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AM Broadcast Spectral Components

Discussion in 'Electronic Design' started by George, Dec 25, 2012.

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  1. George

    George Guest

    Hi all, and Happy Holidays:

    In the Los Angeles area several local AM broadcast stations appear to contain signals at between 100 Hz and 300 Hz away from the carriers, below the main program audio. I have called a couple stations to ask what these signals are, but they haven't called me back.

    I'm wondering what they might be. In Europe there's the AMSS service that's used for low-speed remote signaling, and there's OFDM, but I didn't thinkthey are in use in the US. There's also a Nielsen TV rating signal that'ssometimes used in the US.

    But does anyone have any idea what the signals probably are. A couple of spectrum plots are posted here: 870 spectrum - narrow.pdf 710 spectrum - narrow.pdf

  2. Joerg

    Joerg Guest

    Quitle likely plain old hum :)

    Seriously, local stations can generate a lot of level on house wiring,
    get modulated at all sorts of cheapo power supply rectifiers and then
    re-radiated. Had it many times. In the US you'd mostly see 60Hz, 180Hz
    and so on. In Europe 50Hz and 150Hz.

    You can't really have much in terms of little sub-audio carriers because
    most AM radios would make that annoyingly audible. Also, the audio
    frequency response of an AM channel is something like 40Hz to 5kHz anyhow.
  3. Ralph Barone

    Ralph Barone Guest

    Up here in Canada, we call frequencies in the 100 to 300 Hz range "bass".
  4. Robert Baer

    Robert Baer Guest

    I see nothing unusual about either of those spectrum plots.
  5. Lord Valve

    Lord Valve Guest

    More likely due to frozen nuts...
  6. miso

    miso Guest

    HD (IBOC) is in the guard band, i.e outside the main AM signal. IBOC
    stands for In Band On Channel, but really it is on the channel of
    another station that used to be protected.

    I don't know if it still lives in cyberspace, but there was a weird
    signal coming out of the Nevada Test Site in the early 90's or maybe
    late 80's. When it was traced to the NTS, the signal stopped. No
    explanation was given. IIRC, it was in the 800 to 900 range. Art Bell
    mentioned it on Coast to Coast, so that might pin down the time frame.
    You could hear it on the skip in California.

    I was thinking it might be a weird plasma etcher of some sort,but they
    use ISM frequencies. Usually 13.56MHz. When Triangle Machinery and RA
    Associates were still in business, these ISM RF linears were on the
    surplus shelves. This predated RFID, but still, what would you do with one?
  7. George

    George Guest

    I wondered about it being hum. But the envelope looks more like the shoulders you expect from data modulation rather than line frequency spikes, and they do not occur at exactly 60 Hz or multiples. And -35 dBc is pretty loud for incidental hum. Also they don't appear to vary with the main audio modulation either, based on repeated observations.

  8. Mark

    Mark Guest

    well what does the station SOUND like?

    you would easily be able to hear this

  9. Phil Allison

    Phil Allison Guest


    well what does the station SOUND like?

    you would easily be able to hear this

    ** Hey - don't spoil George's perfectly good troll with a sensible

    ..... Phil
  10. Joerg

    Joerg Guest

    In one plot it looks like 60, 180 ... Hz, the other looks like 120Hz and
    harmonices which you'd have with older single-dide rectifiers. Because
    that results in even-order harmonics. The spectral width depends on the
    load. For example, if this comes from uncle Leroy's old TIG welder while
    he is welding it'll be pretty wide.

    Pretty normal in some situations. Often it is louder on local stations
    than on distant ones.

    Hum modulation usually doesn't. Its amplitude and spectral constellation
    depend on the load that's on the offending rectifier. In the old days it
    was easy to find out the offender by unplugging one device after the
    other. Today that's almost impossible because there is so much of it and
    some stuff needs elaborate re-programming after unplugging, such as the VCR.
  11. Joerg

    Joerg Guest

    European shutters could drive people crazy that way. The expensive kind
    is not plastic but aluminum. Crackle .. ka-crackle ..

    In the days of NTSC you could get some really interesting effects when
    placing an ancient metal-blade fan in the path. With DTV you'll just get
    the usual frozen Picasso or blue screen.
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