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Do you recognize this sound?

Discussion in 'Electronic Design' started by Herbert Blenner, May 4, 2005.

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  1. I have a wave file from a radio reception made four decades ago.

    The sound is familiar to me and wonder how others would characterize
    this signal?

    I withhold my opinion of the sound to avoid biasing the views of


  2. Sounds like a 1 KHz calibrator on an HF receiver.

    With poor selectivity. ;-)

  3. Robert Baer

    Robert Baer Guest

    The sound i got lasted less than a second; way too short to
    characterize more definitively than "noise".
  4. Don Pearce

    Don Pearce Guest

    It is mostly noise, band-limited by what looks like a telephony
    system. It also contains another noise component, more heavily
    band-limited around 900Hz.

    What it comes from is another matter. I have done a little processing
    on it, though, and it is now a submarine ASDIC. Just play it as a


    Pearce Consulting
  5. Same here.
  6. The signal originated from a voice communications radiotelephone system
    connected by telephone lines to a remote recorder.

    I am particularly interested in the 900 Hz component. My signal
    processing tools are limited to a FFT written three decades ago and
    recently translated from FORTRAN. I interpret the transform as showing
    an amplitude and frequency modulation of a 900 Hz sinusoid.
    If your sonar is an accurate representation of the 900 Hz component
    then I would appreciate further details on how you extracted it from
    the other signals.

  7. Don Pearce

    Don Pearce Guest

    That is indeed a valid interpretation - you can make any complex
    signal as modulation on a carrier - doesn't mean it was created that
    way, though. It is much more likely that the 900Hz component and the
    broader band noise are unrelated.
    That was just a bit of fun, created by amplitude shaping, copy and
    paste in the spectral domain and added reverb (Cathedral model). I did
    it with Adobe Audition and the Waves plugin bundle. It took no more
    than 30 seconds.

    It is certainly your original file with no other signals added.


    Pearce Consulting
  8. Don Pearce

    Don Pearce Guest

    If you want to extract the signal yourself using your FFT, then it is
    quite simple. Run the FFT. Set all the values up to the 800Hz point to
    zero. Do the same for points from 1kHz upwards. Now run the IFFT and
    you have a filtered version of the original, with just the 900Hz
    signal and a bit of noise - similar to what I posted, but without the
    extra joke effects.

    Pearce Consulting
  9. Thanks for the advice, Don.

    I have tried the technique and found that transient response of the
    filter to the rapid shifts of the signal dominate the output.

  10. Don Pearce

    Don Pearce Guest

    Then instead of simply zeroing the values, write yourself a
    phase-linear filter and apply that to the signal.


    Pearce Consulting
  11. Mark

    Mark Guest


    sounds familiar to me...

    does this have anything to do with JFK and Dallas?

    i'ts not gun shots!

  12. Don Pearce

    Don Pearce Guest

    Actually, when you look at the file in the frequency domain, it looks
    exactly like a grassy knoll.


    Pearce Consulting
  13. The file is from the Bowles tape of the Dictabelt and does not contain
    the alleged shots.

  14. When I look at the file in the frequency domain, it looks like a strong
    and nearly narrow band sinusoid surrounded by lesser noise. To me this
    spectrum shows simultaneous amplitude and frequency modulation of

  15. Like I said almost a week ago, a 1 KHz calibrator on a really crappy
  16. I have seen 1 ms square wave sources on scopes and 100 kHz crystal
    oscillators on receivers. What in the world does a 1 kHz calibrator do
    on a receiver?

  17. Don Pearce

    Don Pearce Guest

    No it doesn't. There is no information in that file that tells you
    *how* that noise came to be there. It could indeed be sidebands
    imposed by a modulation process, or it could be simply noise
    accompanying the signal.


    Pearce Consulting
  18. Don Pearce

    Don Pearce Guest

    Except it isn't even close to 1kHz.


    Pearce Consulting
  19. I seem to have missed your response.

    The modulation index of the 900 Hz signal is relatively small. This
    enables my function generator to almost lock on the dominant signal
    from the file. Connecting the signal from the file to the X-axis and
    the output of the nearly locked generator to the Y-axis produces a
    nearly stationary scope patten. Examination of the sine wave output of
    the generator shows the frequency modulation.

  20. Don Pearce

    Don Pearce Guest

    It shows the other signals, certainly. To call it frequency modulation
    is nothing more than your chosen interpretation. There is no
    justification for that interpretation.


    Pearce Consulting
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