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Need help understanding why computer/headphones act as FM receiver

Discussion in 'General Electronics' started by Dale M., Apr 27, 2004.

  1. Dale M.

    Dale M. Guest

    I just don't understand how this is possible. I've got a relatively
    new Gateway laptop, with a 802.11g wireless card, a USB mouse, and
    some cheap noise cancelling (Jensen) headphones that I use at home to
    listen to some graduate classes. I've setup a rolling cart so I can
    sit in bed and work as needed. When I connect the headset to the
    computer, I hear (faint) music in the background.

    Today, I discovered that the music is actually a radio station, the
    same radio station that my clock radio (GE) is set to.

    When I unplug the headphones, the music stops. When I select Mute in
    Windows, the music keeps playing in the headphones. The music does
    not come out of the laptop speakers. There is no connection between
    the clock radio and the computer, other than they both are using home
    power. (I checked this and unplugged the laptop from the power
    adapter, but still hear the music).

    Can someone explain what is happening here?

    Driving me crazy trying to understand this...Dale
     
  2. Art

    Art Guest

    ? Have you tried to unplug the Clock Radio to see if then it stops?? Is the
    FM Radio Station relatively close to your residence?? Have you tried a
    different set of headphones to see if the same effect occurs?? I suspect you
    are probably picking up the signal from the FM station, which must be a
    moderately strong signal in your area. I have experienced commercial radio
    stations material received thru PA systems, Guitar Amps, Etc.
     
  3. Ken Taylor

    Ken Taylor Guest

    Is the clock-radio on the PC? Maybe you are coupling across to it. Try to
    physically separate them more.

    Ken
     
  4. Externet

    Externet Guest

    Hello Dale.
    I believe the key is in your words... "some cheap noise cancelling
    (Jensen) headphones"... Well, noise cancelling headphones are
    supposed to have a microphone to pick ambient noise in order to know
    what to cancel.
    They are picking your clock radio audio. No misteries there.
    The cancelling levels may be poorly calibrated, but that has to be it.
    Miguel
     
  5. Dale M.

    Dale M. Guest

    I cut power to the Clock Radio, but still hear the station in my
    headphones. The FM station is over 60 miles away, and I suspect more
    than that...it's not easy to dial it in on my radio, and depending on
    the type of day depends on my normal reception (on the radio). I
    don't have a different set of headphones, but like I said before, when
    I pull the headphones out of the computer's headphone jack, the
    broadcast stops playing thru the speakers (i.e. it's not the headset
    alone...)
     
  6. Dale M.

    Dale M. Guest

    The clock radio is only about 1-2 feet away from where the laptop sits
    when this happens. Actually, I've noticed today that how the headset
    cords that run from the plug to the earpieces hang in relation to the
    radio seem to make a difference. The few samples I've taken indicate
    that the closer to the radio, the stronger the reception.

    What's coupling across it mean?

    Dale
     
  7. Dale M.

    Dale M. Guest

    How can that be if the radio portion of my clock radio is off?
    There's no sound coming from the clock radio speakers for the headset
    to pick up, and if that theory were true, I think I could expect to
    pick up the music even when the headset was disconnected from the
    laptop, which it does not.

    Still searching...
     
  8. Ken Taylor

    Ken Taylor Guest

    It means you're picking up one from the other. You mention in another post
    that this can happen even when the radio is 'off', however many
    (particularly cheaper) clock-radios don't bother to power down the radio
    portion of the set, they just switch the audio out. It definitely appears
    that you are just picking up the radio's emissions, probably from the IF or
    detector stages. If it bothers you to have to move radio or laptop, go to a
    Radio Shack or similar parts place and get a ferrite bead to go over the
    headphone lead, or wind the lead around a ferrite bead. Here's what you're
    looking for:
    http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=273-105

    (Yes, I'm sure many will say they know of cheaper/better components, but I'm
    betting this will be easiest for a novice to get hold of).

    Cheers.

    Ken
     
  9. iQbal

    iQbal Guest

    This is what i think is happening,

    The cable length of your headphone is acting as a suitable antenna for
    the radio station frequency. Are you sure its FM and not a AM radio
    station.

    All you need to demodulate an AM signal is a certain non linear
    element in the chain within the active electronics. This element could
    act as a demodulator and recieve a radio station if a signal of
    sufficient amplitude is presented at the input.

    You could avoid this by simply bypassing your headphones with a small
    22 - 47 pf capacitor.

    iQbal
     
  10. Earl

    Earl Guest

    Dale,

    This is a common problem with self-powered computer speakers, and it might
    the same with your headphones. Do your headphones have internal batteries
    to power them? Is the signal (input) cable the only cable that physically
    attached to the headphones?

    I work for an FM radio station that has the transmitting antenna on the
    roof, and the top floor tenants had this problem with their computer
    speakers. I had to rewire a few computer speakers. Not a big deal, and it
    fixed the problem completely.
     
    Ivica likes this.
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