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FM stereo?

Discussion in 'Electronic Basics' started by MRW, Jan 13, 2006.

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  1. MRW

    MRW Guest

    Hi,

    Let me get get this straight. If I have an FM stereo signal, say at
    101.5 MHZ, then I should have a signal tone at 101.5190 MHz and a
    sub-carrier signal at 101.538 MHz?

    Thanks!
     
  2. Phil Allison

    Phil Allison Guest

    "MRW" gmail.com
    ** We have fair warning......


    ** No.

    The 19 kHz stereo pilot tone is a frequency modulation.


    ** No again.

    Modulations of an FM carrier are about the centre frequency.

    They are not separate frequencies.



    .......... Phil
     
  3. MRW

    MRW Guest

    Thanks, Phil!

    So the FM stereo signal will still get decoded through the demodulator?
    So my demodulated signal should be in the range of 1Hz u to 50kHz and
    then my stereo receiver should be able to separate the channel signal
    centered around 38kHz?
     
  4. Phil Allison

    Phil Allison Guest

    "MRW"
    ** Wow.


    ** Stereo FM is a bit complicated.

    In reality, the original Lch and Rch signals are separately *analogue
    sampled * and used to modulate the VHF carrier frequency *sequentially* at
    the rate of 38,000 times per second.

    The exact 19kHz "pilot tone" is sent to make it possible for the
    receiver's decoder to lock onto this sequential sampling and then switch the
    recovered audio to Left and Right audio outputs at the exactly right
    moments.

    The upshot is that a stereo FM receiver must have at least 53 kHz of
    recovered modulation bandwidth - requiring a 200 kHz radio spectrum
    bandwidth.

    Care to ask a more precise Q ?



    ......... Phil
     
  5. MRW

    MRW Guest

    Thanks again, Phil!

    Do you know what it means when a datasheet mentions that the RF signals
    are fed into two balanced inputs? I'm not quite sure. As you can tell,
    I"m a newbie.
     
  6. Fred Abse

    Fred Abse Guest

    'Fraid you're wrong, Phil. The +38KHz stereo (L-R) signal is a suppressed
    carrier, double sideband signal, not part of the FM. You can actually see
    it on a spectrum analyzer hooked up to an antenna, quite separate from the
    cluster of FM sidebands.

    That's why the pilot tone - to locally regenerate a demodulation carrier.
    Unlike SSB, DSB needs the reinserted carrier to be phase locked.
     
  7. Oh man, you're way off base. The L+R signal is sent on the main carrier
    just like any mono FM broadcast. The L-R signal is a supressed carrier
    double sideband signal FM modulated onto a 38kHz subcarrier. The 19kHz
    pilot tone is doubled in frequency inside the receiver and then used to
    demodulate the subcarrier signal to retrieve the L-R signal. This is
    then mixed with the L+R signal to seperate the L and R information.
    Actually there are subcarriers commonly used at 67kHz and 91kHz pretty
    much using up the rest of the channel allocation. Sometimes data is
    sent on a 71kHz subcarrier. If you have a shortwave receiver that does
    FM and tunes below 100kHz, you can take the base band audio from the
    discriminator output and feed it into the antenna jack and tune it to
    67kHz and see for yourself. It's called SCA and I figured you'd at
    least heard of it before.
     
  8. Phil Allison

    Phil Allison Guest

    "Fred Abse"

    ** No - YOU are totally wrong here.


    ** Complete nonsense !!!!

    It arrives at part of the recovered output from the FM detector.

    Ergo, it was part of the *modulation* signal.


    ** Makes it a high frequency modulation - not a separate signal.



    ** FM stereo receivers made since the mid 70s use *switching* decoders.

    The LM1310 and LM1800 and Asian equivalents.

    Better go look them up.




    .......... Phil
     
  9. Phil Allison

    Phil Allison Guest

    "Anthony Fremont"

    = a net stalking, wanking, bloody IDIOT !!!!


    ** WRONG !!!

    See: http://www.smoke.com.au/~ic/mpx.html


    " In addition a difference signal (Left - Right) is generated and then used
    to
    modulate a 38 Khz subcarrier using Double sideband suppressed carrier
    (DSBSC) modulation.
    This is an AM modulation of the subcarrier. "
    -------------------------------------------------

    " This is the 'Classic' method. An easier to build method simply switches
    between the left and right channels at a 38 Khz rate. The mathematics of
    this produce a signal the same as the classic method but there is less
    likelyhood of differences between channels sneaking in. "


    ** FM stereo receivers made since the mid 1970s use *switching* decoders.

    The LM1310 and LM1800 and several Asian equivalents.

    Better go look them up and learn something.



    FUCKHEAD !!!!!!!



    ......... Phil
     
  10. Phil Allison

    Phil Allison Guest

    "MRW"

    ** One would have to work out exactly what was meant by knowing the
    context.

    Can you post a link to the sheet?




