[sdiy] Build a board to restore audio

[Richard Wentk]

At 07:53 28/11/2002 -0800, you wrote:
>     This is a task that is probably best accomplished in the digital
>domain.  I have not actually tried any of the programs that remove
>"noises" from records, but a friend who has tried a couple told me to
>save my money.  He felt that they made the recordings sound muddy.
>Removing big scratches is fairly easy, however, but it is time
>consuming.  I just use Cool Edit.  A scratch is generally only a couple
>of samples long.  I just grab the peak and drag it back in line.

Cool Edit Pro has some *excellent* automated click 'n sizzle noise 
reduction tools. I use it to get rid of pops and glitches for mastering 
CDs. No mud at all. Doesn't seem to screw up the transients either. It's a 
bit more expensive than the other products, but it's still going to be 
cheaper than trying to build something as good in hardware.

Obviously if you're trying to pull audio out of vinyl that's seriously 
fried, it's going to be hard to do that without some processing artefacts. 
If the information is lost, it's pretty damn hard to recover it. But if you 
have mud, you can always add some sparkle back with an exciter. BBE are 
doing a plug-in version of their Maximiser now. It works pretty well.

>    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. (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.

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. :)

Richard