Get rid of electrical hum in audio?

Discussion in 'Amateur Video Production' started by funsoul, Sep 18, 2003.

  1. funsoul

    funsoul Guest

    Hi All

    Captured a tape and although the video quality is decent, the audio
    has a tremendous hum (intermittent but very loud at some points).

    Is there a relatively easy way to reduce or eliminate it? It's a
    stable frequency (60-ish Hz, maybe?) so some pretty narrow-pass
    equalization might do it but I'm a total newbie on the audio side of
    things.

    Any tips/hints/suggestions?

    tia!
    -funsoul
     
    funsoul, Sep 18, 2003
    #1
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  2. funsoul

    Jay Rose CAS Guest

    Go to DV.com and read the article on noise reduction. It covers how to
    tune a parametric equalizer - which you already have in your NLE - for
    this kind of thing.

    But while the frequency is stable, I doubt it's just at 60 Hz - if it
    were, you probably wouldn't hear it. There are most likely harmonics all
    over the band. A parametric won't cut it.

    That same article has pointers to another piece about comb filtering,
    which can zap high-harmonic AC buzzes such as dimmer hum almost perfectly.

    And of course the article also covers how to get the best from Noise
    Reduction software, which is the last resort here.
     
    Jay Rose CAS, Sep 18, 2003
    #2
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  3. funsoul

    Rick Guest

    The two programs I use (SoundForge 6, Cool Edit Pro 2) both
    have super noise reduction filters. I prefer CEP's user interface,
    it's just a matter of selecting a small portion of the background
    hum and it will create a filter from the selection. The result is
    complete elimination of the hum with virtually no audible signal
    loss. Great, great design.

    Rick
     
    Rick, Sep 18, 2003
    #3
  4. funsoul

    Jim Davis Guest

    I had this problem on an old VHS tape I was converting to DVD. I used
    the notch filter in Premiere 5 set to 60Hz. The odd harmonics
    (1,3,5,...) are where you'll likely have the problem. I also had to
    set notch filters at 180Hz, and 300Hz (3rd and 5th harmonics) and it
    cleaned it right up. Keep adding at the odd harmonics until you're
    satisfied.
     
    Jim Davis, Sep 18, 2003
    #4
  5. funsoul

    funsoul Guest

    Thank you Jim!!!!!! :)
     
    funsoul, Sep 19, 2003
    #5
  6. funsoul

    Warren Young Guest

    First, capture another tape and see if you get different results. If
    you do, the noise is on the tape and you'll have to do your best to
    remove the noise after the fact. If it's caused by your system, the
    smart path is to fix your system and re-capture. Look around on the net
    for tips on eliminating system hum and ground loops.
    If you have a complex audio track (more than just people talking) you're
    likely to damage good audio when reducing the noise. The tips others
    have given about using equalizers and noise reduction filters is a good
    way to go when you have no alternative, but don't make these your first
    choice when solving this kind of problem. Filters can cause more damage
    than they fix.

    Before starting work on this, you might load the audio track up into a
    good audio editor and run a spectrum analysis on it. You should see a
    peak where the noise fundamental is. If you can't find a peak, you'll
    have a tough time using an equalizer to reduce the noise. You may have
    to resort to noise reduction, which results in easily audible artifacts
    if you use it in a heavy-handed way. Cool Edit (now Adobe Audition) can
    do all of this. Sound Forge can as well, though the noise reduction is
    an additional-cost plugin.

    One guy said that you might not hear the noise if it's 60 Hz, and
    another said that filters cause almost no damage. When doing this sort
    of work, you owe it to your self to use a decent headphone setup. Sony
    MDR-7506es would be perfect for this. You want headphones that go more
    for accuracy than "niceness". If the system hides things from you, you
    can't fix them. Or worse, you may cause damage to the audio track that
    you will only hear later on better systems. I assure you, a decent
    headphone system will easily show a 60 Hz hum and it will also show you
    flaws in the audio caused by overfiltering.

    With good headphones, good software, and a lot of work, you can find the
    noise, and find out how much you can reduce it before the cure starts
    becoming worse than the disease.

    TANSTAAFL.
     
    Warren Young, Sep 20, 2003
    #6
  7. funsoul

    peter Guest

    The shareware "goldwave" can also do this.
    Open the audio in goldwave
    find the largest section in the audio with only the hum, select and copy it
    deselect
    go to filter, noise reduction, choose "clipboard" and go
    this should get rid of the hum (you may want to experiment with the various parameters)
    save the file, re-encode and re-mux with the video.

    I have done this many times on video with AC hum. The end result is video with no hum.
    goldwave is cheaper than cooledit or soundforge.

    Even cheaper is the GPL-ware called audacity. No idea if it has noise filter capability.
    You can download it for free.
     
    peter, Sep 24, 2003
    #7
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