    ........... Phil
     
  11. You're right on it being an AM process to create the L-R sidebands using
    a 38kHz carrier which is then suppressed, but the results of that
    process is FM modulated onto the main carrier just like the L+R stuff.
    Learn something new every day I guess. Never before heard that just
    switching back and forth at 38kHz would give the same results. I'm
    still not convinced on that as the datasheets I've seen so far (for the
    devices you mentioned) seem to work in the so-called "classic" way. I'd
    like to see something a little less vague that "simply switches between
    the left and right channels". What, precisely, is being switched?
     
  12. Phil Allison

    Phil Allison Guest

    "Anthony Fremont"
    = a net stalking, wanking, bloody IDIOT !!!!



    ** The L+R, 19kHz tone and the DSBSC signals are simply added together to
    form a composite signal and then used to frequency modulate the
    transmitter's carrier.

    Same goes for any additional supersonic signals for extra stuff - just
    added into the modulation signal.



    ** The signal coming out of the FM detector is switched ( ie directed) to L
    and R outputs alternately.

    Go do you own Google search for the details - fuckhead.




    ........ Phil
     
  13. I understand that, they just use a ~100kHz wide audio signal to FM the
    main carrier.
    That doesn't sound like it would do anything usefull at all. How will
    that make the L-R information relocate itself down to human audible
    frequencies from the 38kHz carrier? It's Seems to me that you'd at
    least have to demodulate the L-R stuff to convert it back to its
    original form and then mix that with the L+R stuff. I presume that the
    switching is phase locked to the 19kHz pilot tone which you'd have to
    filter off as well. If that's all the case, then it sure doesn't sound
    all that much easier.
    Good luck on that anger management.
     
  14. Phil Allison

    Phil Allison Guest

    "Anthony Fremont"

    = a vile, net stalking, public wanking, autistic bloody IDIOT !!!!



    ** Bad luck - you dumb as shit, criminal FUCKHEAD.

    Go do your own Google search for the details.

    Plenty there.

    FUCKWIT !!



    ......... Phil
     
  15. You really should try to grow up a little phil.
    I have. The only thing I've seen so far that agrees with you (about the
    rapid switching of the baseband signal) is the one link that you
    provided. I don't suppose you could supply something more technically
    detailed could you? Like I said in the stuff that you so cleverly
    snipped away, AFAICT there is no way that simply switching the output of
    the discriminator rapidly will magically transpose the freqeuncy shifted
    L-R stuff down to recover the stereo channels. It, surely, must be much
    more complex than you are inferring.
    That's not what I'm finding after scads of searching.
     
  16. Fred & Anthony are correct...

    From the LM1800 test data:

    "Note 2: The stereo input signal is made by summing 123 mVrms LEFT or
    RIGHT modulated signal with 25 mVrms of 19 kHz pilot tone, measuring all
    voltages with an average responding meter calibrated in rms. The resulting
    waveform is about 800 mVp-p."

    This explains it quite well.

    http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part21/page1.html

    It probably doesn't help using the term "multiplexing" in a
    slightly confusing way.
     
  17. Bob Myers

    Bob Myers Guest

    Consider how DSBSC is recovered in the first place, the 19 kHz pilot
    tone's role in that - and then realize that "sampling" a signal at 38 kHz
    and amplitude modulation of a fixed-amplitude 38 kHz carrier are to
    a great degree exactly the same thing. That should make both the
    encoding and decoding processes much clearer for all concerned.

    There seems to be a huge argument going on here about two apparently
    different things which really do not differ at all. "Sampling left and
    right
    channels alternately at 38 kHz" results in PRECISELY the same signal
    as the original "L+R at baseband, plus L-R DSBSC around 38 kHz" -
    they're just essentially two ways of doing the same thing. Either way,
    you take the result and use it (with a 19 kHz pilot tone added in) to FM
    the VHF carrier. At the receiving end, the output of the FM demodulator
    is as it always was - L+R at baseband, and L-R DSBSC up around
    38 kHz. Sampling THAT whole shebang at 38 kHz equates to "demodulate
    the DSCSC L-R signal and combine the recovered signal with L+R to get
    L and R separately." (Note that sampling the total signal at only 38 kHz
    is actually UNDERsampling it, per Nyquist - which you are counting on,
    since the aliasing of the various components is what's going to
    bring those together and properly combine them, mathematically,
    and so result in L and R separated and at the correct audible frequencies.)

    Bob M.
     
  18. Phil Allison

    Phil Allison Guest

    "Bob Myers"


    ** A small point to keep in mind is that the FM stereo signal is at no time
    actually " sampled " in the way that digital audio samples the waveform.

    The Nyquist theorem strictly applies to a stream of *instantaneous* samples
    of a waveform, ie fixed voltage samples taken at exact moments in time.

    FM stereo involves a different method known as "analogue sampling" - where
    a signal is switched at some rate onto a link for a small increments of time
    during which it can and does vary in voltage.



    ......... Phil
     
  19. frequencies.)

    Thanks Bob, I think I'm getting it now. I had never before seen this
    described as a plausable method to encode/decode stereo. I still can't
    help but think that this method would introduce allot of intermod type
    distortion.
     
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