# [sdiy] Low Cost Comb Filter Methods

Andrew Simper andy at cytomic.com
Thu Jun 14 15:32:18 CEST 2018

```On Tue, 12 Jun 2018 at 16:00, <rsdio at audiobanshee.com> wrote:

>
>
> Meanwhile, the help I was looking for was commentary on whether a
> 3-capacitor op-amp circuit would actually be straightforward to simulate /
> emulate in software. One of these days I’m going to have to learn how to
> use the Z-transform to convert a circuit diagram into lines of code. Seems
> like it should require only three words of state memory and a few cycles of
> math.
>
> Brian
>
>
That would be my cue :)

Yes, very easy to model in the linear case, but you will most likely be
better off just using a linear SVF if you want a whole bunch of them as it
will be way more efficient. The point of the design I think is you could
use a single op-amp per resonator, an SVF uses 3, but in digital it's
cheaper to just use an SVF since perfect opamps are free. The only point I
can see in preserving the structure of the filter in the paper would be if
you used Boyle macro model for the op-amp so you would get some non-linear
saturation effects in the implementation, but then things get quite a bit
more complicated.

If anyone is interested in the linear case then here are the equations to
solve the circuit:
https://cytomic.com/files/dsp/comb-filter-resonator-equations.pdf

and here is an implementation in python generator by my circuit solver:
https://cytomic.com/files/dsp/comb-filter-resonator.py

and a plot of the filtering of a sawtooth given one set of component values:
https://cytomic.com/files/dsp/comb-filter-resonator.png

Cheers,

Andy
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