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FM Stereo (the truth?)

Discussion in 'Electronic Basics' started by Anthony Fremont, Jan 14, 2006.

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  1. Can anyone verify that stuff that phil stated? For those that haven't
    seen it, he claims that rapidly switching the baseband output from an FM
    discriminator rapidly (38kHz) between the L and R channels (speakers,
    preamp in, or whatever floats your boat) will separate the L+R signal
    into its L and R components. Phil provided one link making that claim,
    but it is severely lacking in any technical explanation of that type of
    decoding. I've searched and searched and I can't find anyone else
    claiming the same thing. Seriously, this is no troll, I've never heard
    of this before. Is it really true? Can anyone elaborate on how/why
    that would work? Wouldn't Nyquist (sp?) limits apply here?
  2. 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
  3. Jasen Betts

    Jasen Betts Guest

    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.

  4. Guest

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


    Parallel computing for Apple II's
  5. Bob Eldred

    Bob Eldred Guest

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

    Bob Masta Guest

    On Sun, 15 Jan 2006 16:30:53 GMT, "Anthony Fremont"

    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

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  8. Rich Grise

    Rich Grise Guest

    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!
  9. Fred Abse

    Fred Abse Guest

    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.
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