[TriLUG] Sound Processing

[Joseph Mack NA3T]

On Tue, 10 Jul 2007, Brian McCullough wrote:

> I have an issue that has been causing me a great deal of grief over the
> past while.
>
> I have some sound files ( telephone recordings ) that have a VERY loud
> 60-Hz hum.  They are, at the moment MP3 files, but were WAVs coming out
> of the recorder.
>
> I am wondering if anybody has and bright ideas for inserting a
> 180 degree out of phase signal to cancel the hum, or any other ideas to
> clean up these files.

Cancelling the hum sounds a smart idea, but I've not heard 
of anyone feeding an out of phase signal. It might be hard 
to keep the cancelling signal near enough to out of phase. I 
expect you could get 10db, but that's about all.

The hum may have harmonics, if it came from a power supply, 
which an out of phase signal will only partially cure. Have 
you looked at the spectrum of the audio to see? I haven't 
kept track of the packages, but back in the mid '90s there 
used to be linux code that gave you spectra of sound files. 
Presumably they still exist.

FFT filtering code is routine in embedded machines (usually 
with DSP chips for speed, eg for TV, which you don't need 
for audio) and EEE people learn FFT filtering in undergrad 
classes. I expect code is available almost anywhere to do 
this.

You could try a notch filter (if it's a single frequency), 
or a high pass filter (if there are harmonics).

Alternately an analogue notch filter can be built out of an 
op-amp and a few resistors and capacitors if you're familiar 
with this mode of working. An analogue high pass filter with 
a steep drop off (which you'll need) will be a little more 
work.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
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