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removing loud audio hum from voice recording

Discussion in 'PC Hardware' started by OhioGuy, Feb 8, 2009.

  1. OhioGuy

    OhioGuy Guest

    I'm working on a magazine article, and I used a Sandisk Sansa to
    record a ~45 minute audio interview to use as my source material. Now
    that I get around to reviewing it, it sounds ok for the first second or
    two, then an audio hum overwhelms the audio to the point where I can
    just barely make out the speaking, but not well enough to get anything
    out of it, no matter how long I spend straining. I originally thought
    this was an issue with the audio out headphone jack on the Sansa
    Express, but the problem persists even after I transfer the file to my PC.

    The Sansa Express records Microsoft ADPCM, 89 kbps bitrate, 4 bit
    sample size, in mono, and a sample rate of 22 kHz. It appears to be
    saved in a *.wav package.

    Anyway, when I play the file with VLC Media Player, and I choose the
    oscilloscope visualization, it shows two distinct wave patterns. I'm
    wondering if one of them is the audio hum, and if there might be some
    way to use software to edit the adpcm file so that the audio hum is
    removed, or at least lessened, and I can then listen to the recording
    and use it for my article.

    Anyone have experience with something like this? Thanks!
    OhioGuy, Feb 8, 2009
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  2. OhioGuy

    OhioGuy Guest

    Now I'm not sure it is a specific frequency. I looked at the
    waveform, and it looks like little 'spikes' going down into negative
    territory - about 100 per second. However, when I play it through my
    stereo and use the graphic equalizer, a bit more of the noise disappears
    when I move the upper frequency sliders down. When I zoom in even more
    to the wave, I see that the noise is a pulse that lasts 3 ten
    thousandths of a second, and begins every hundredth of a second. It
    looks like a v, with the bottom of the trough going down to about -.18
    from zero.

    No matter what I do, though, it appears that since the hum is 2 to 3
    times as loud as the people speaking, I am having a really tough time
    making out what they say. The annoying thing is that the Sansa Express
    recorder will not duplicate this problem - it appears that the thing
    only had this problem when I was sitting in their office near the
    computer and a bunch of other electrical equipment. Hence, I'm thinking
    it might have been RF interference of some kind. Every recording I do
    now is quite clear.

    I used Audacity to apply a high pass filter, then tried a low pass
    filter. No help.

    I think next time I'm just going to take a backup recorder, and have
    two of them running, just in case.
    OhioGuy, Feb 8, 2009
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  3. OhioGuy

    dontdont Guest


    If the interference is as consistent as you describe and you wanted to
    email me with an attachment containing the first minute of your
    original unmodified recording in .wav format then I would take a few
    minutes and apply a chopper to try to slice away the interfering
    signal. I would reply by email with the processed minute of .wav
    file. If the results were good enough then perhaps we could try the
    chopper on your whole file.

    Or if you have the right tools you could try writing up your own
    signal processor snippet to do this. From your description I'd
    suggest your code look for about .3ms wide negative going pulses,
    remove those, and fill the gap with a straight line between the two
    endpoints where you chopped out the pulse.

    No obligation, no credit, usually I learn something and that is enough
    for me.
    (no really)

    And next time I'd suggest doing a quick sound check on at least one
    recorder to confirm you have an acceptable recording under the
    dontdont, Feb 9, 2009
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