# [PD] 2 pole lpf with resonance?

Frank Barknecht fbar at footils.org
Wed Feb 14 10:22:43 CET 2007

```Hallo,
Vreahli the Audio Bandit hat gesagt: // Vreahli the Audio Bandit wrote:

> Hiya. Okay - so here's the skinny. I set out about a half hour ago to
> figure out how to set up a 2 pole low pass filter with resonance in
> Puredata. Looks like that's apparently set up with a biquad~ filter. I
> read through the rather handy looking information over here at
> http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt. I came out of that
> a bit more confused than when I went in. I also bumped in to
> http://www.creativesynth.com/MAXMSP/001-SynthBuildingMSP/sbm_03.html -
> which left me drooling quite a bit more than when I went in. Is there
> any way to set up a LPF with a live-tweakable audio rate resonance and
> frequency setting while avoiding doing the biquad math by hand? I can't
> even figure out how to get my computer to crunch those numbers for me.
> Thanks in advance for helping me out with this seemingly simple yet
> convoluted question.

The biquad~ math isn't extremely hard, but of course it's still hard
if you have no experience/knowledge about filter theory yet. Actually
there was a long discussion about biquad two or three weeks ago, you
may want to check the archive.

The quickest route to getting useful biquad~ coefficients without
going through the math would be the filter objects in the ggee-library
(CVS at /externals/ggee/filters). They implement the equations from
the Audio-cookbook.

However [biquad~] doesn't have audio inputs to set the coeffs.
Alternatively you could use the elementary filters [rpole~], [rzero~]
etc. They accept signals for their parameters. Maybe I should sit down
and build a 2plp now ...

Ciao
--
Frank Barknecht                 _ ______footils.org_ __goto10.org__

```