[LAD] Mixing audio: Noiseless volume changes

[Fons Adriaensen]

On Tue, Mar 19, 2013 at 08:15:41PM +0100, Tim Goetze wrote:
> [Fons Adriaensen]
> >Exactly the same with the form I proposed, w, a, b need to be computed
> >just once, not for every gain change. In fact only w depends on the
> >sample rate, a and b are fixed constants. 
> 
> Ah yes, sorry, I see that now.  
> 
> If that extra operation comes around to bite hard enough, I'll be sure
> to chain two 1st-order lowpass filters.

There is a much more important (in some use cases) feature of
this form: you can change 'w' (cutoff frequency c.q. settling
time) instantly without introducing transients into the signal.
Doing this with a biquad would generate a step in the output.

So it's the ideal form for a 'voltage controlled' filter.
In fact the famous 'Moog ladder' is just the same but using
four first order sections instead of two.

Ciao,

-- 
FA

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