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Homework Help: Stereo indicator light in FM radio

  1. Dec 3, 2012 #1


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    1. The problem statement, all variables and given/known data

    How does the stereo light in an analog FM radio turn on when you tune into a stereo station?

    3. The attempt at a solution

    I know that radio tuners work on the concept of resonance. Current and voltage levels spike up at and near the resonant frequency of a circuit.

    My initial assumption would be that there is some kind of switching mechanism that when the voltage or current hits a certain level (at resonance), the switch is flipped and the light is turned on.

    In newer radios, I'm assuming the switching would be done by a transistor, but what was used in the older analog radios to turn on the stereo light?
  2. jcsd
  3. Dec 3, 2012 #2


    Staff: Mentor

    I think the circuit determines that both channels have a signal before the light is turned on meaning you now have the left and right channels lined up.
  4. Dec 3, 2012 #3


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    So does a stereo station actually transmit two separate frequencies and then the radio has two separate tuners and when both are in a condition of resonance, the voltage levels are enough to turn on the stereo indicator light?
  5. Dec 3, 2012 #4
    When Broadcast FM was first created, it was monaural. Sometime later when stereo FM was invented, it needed to be compatible with monaural FM. With stereo FM, the Left + Right channels are transmitted in baseband (100 Hz to 15 kHz) thus making it compatible with monaural, and the L - R is modulated on a 38 kHz subcarrier. The L - R signal is detected from the subcarrier. When L + R and L - R are added together you get the left channel and when one of the two is inverted and the two are added, you get L + R plus -L + R or the right channel.

    The red light comes on when the 38 kHz subcarrier is detected. A high school student once told me that he was station engineer for his high school's monaural FM station. He understood everything I described above and added a 38 kHz subcarrier to the signal (unmodulated) and the red light came on in the instructor's receiver. The instructor was intrigued knowing the FM signal was monaural.
  6. Dec 3, 2012 #5


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    "The right light comes on when the 38 kHz subcarrier is detected."

    When you say "detected," what does that mean exactly? Is there a voltage rise that is set off when this subcarrier is detected?
  7. Dec 3, 2012 #6
    You must have seen the "right light" before I corrected it.

    I believe the 38 kHz subcarrier is amplitude modulated before being added to the audio which frequency modulates the carrier. After the composite FM carrier is discriminated, the 38 kHz subcarrier exists again as an amplitude modulated carrier. It must be filtered to reject the baseband audio and then the audio recovered with a typical AM detector. Yes, after AM detection, there will be a DC rise in voltage that is attributable to the 38 kHz subcarrier. This rise in voltage will fluctuate with the L - R signal but it will be higher than if there were no 38 kHz subcarrier.
  8. Dec 3, 2012 #7
    skeptic is right but the carrier transmitted is actually at 19kHz (the L-R component *is* modulated up to 38kHz) in order to keep it better separated from the L+R and L-R bands.

    The 19kHz carrier is detected with a narrow band filter (which could be used to light up a stereo indicator) and its frequency is doubled to 38kHz in order to demodulate the L-R band at 38kHz.
  9. Dec 3, 2012 #8
    I must correct what I've posted. The 38 kHz is not transmitted. Instead the L - R signal is double sideband modulated on a 38 kHz subcarrier with a balanced mixer. This means that only the sidebands minus carrier are transmitted. The 38 kHz subcarrier is then frequency halved to 19 kHz. L + R baseband, the 19 kHz subcarrier and the 38 kHz double sideband signal are all combined and frequency modulate the carrier.

    On the receive side the 19 kHz carrier is frequency doubled, injected into the 38 kHz double sideband, the L - R signal is recovered. The red light come on when the 19 kHz subcarrier is detected.
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