Got a trick to mask clipping?

[Kevin Spahr]

I shot a live event and I had to set my wireless (room) mic levels without know just what would happen or having any sort of rehearsal. But I knew it would be people speaking and people playing music. Well, at one point a "community drum" was brought out. This is a drum between 3' and 4' in diameter and a bunch of people gather around it and pound on it with mallets. My camera tripod was vibrating 50' away! Needless to say my mics were clipping.

So before I start in on the audio, does anybody have any idea how to mask this clipping to make it less nasty? I know it can't be fixed so I'm looking for a way to reduce the harshness. There's chanting along with this drumming so I can't use a sidechain and replace it completely with something similar - although I may try to add some drum sounds using this method in conjunction with trying to filtering out the harsh frequencies. This video is not going to be used "professionally", it is going to be shared with about 50 people that could not attend, so an elaborate, expensive operations will not be an option.

There are a lot of experienced people here and I hope somebody might know a trick they could share...

[Greg Miller]

If you have access to the drum, you could always try ADR... Automatic Drum Replacement. ;-) Of course you'd lose the chanting. Maybe you could convince the viewers that the "chanters" were actually mimes.

More to the point, some programs (like CoolEdit / Audition) have tools to reduce clipping. It works with infrequent clipping that's not too extreme, but can't fix every situation. Can you post a sample as a WAV file? (If you compress it to MP3, the waveform will change slightly and the tool will be much less effective.)

[Reid Vanvoris]

Audacity is freeware and good quality. here's a tutorial. you already know if it's really bad, there's not a lot you can do.
Audacity 1.3 Clipping Removal Tutorial - YouTube

[Graham Bernard]

If you want to forward me a sample I'll gove it a go and you can see/hear if there's an improvement.

Cheers

Grazie

[Kevin Spahr]

The mics were doing the clipping, not the recorded signal - that peaked around -12dB.

Today I finally got around to working on the audio. (I had one of those wonderful "General Errors" in FCP when I would render the video, that took awhile to track down - some projects just don't want to cooperate.)

In Soundtrack Pro I EQ'ed most of it out and then raise some of the good base freqs to hide it a bit more. That should do it.

Thanks guys!

[Greg Miller]

I'd still like to hear (and fiddle with) a sample, if you'd post one.

[Kevin Spahr]

Sure Greg, here's a 15 second clip:

[Warren Kawamoto]

To me, it sounds like your mic is vibrating against something rather than distorting, especially at the end of the clip.

[Greg Miller]

Ouch! That's pretty severe. The waveform at the end is certainly different from that at the beginning. The problem is not just clipping of the peaks. It's a complete change of waveform. No "unclipping" tool can begin to repair that.

Yes, I can make it sound a lot better with a LPF set down pretty low in frequency. But that's not a useful treatment at times when there is chanting along with the drum. (Did I understand you correctly, that the chanting and drumming occur simultaneously? There's no chanting in your sample file.)

I'm still pondering and fiddling with it, but I don't feel I have a "universal" solution that will encompass everything.

[Roger Shore]

The problem with audio clipping is that there are 3 primary causes. I think that this example suffers from all 3, to some extent.

The one that 'clip restoration' software can help with is where the A/D converter is overdriven, and simply runs out of digital headroom. There, the audio will tend to 'clip' cleanly, and clip restoration software can be quite effective.
Running the sample clip through that process makes some (not much !) improvement to the earlier part which is probably suffering primarily (but sadly not exclusively) from that aspect.
(Copy attached -see below.)

The latter part especially seems to suffer from both the microphone capsule itself being overloaded, and/or the analogue mic preamp being overdriven.

The 'clipping' distortion introduced by those two processes is much more difficult (if not impossible) to remove. In addtion to introducing clipped 'tops' to the waveforms, which is correctable, there is substantial harmonic distortion introduced into the analogue signal itself, which the A/D then - unfortunately- faithfully reproduces.
The analogy of trying to remove this kind of distortion is like the famous example, often quoted, of trying to remove the egg intact from a baked cake -- not really an option!