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

From: Steve Harris <S.W.Harris@email-addr-hidden>
Date: Fri Oct 06 2006 - 10:24:10 EEST

On Thu, Oct 05, 2006 at 08:14:35 +0200, David Olofson wrote:
> On Thursday 05 October 2006 19:59, Paul Winkler wrote:
> > On Thu, Oct 05, 2006 at 07:07:34PM +0200, Fons Adriaensen wrote:
> > > On Thu, Oct 05, 2006 at 05:12:20PM +0100, Steve Harris wrote:
> > >
> > > > The SC* plugins do the same as TAP (calculate the gain every 4
> > > > samples),
> > > > but I interpolate the gain values between each computation. The
> > > > attch/deacay times were slow enough in my testing that it was OK
> > > > to do
> > > > that.
> > >
> > > It should be OK for all practical attack/release times. The only
> > > penalty is 3 samples of delay on the gain change and maybe that's
> > > to be avoided for a hard limiter. For a normal compressor it
> > > should not matter.
> >
> > That is what, 90 microseconds at 44.1 kHz? I don't think there are
> > any analog compressors that react anywhere near that fast. Don't
> > worry about it :-)
>
> I don't know how fast it *actually* is, but FWIW, my old Behringer
> compressor/limiter has a lowest attack setting of 0.1 ms, and a
> lowest release setting of 50 ms.

So that would be a little iffy. SC4 only advertises that it goes down to
1.5ms, which gives something like 90 segments in the attack segment.

Looking at the code, the compressors use a pretty expensive linear -> dB
conversion routine (cubicly interpolated lookup table) to work out the
gain changes, maybe I could substitute a cheap approximation function.
I'll bet that analogue compressors didn't use accurace logarithmic
approximations. I'm not sure It'd be worth dropping the calcualtion period
below 2 though.

- Steve
Received on Fri Oct 6 12:15:01 2006

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