How to reduce noise in audio recordings?

[mr_lou]

When I connect my CPC audio-out to my mixer, and play music, I hear quite a lot of noise / interference. (Well it's there even without playing music too).

How can I reduce this?

First I was thinking about connecting ground of the CPC to a radiator. Then I was thinking about just connecting the two grounds of the CPC and the mixer together.
And then I thought I better ask on the forum instead.

Any experts on this field?

[Bryce]

What type of noise is it?

A) A permanent low pitched buzzing in the background.
B) A high pitch whining.
C) The "processing" sounds of the CPU?

As I know that you own a DigiBlaster v3, try connecting to the mixer through the DigiBlaster. The Digiblaster doesn't need to be connected to the printer port, just use the Audio-In / Audio-Out connections. The DigiBlaster has two filtering capacitors which might filter out the noise you're hearing. If this does work, then I assume your answer to the question above was (A). To solve your problem without having the DigiBlaster connected you'll need to add some ceramic capacitors in series on you connection cable. This will filter out any DC signal that the AY is leaking onto the AC signal.

Bryce.

[mr_lou]

I think it's C, although A could also fit the description.

I tried connecting the Digiblaster in between the mixer and CPC, but it changes nothing.

So connecting ground to a radiator or connecting the two grounds to eachother is not the way to go?

[Bryce]

The problem is.... Real Audio gear is made for "real" analogue sound, where the signal varies both above and below the ground plain, ie: the current flows in both directions. However, the sound coming from the CPC is a positive signal referenced to ground (see badly drawn diagram below). So connecting the grounds together could cause all sorts of problems and possibly even damage the mixer, because essentially you're short circuiting half of its amplifier stage.

It's very difficult to get a clean signal from a device that's full of unscreened ICs clacking away at 4Mhz. You could try putting a good equaliser between the CPC and the mixer and filter out the most annoying frequencies that way, or feed the signal into a PC with a decent soundcard and filtering software. Otherwise there's not much more you can do.

Bryce.

[mr_lou]

Hmm bummer.
Well...  I guess that means that it's actually best to record from e.g. WinApe, and only in cases where WinApe doesn't produce the correct sound, use a real CPC and try to remove the noise post recording. Audacity can do that, but it affects the quality of the sound. With Audacity you can record the noise first, to tell Audacity how it "looks" like, and then it'll remove that pattern from the audio. It's not optimal though, but apparently there aren't really any optimal solution....

But you still believe some capacitors in series can remove the noise? And they have to be cheramic? Such sound cables aren't made by any factory by any chance?

[Bryce]

The capacitors just block DC signals from getting through, because AC signals pass through a capacitor, whereas DC signals don't.
No there are no cables made like that because those capacitors are built into most decent audio equipment.

Bryce.

[steve]

This may be a silly idea, but would putting the circuit board in a re-sealable plastic bag with holes where the sockets are and then wrapping the bag in foil and connecting the foil to earth/0v, reduce the "processor noise".

[Bryce]

No, it would probably make it worse, because the interferring signal is inside the bag. It's getting to the mixer through the wires, not through the air.

Bryce.

[Badstarr]

Just a quick question are you just recording the signal from the cpc and not using it for live performances?  If so i may have a solution for you but will need some input from Bryce to get you up and running. What you need to do is find a way to tap the noise signal ie the cpu sounds separate to the audio from the AY chip. I guess Bryce will know how to do this.

You need to record the noise to a separate channel at the same time as the music. This means you will either need to record the cpc music to a mono channel eg the left and the noise to the right or visa versa. Obviously if you have multi channel inputs on you sound card you can just record the stereo signal and the noise to a third channel.

You need to make sure as best you can that your seperate noise signal is the same gain as the noise signal included in the cpc stereo output. Now record the noise to one track at the same time as recording the music to a second track. When you have finished, this is when the magic happens you need to reverse the phase of your noise track now all you need to do is play or sum all channels together and the noise will have vanished!

[Bryce]

That type of noise cancellation works fine for uniform repeating noise, but if the noise is really the CPU processing noise, then it is constantly changing and will not be exactly the same each time, so you might reduce the noise a little, but far from completely and it may even add noise in certain conditions.

Bryce.

[Gryzor]

The problem is.... Real Audio gear is made for "real" analogue sound, where the signal varies both above and below the ground plain, ie: the current flows in both directions. However, the sound coming from the CPC is a positive signal referenced to ground (see badly drawn diagram below). So connecting the grounds together could cause all sorts of problems and possibly even damage the mixer, because essentially you're short circuiting half of its amplifier stage.

It's very difficult to get a clean signal from a device that's full of unscreened ICs clacking away at 4Mhz. You could try putting a good equaliser between the CPC and the mixer and filter out the most annoying frequencies that way, or feed the signal into a PC with a decent soundcard and filtering software. Otherwise there's not much more you can do.

Bryce.

Ooh. You learn something new on this site every day...