[sdiy] Pole Mixing (with Resonance)

Mattias Rickardsson mr at analogue.org
Sat Apr 10 16:07:29 CEST 2021

Great web app! Also great timing - I've spent some of the last weeks
simulating similar pole mixing in LTSpice. Now if the results get useful it
will seem like I just nicked the ideas from the internets! ;-)

Please do try implementing the bandpass feedback suggestions (there are two
of them - 2-pole and 4-pole bandpass feedback) from the AN701 application
note for the SSI2164 quad VCA.

It seems like the HP modes are still messed up, but not as severely as with
conventional feedback. I'm actually a bit surprised how bad it looks - does
the Xpander/Matrix-12 really have these problems with several of their
filter modes when resonance is turned up? Having perfect response only at
zero resonance leaves some room for improvement to say the least. I always
expected them to be mathematically perfect, but hardly ever tried them out.


Den lör 10 apr. 2021 12:09Tom Wiltshire <tom at electricdruid.net> skrev:

> It occurs to me that using bandpass feedback as described in the SSI
> application note might fix the problem with highpass modes. They propose it
> as an alternative way of doing passband droop compensation, but it might be
> a better way of doing resonance in a pole-mixing filter too.
> Clearly when you use feedback from the LP4 output, you’re feeding a lot of
> low-end signal back into the filter which then screws up the balance
> required for good stop-band performance in the highpass modes.
> I’ll give it a try one day!
> Tom
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
> > On 9 Apr 2021, at 21:47, David Moylan via Synth-diy <
> synth-diy at synth-diy.org> wrote:
> >
> > 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
> 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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