# [PD] biquad~ with elementary filters [was: Re: dinosaurs ...]

Damian Stewart damian at frey.co.nz
Mon Sep 15 14:49:32 CEST 2008

```Charles Henry wrote:

>> wow - this is a bit above my head at the moment. i can go from equations in
>> C to pd, but only if i have the equations themselves to look at...
>
> It's easy.  I've done it half a dozen times.  For example,
> http://lists.puredata.info/pipermail/pd-list/2007-01/046315.html
>
> Look at the help file for biquad~ in Pd.  The equations are there.

last time i looked at the equations in biquad~'s help patch, my brain melted.

> Here's the transfer function:
> Y/X=(ff1 + ff2*z^(-1) + ff3*z^(-2))/(1 - fb1*z^(-1) - fb2*z^(-2))
>
> or
>
> Y/X=(ff1*z^2 + ff2*z^1 + ff3)/(1*z^2 - fb1*z^1 - fb2)
>
> All that you need to do to find the complex zeros is to use quadratic formula
> on the numerator (ff1*z^2 + ff2*z^1 + ff3)
>
> and do the same to find the complex poles on (1*z^2 - fb1*z^1 - fb2)

dude, that's not 'easy'. i can barely remember how the quadratic formula
works with pen and paper, let alone in C, and let alone to the point where
i could confidently transfer from C to Pd. i'd have no idea where the bugs
were... and indeed, no idea

and once i've got these 'complex zeros' and 'complex poles' i don't have
any idea how to plug them into czero~ and cpole~ (or is it rzero~ and
rpole~?) objects.

--
damian stewart | skype: damiansnz | damian at frey.co.nz
frey | live art with machines | http://www.frey.co.nz

```