Re: [Audacity-devel] Reducing clipping during playback

[<cr...@de...>]

Brian Gunlogson:
> Is it somewhat trivial to reduce clipping during playback?  My idea
> is to lower the value of the sample if it clips, but do nothing to
> the other samples.

Unfortunately, that is the definition of clipping.  The harsh clicks associated
with (e.g.) clipping drumbleats are due to the sharp corners at the edge
of the adjusted region. 

The right answer is to impose some sort of more gentle automatic-gain-control
around the clipping samples.  Traditionally that's done with a feedback
circuit that reacts rapidly to clipping (or near-clipping) and slowly to 
too-quiet sounds. 

The usual auto-volume AGC, though, has more restrictions than Audacity:
a physical circuit can't react to a signal before the signal gets to the
circuit (duh), but within Audacity the code has access to the whole
audio track -- so it can react in advance of the high amplitudes that cause
the clipping.

On my wish list is an "AGC" tool with two slider parameters and a
checkbox: a width (adjustable in log scale from about 10 ms to about
100 seconds); a "hardness" (adjustable from "do nothing until the
signal would clip, then just maximize it" to "start reacting when the
signal gets within about 6 dB of clipping"); and an "increase volume"
checkbox that indicates whether the sound should be amplified as well
as attenuated.  The AGC tool would impose a (unity minus Gaussian)
envelope around clipping points in the signal, reducing their volume
accordingly.  The 10ms width would be good for (e.g.) getting rid of
the two or three highest-amplitude drumbeats in a track, while the
100sec width would be good for processing live recordings or ripped
LPs without having to twiddle the volume much.