[LAD] A short technote on digital state-variable filters

Gordonjcp gordonjcp at gjcp.net
Sat Nov 21 23:17:31 CET 2020

On Thu, Nov 19, 2020 at 10:05:00PM +0100, Fons Adriaensen wrote:
> Hello all,
> Some people have asked me to provide a bit more documentation
> on the state-variable filters I used in zita-eq1 and zita-jacktools.
> So I've written a short technical note on this. You can find it at
> <http://kokkinizita.linuxaudio.org/papers/digsvfilt.pdf>

I wish I was good at maths!  This makes it all look so simple, and it works well.

I guess with the corrected parameters, you can't get it to self-oscillate without Q really being infinite?  With the uncorrected parameters it seems to go "over unity" with Q set to about 10, but a soft clipper would stop it blowing up.

It behaves well if you interpolate the c1 and c2 values to sweep the filter across a block of samples, which is handy.


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