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Filtering by amplitude, not frequency??

Discussion in 'Electronic Basics' started by Don A. Gilmore, Jan 17, 2004.

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  1. Hi guys:

    I have an audio signal that has a low-level 120 Hz interference in it. In
    my application there is no way to eliminate the 120 Hz at its source. The
    interference is at a low level in comparison to the audio signal, but is
    still an annoyance, especially at high gain. Since my audio signal (music)
    can often involve desirable signals around 120 Hz, I can't really notch it
    out without harming the music.

    Is there an easy way to filter out waves according to their amplitude rather
    than their frequency? In other words, if a signal is not above a certain
    threshold, can it be filtered out while allowing the louder signal through?

    This may be a stupid question with a simple answer, or you may tell me that
    it is impossible, but fire away nonetheless. Thanks.

  2. Ian Bell

    Ian Bell Guest

    Sounds like what you need is a multi band compressor/expander, particularly
    the expander. I have been usign one to clean up some cassette recordings
    and basicaly I have used just the expander settings for this. SO for
    example where a track has low level 50/60Hz hum but also bass guitar yo can
    set one band of the expander to reduce signals below a certain threshold
    and leave anything above it untouched. Works a treat with hiss to.


  3. John Fields

    John Fields Guest

  4. Goran Tomaš

    Goran Tomaš Guest

    Well, you would need a multiband gate or downward expander... But you
    will have a hard time finding (and affording) such a unit.

    Other suggestion is much better - kill the 120 Hz noise with notch
    filter. If the notch is very narrow you won't hear _any_ difference
    with music! All you would need is one parametric equalizer and that
    should be much easier to come up with.

    Goran Tomas
  5. Why is that you say you can't eliminate the 120 Hz, tellus its source?
    hank wd5jfr
  6. Assuming its not a ground loop problem which might well be fixable by
    removing various grounds, you could take a bit of the power supply
    ripple and add/subtract it in to the signal with a pot to cancel the
    existing signal. You might need to have a bit of phase shift on it to do
    it accurately.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "That which is mostly observed, is that which replicates the most"

    "quotes with no meaning, are meaningless" - Kevin Aylward.
  7. It's not from the power source. This is a battery-powered device. The 120
    Hz interference is literally 120 Hz sound. It is inherrent in this
    particular environment and can't be consistently eliminated in all cases.

    I may try the notch filter that was suggested. Any ideas on the best way to
    get a good, sharp notch without a lot of components? A "twin T" maybe?

  8. John Fields

    John Fields Guest

  9. Greg Neill

    Greg Neill Guest

    If you could get a clean signal of the pure 120Hz in some way,
    you might be able to invert and add it to the channel at the
    appropriate amplitude, effectively canceling it.
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