FM Stereo

Discussion in 'General Electronics Chat' started by AnalogKid, Jun 1, 2015.

  1. AnalogKid

    Thread Starter Distinguished Member

    Aug 1, 2013
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    HAPPY BIRTHDAY FM STEREO!!!
     
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  2. crutschow

    Expert

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    Ahh, a true Nerd birthday celebration. :cool:
     
  3. Hypatia's Protege

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    Aye! --- Attempting to 'wade through' the 'traditional' analog signal construction technique precipitates migraines to this day!:eek::D

    With nostalgic regards
    HP
     
  4. AnalogKid

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    It could have been worse. Thre were two systems competing for FCC approval. The RCA/Zenith system we have today has the AM subcarrier, but the (Bing) Crosby-backed system had an FM subcarrier. Better signal quality but less range. Think about the sideband equations for an FM subcarrier on an FM carrier...

    Massive amounts of under-the-table money over 5 years, RCA won. Their AM subcarrier is subject to all of the problems that led to the development of FM in the first place, including fading away into noise. That's why the STEREO light flickers out at the fringes while the mono signal still is strong.

    ak
     
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  5. crutschow

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    That's not really related to it being an AM subcarrier since all the AM subcarrier information is encoded in FM before transmission.
    The reason for the Stereo signal having more noise is that the audio bandwidth has to twice as high for the stereo audio information meaning the S/N is reduced by 3dB.

    I used to have a Audiovox (I think) car radio that gradually mixed the left and right channels together, going from stereo to mono, as the signal faded.
    Worked quite well as you never got the instant jumping from stereo to mono and back that can be annoying at the fringes.
     
  6. RichardO

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    May 4, 2013
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  7. AnalogKid

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    Don't think so. The baseband mono channel in stereo has almost the same bandwidth as the original mono channel, 15 kHz vs. less-than-20 kHz. Original mono FM had many more possible subcarriers for commercial and public services. These were rearranged to make room the the difference channel and its subcarrier. Bandwidth is the same as the baseband channel. The two channels are functionally independent within the 75 kHz modulation bandwidth.

    But even though the AM subcarrier is embedded in a FM signal, its sidebands are just as susceptable to noise and attenuation. The competing Crosby system with its FM subcarrier held full stereo separation right up to the point where the entire signal disappeared. The bouncing in and out of the entire signal at the fringes was judged less pleasant than the switching from stereo to mono. Also, the Crosby system had less range for the same transmitter power in tests because its FM subcarrier ate up more power than the Zenith AM subcarrier.

    Correction, the big bad guys were GE, not RCA,.

    ak
     
  8. crutschow

    Expert

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    Think so. The bandwidth I'm referring to is the audio bandwidth after the detector, not the FM bandwidth.
    To recover the stereo information requires double the audio bandwidth since the left and right channels has to be double the mono bandwidth to recover the sidebands around the 38kHz carrier. It's that bandwidth that determines the amount of circuit thermal noise in the demodulated signal.

    If the AM sidebands are embedded in the FM signal then they cannot be any more susceptible to the FM carrier amplitude noise or attenuation then the primary signal. Don't see how they could be. :confused:
    Functionally all the signals, both the primary audio and the sidebands, are combined together and then used to FM modulate the carrier. The are indistinguishable from each other once they are modulated into the carrier.
     
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