[sdiy] Pole Mixing

David Moylan dave at expeditionelectronics.com
Fri Apr 9 20:03:46 CEST 2021

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 response?
> 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 dynamics).
> Brian
> 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:
>> https://expeditionelectronics.com/Diy/Polemixing

David Moylan
Expedition Electronics
sonic adventures!

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