FM Stereo (the truth?)

Yes, it it a "reversible" process. Good separation depends on good phase lock of the demodulating switching signal to the 19kHz pilot.

-michael

Parallel computing for Apple II's

FM

claim,

of

heard

After doing some more digging, I've found that there is a (somewhat cheesy IMO) method to GENERATE the baseband signal by "chopping" the L and R channels at 38kHz. You then have to filter all the harmonic trash above 53kHz that is created by this "alternative" process, and finally insert a 19kHz pilot tone. You can then feed this to the equivalent of a wideband (allowing freqs to 53kHz) mono FM transmitter to send out a reasonable facsimile of a "stereo" signal.

I could find no such method being used to demodulate (detect, discriminate or whatever you wish to call it) an FM stereo signal. After what is required to create the baseband signal using the method above, I am not sure that you can run this procedure in reverse and achieve any significant channel separation. Can anyone elaborate on this?

consider the L-R carrier it's an AM signal

when it is at it's peak the total signal is (L+R)+(L-R) = 2L

but when it is in the trough the total is (L+R)-(L-R) = 2R

simple eh?

Nyquist limits apply, yes, that's why FM quality is what it is.

Bye. Jasen

I think it is true. Is not the base band signal L + R + (L - R)sinwt where w is 2*Pi*38KHz? Now, when the sine is positive you have to a first approximation L + R + L - R = 2L and when the sine is negative you have L + R - L + R = 2R So if you switch phase coherently at 38KHz you should get 2L and 2R separated assuming the relative amplitudes are correct. The fact the the sine is not a square wave for switching but only the first harmonic of the square wave produces an ampitude term that affects the depth of separation. For this to work you have to have the full base band including the 38KHz sub carrier side bands. Obviously it won't work on the L + R audio signal alone. Bob

haven't

an FM

(speakers,

signal

claim,

of

heard

where w

Now see, I don't know. I guess that's the problem with being a hobbiest. ;-)

have L +

get 2L

fact the

harmonic of

including

audio

So the 19kHz pilot tone doesn't need to be removed before the "chopping" begins? I have a decent grasp of heterodyning signals and the results, but this 38kHz chopping and its consequences are very new to me. So what you are saying is that the receiver can employ the same chopping technique (as long as it is phase locked to the pilot tone) and the L-R sidebands centered around 38kHz will demodulate and restore the separation? Man that's just too weird. ;-) If that's the case, I guess I owe philth an apology. I still haven't found any web sites that demonstrate this technique being used within a receiver. I guess the key piece that leads me to believe this may be true (that it's a reversable process) is that I've seen several references to the importance of the phase of the pilot tone. In SSB reception, the phase of the injected carrier is not important for recovering the information, so there must be some reason for its importance in FM stereo reception. Being a ham, I never got much chance to experiment with FM stereo. :-)

I don't know the details of broadcast FM, but you can consider a chopper to be multiplication by a square wave, as opposed to the more typical multiplication by a sine wave we think of in heterodyning. So you would get the desired spectrum, plus images at 3x, 5x, etc with decreasing amplitude. The trick would be removing the unwanted images, or using some clever arrangement to move them out of the desired band.

As a historical note, a laboratory instrument called a "lock-in amplifier" used choppers for synchronous detection of extremely faint signals. A reference signal provided the chopper drive, and the output of the chopper was then simply low-pass filtered to give a DC value proportional to the amount of reference-frequency component in the input signal. They typically had 2 quadrature channels, since one channel can't resolve a quadrature input, and they were known to give false responses to harmonics. But a lot of real-world lab work didn't have problems with that. The typical application I recall was with a light stimulus chopped by a slotted wheel to provide pulsed stimulus to the experiment, and the lock-in used to determine the response (be it photodetector, physiological, etc.) The main virtue of lock-ins was that they could resolve signals buried in copious amount of noise. The same process is now done digitally with synchronous averaging, where the entire response waveform can be recovered.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

You say you've done ham radio. Look at it this way. You've already demodulated the FM, so you've got your L+R on the "baseband", the

19KHz "pilot tone", and L-R DSBSC modulated on a 38KHz subcarrier. (double-sideband, suppressed carrier).

Well, imagine you've reinjected the 38KHz subcarrier in this DSB signal, and you've got straight AM, just like an AM radio signal, but at 38 KHZ.

Now look at the instantaneous value of that. 38,000 times a second, it goes negative or positive, by more or less, which is the amplitude of the L-R signal.

But that's if you filter it out from the baseband L+R and are looking at it all by itself. You don't do that - you chop just what's come out of the FM discriminator, baseband, modulated subcarrier and all, (you might want to suppress the 19KHz subcarrier), and if you did a time-domain display and took the amplitude at each excursion of the subcarrier, you'd find that their sum (which the demodulated FM is, after all) is 2R on one half-cycle and 2L on the other.

Hope This Helps! Rich

Reinserted carrier phase doesn't matter in the case of SSB, but it does in the case of DSB, which is what the Zenith-GE system uses to modulate the (L-R) component. It's probably 30 years since I had to do the math. I shall have to go dig it out, if I can still find it.

I've spent a little time during a very busy couple of weeks trying to find definitive references to Zenith-GE, and come up with not much beyond superficial descriptions. Anybody know if it was ever patented? Searching classification 179 produces quite a few "interesting" ideas ;-). The nearest I can find is the Crosby patent #2851532, but that's not the whole nine yards, using as it does, a frequency-modulated subcarrier.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)