Reducing crackle in old audio recordings

Reducing crackle in old recordings is not usually necessary unless the original is on disc. This is relatively rare with drama.

Old tapes do not usually suffer from crackle; it's more common to have an incorrect balance between treble and bass, or excessive background hiss or rumble.

This article is written for those who are familiar with Cooledit Pro and is a way of reducing crackle significantly using a bit of lateral thinking. There are other ways of doing it- for example, by tinkering with the settings on the Pop and Click eliminator, and using repeated passes.

This is just another tool.

The method ... sample the non- sound part of the recording (noise plus crackle) .... extract it from the waveform, do an aggressive noise reduction on it. Decrackle it, then add it back to the original recording. Experiment with a 2-3 second sample to begin with, until you've got the hang of it.

Here's the outline. It is not written for novices, so you'll have to work out which buttons to press.

1.Get a noise profile from the non-sound part of the recording. Using this ...

2. Reduce the noise on a small bit of the file, fairly energetically- say 40dB. Keep only the noise and copy it to the clipboard.

3. Declick this noise-only file (which is now on the screen), energetically, using the pop and crackle eliminator.

4. Isolate the crackle by doing a mix-paste / invert from the clipboard, which still contains the information from step 2.. Now copy the resulting file to the clipboard. It should have a few vertical spikes and very little else.

5.Mix-paste this, uninverted, back into the original file. You'll have to skip back carefully, using edit-undo two or three times, to get to the original.

I worked through this on an old '78' recording and was surprised by the result.

Nigel Deacon / Diversity website

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