[sdiy] Pole Mixing (with Resonance)
dave at expeditionelectronics.com
Fri Apr 9 22:47:56 CEST 2021
Got a resonance slider added. I had to attack the math a few different
ways, but finally got the proper cancellations and looks very simple in
retrospect. Definitely ruins the HP modes. :)
On 4/9/21 2:03 PM, David Moylan via Synth-diy wrote:
> Hi Brian. This was inspired by the Xpander/Matrix filter but is more
> of a general exploratory tool at this point. I think most of the
> names overlap but there are definitely some not implemented on the
> Obies, such as HP4 and the double notch curves. Those require all 4
> poles plus a dry signal and the Obies don't have that capability. The
> curves are calculated on the fly from the theoretical transfer functions.
> I'm in algebra hell right now trying to work the math out to handle
> the feedback signal. It would be a lot easier with no dry signal but
> once you start mixing that in you get different denominators that have
> to be resolved and suddenly I'm looking at 8th order polynomials :(
> On 4/9/21 1:53 PM, Brian Willoughby wrote:
>> This is excellent! Thank you for sharing this online.
>> As a Matrix-12 owner, I appreciate the list of presets that cover the
>> available modes. It looks like you've even included some modes that
>> aren't available in the Oberheim. Is that correct?
>> Did you use impulse response measurements to obtain the frequency
>> The reason I ask is that I wrote a macOS CoreAudio application that
>> can load various AudioUnit plugins and measure their frequency
>> response by simply feeding a short, 1-sample impulse into the plugin
>> and doing a Fourier transform on the result. This only works for pure
>> LTI (Linear Time-Invariant) plugins, but it's spot-on for those that
>> fall into the category (i.e. no distortion or envelope-controlled
>> On Apr 8, 2021, at 15:24, David Moylan wrote:
>>> Hi All. I banged together a little web app to play around with
>>> filter pole mixing, of the Oberheim Xpander type. You can mix poles
>>> in varying amounts and see the output magnitude shape as well as the
>>> transfer function. Y axis is Db and X axis is log scale based on
>>> normalized frequency (so basically 1 equals the cutoff frequency).
>>> Haven't done phase plot yet.
>>> If you have an interest in this sort of thing check it out:
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