linuxaudio.org

[Harry van Haaren]

quoted text
>On Mon, Mar 18, 2013 at 11:45 PM, Fons Adriaensen 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:

http://en.wikipedia.org/wiki/Rise_time

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