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I've recorded a living room concert of two singers with two guitars with a Zoom H2. The recording is fine 90% of the time, but there is some clipping distortion. I think this is caused because I had my Zoom H2 set to high gain, instead of mid-gain.

But as I can not re-record the concert, is it possible to fix this (even a bit) in post production?

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I think it is possible, but depends in how clipped the signal is. Let me explain: think about a softly clipped signal. Clipping is present only in the greatest peaks, and therefore appears for a short time lapse. enter image description here

This kind of method could detect the clipped intervals and ''soften'' them, based in the previous behaviour of the signal. An statistical approach would probably work, but it is not a trivial task. Of course, you have a probability of guessing how the original signal actually was, so you will probably add some distortion. Hopefully, the result will be better than the clipped version.

On the other hand, if the signal is heavily clipped, the chance of guessing it right is unarguably lower. The signal could be anything but a peak: it has time to do lots of things until the clipping stops.

enter image description here

The recovered signal using the stated method would be the blue one, while in fact any signal (e.g. the green one) could have actually happened in the lapse of time we don't have info about.

Just when I thought I had come across a great idea, Wikipedia told me that I'm not the first to think about this:

Several software solutions of varying results and methods exist to counteract this problem: Sony Sound Forge, iZotope Rx2, Adobe Audition, Nero Wave Editor, and a plugin in the Audacity LADSPA package come with clip restoration software. There is also an Audacity plugin called Clip Fix that uses cubic splines to attempt to restore a continuously differentiable signal.

Please excuse the poor quality of my hand-drawn figures ;)

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+1 for the hand drawn wave forms. Good show!! Also, good answer :) –  JoshP Feb 22 '13 at 13:26
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Yeah, if the cliping is very light, this can work, but it generally lacks clarity and detail. In general, a clipped source is still a broken source if it is anything more than very minimal clip recovery needed. (This is effectively the acoustic equivalent of blurring an image.) (At least in my personal experience.) Great effort on the hand drawn waveforms. –  AJ Henderson Feb 22 '13 at 16:46
    
Nice drawings! I suspect that a fractal approach would yield better results than cubic splines. Create a catalog of wavelet shapes, and then do affine transformations on them until you find the best match for the clipped wave and apply that. Compute intensive, though. –  ObscureRobot Feb 22 '13 at 22:41
    
@AJHenderson Your image blurring comment made me have the following idea: would adding reverb (also a sort of blurring) be an option to smooth out the light clipping? –  Bart Arondson Feb 23 '13 at 2:01
    
@BartArondson - Hmm, going with the graphics parallels, I'd say that reverb is more of a clone brush than a blur. Reverb is repeating something you have where as a blur is more of something you do to cover up a lack of having something. In practical terms, reverb would just help make it less obvious that it is clipped because the sound just before the clipping would be mixed in with the clipped waveform, but it wouldn't guess at recovering what had been there. –  AJ Henderson Feb 23 '13 at 6:32
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Nope, the sound information is missing and there is no way to recover it. (At least not that I'm aware of.) Even if there was, it would have to basically be completely guessing at what should be there. (Edit: there is software that will make the guess, and that's what the answer with the waveforms is illustrating. It is worth noting that it is a guess though and is basically just making up something to fill in the gaps. If the clipping is severe, it likely will not be capable of producing a crisp sound, but more like the acoustic equivalent of a heavily blurred image.

Basically what happens with clipping is that the waves of the signal exceed the amplitude it can record and thus the information is lost as it is physically incapable of storing the values that correspond to the signal. If it was actually an analog clip, then it is a similar principal where the circuitry can't process or create a large enough signal.

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