PhilJSmith67]

One other thing you can do to clear up the perceived noise and smooth the receiver blend to mono is to adjust the stereo separation, if your audio processor or stereo generator will allow it.

The Omnia's will let you do it and the effect is pretty noticeable on the fringe.

Another thing you can try is reducing the power for 19kHz pilot, while maintaining the normal power level for the 38kHz DSBSC signal. This will cause recievers to prematurely revert or blend to mono in fringe areas, where they would normally produce hiss from the stereo subcarrier. Your fringe listeners end up getting the cleaner mono signal, without penalizing your nearby listeners who can get a clean stereo signal.

Don't lower the pilot too much though, especially if your main channel isn't REALLY brick-walled at 15kHz, or is maxed on modulation. A really nasty thing can happen where the weaker pilot gets "slightly trounced" from the audio, producing a lovely stereoscopic crackle with some receivers.

 

[Turntable]

II'm sure I've heard at least one LP "greatest hits of the ?'s" where the dub to disc had somhow been out-of-phase on one song...how THAT happened, I can't guess.

Easy. Either the pinouts on the phono cartridge were reversed, or the tonearm wires were wired out-of-phase on the board.

The same is possible with cart machines where the cables got reversed on the barrier strip on the board. Always remember: when a balanced connection is used, XLR pin 1 is shield, pin 2 is hot, and pin 3 is negative.

-A

 

[littlejohn]

A cd occasionally shows up out of phase. We've an all digital plant, there isn't any way to get a reversal save in the proc amps. Even worse, however, is to get one side 90 degrees out. It's obviously out of phase, so you reverse a channel and it's >still< out of phase. You could manage this if you set the jumpers just wrong on the Moseley 6000 series STLs. Which I managed while converting one from compsitie to discrete. Took forever to figure out what was wrong, and almost as long to find how and fix.