[sdiy] Build a board to restore audio

[Tom May]

Richard Wentk <richard at skydancer.com> writes:

> >    Removing HUM is another problem.  To do this, you need a notch
> >filter, probably several.  Trouble is, these will introduce a lot of
> >phase shift around the notch.  Right at frequencies you don't want to do
> >this (60,120,180,240Hz).  You might be better off living with the hum.
> 
> The usual way to do this is by sampling the noise spectrum and
> subtracting that in FFT-land from the source spectrum.

The usual way actually seems to be do an FFT, gate each band
independently or semi-independently, then IFFT.  The noise sample is
used as a starting point for the gate threshold.  When the band is
above the threshold, the signal is assumed to be masking the noise in
that band.  If you're not careful with the parameters, and even if you
are, you can get artifacts that sound worse than the original noise.
But you can also do some amazing things.

> (That's kind of
> how Cedar works, but it's a bit cleverer than that.) Cool Edit Pro and
> Sound Forge have this kind of semi adaptive filtering built in. You
> have to sample the noise spectrum manually, but that's easy enough to
> do.
> 
> This works extremely well for hum, and other quasi-periodic
> signals. (Air conditioner noise, assorted RF whistles, and so on.)
> It's not so perfect for serious tape hiss, but serious tape hiss is a
> Hard Problem, so that's no surprise. There's no neat analogue solution
> there either. (If you want to talk about mud, try one of those dynamic
> low-pass filtering boxes.)
> 
> You can also use Cool Edit's FFT filter to build a set of super
> accurate notches, which will filter the sound without all the usual
> phase problems you get with analogue. You'll find you always need a
> lot more notches than you think you do, because hum often contains
> surprisingly high harmonics.

I've always gotten the best results on hum with wideband noise
reduction tools.

> For any audio editing the FFT filter is a blast. I use it all the
> time. It's such a cool tool, with a zillion applications. Everyone
> should have it. :)

I'm wary of any FFT/IFFT process, because of leakage and whatever
other bad things may be going on.  Maybe I'm just paranoid though.

Tom.