[LAD] Mixing audio: Noiseless volume changes

Tim Goetze tim at quitte.de
Tue Mar 19 03:26:21 UTC 2013

[Harry van Haaren]
>On Mon, Mar 18, 2013 at 11:45 PM, Fons Adriaensen <fons at linuxaudio.org>wrote:
>> A critically damped
>> second order lowpass with a rise time of 30 ms or so will eliminate all
>> audible artefacts. It's very low on CPU and you only need to run it while
>> the gain is changing.
>Although I understand the words & their meaning, I have no idea how to
>start implementing such a filter.
>I've found a 1st order algorithm on musicdsp:

A 2nd-order IIR filter is often called a "biquad"; at musicdsp, look
for that instead.  There should also be plenty of formulae for
computing 2nd-order lowpass filter coefficients.  Those following
tradition (like RBJ's lovely cookbook at musicdsp) will ask for filter
Q, which is 0.5 for critical damping.

>How is the "rise time" determined here?

As a function of the filter's damping (zeta = 2*Q) and frequency:

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