[linux-audio-dev] Paper on dynamic range compression

[Jens M Andreasen]

On Wed, 2006-10-18 at 08:28 +1000, Erik de Castro Lopo wrote:
> Dan Mills wrote:
> 
> > 
> > --- Erik de Castro Lopo <mle+la at mega-nerd.com> wrote:
> > 
> > > You need a low pass filter on the control signal. It
> > > should
> > > be somewhere well below 1kHz.
> > 
> > Agreed that you need the filter, but a 'brick wall' at
> > 1Khz means that anything faster then 50ms or so as an
> > attack time (and there are legit uses for such), will
> > itself overshoot horrribly due to the gibb effect of
> > bandlimiting the control signal.
> 
> This is the way analogue compressors work. If you have a 
> sound with a fast transient going into a slow attack 
> compressor, the transient passes throught pretty much
> untouched (apart from any clipping that may occur due
> to other parts of the design).
> 
The clipping of the signal could be designed to sound resonably good by
having the area around zero to be linear, but as you get closer to the
extremes, the response curve bends inwards, fitting a virtual headroom
of say 36 dB into only 12 dB of the mastered 16bit signal. Overdrive I
think is the proper name?

A sine wave transient too fast for the compressor to handle immediately
would then start out as a square with gentle rounded corners instead of
a square with a flat-roof haircut. The distribution of the added
frequencies is thus moved to lower octaves where the distortion is less
dangerous/disturbing.

An improvement could then be to apply a 2 dB/oct high-pass before the
overdrive circuit followed by the inverse after the circuit, emphasizing
the effect on the faster transients and leaving deep sine waves to use
the full dynamic range unaltered, without distortion. This would also be
an approximation of magnetic tape dynamic response.

50 ms still appears like a long time. Was there a technical reason not
to go down to around 20 ms to reach the psychological sense of
"simultaniously"? 
Or perhaps 50 ms is a good value? Something very dramatic obviously
happened in the signal when the compressor kicked in, so moving the
"drama" from dynamics into short bursts of distortion  might actually
help in preserving a sense of "naturalness."




> Erik
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