[sdiy] Filter slopes

[David G Dixon]

The Polaris filter takes a slightly different approach to the Oberheim
matrix idea published on the web.  The main difference is in how the
feedback input is handled.  Some of the modes require the fed-back input to
be summed into the output.  This is important for obtaining highpass and
allpass modes.  I was doing this wrong until someone on this list drew me to
the light.  (I think it might have been Rutger Vlek!)

Also, the Polaris uses input feedback to maintain bass response, and the
amount of this feedback is adjustable with a trimpot.  Hence, the resonance
character is continuously adjustable from wimpy (Roland-like) to
over-aggressive.  Somewhere in the middle of this trimpot, one obtains a
response a lot like the "Quad" mode of my earlier four-pole filter, the Dr.
Octature, which switched in stage-by-stage amplifiers to boost the bass
response (and give equal sine waves from each stage output in
self-oscillating mode).


  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Guy
McCusker
Sent: Monday, June 11, 2018 3:12 AM
To: synth-diy at synth-diy org
Subject: Re: [sdiy] Filter slopes




On Mon, 11 Jun 2018 at 10:35, Tom Wiltshire <tom at electricdruid.net> wrote:



> On 11 Jun 2018, at 09:32, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> 
> For a practical implementation hunt out the Oberheim multimode filter
circuit:
>
http://www.synfo.nl/servicemanuals/Oberheim/OBERHEIM_MATRIX-12_SERVICE_MANUA
L.pdf
> 
> See page 4.

While the Oberheim is definitely the original source of this idea


I don't know the dates of the various discoveries of this idea but Bernie
Hutchins wrote an article on how to create various responses by mixing the
stages of a four pole low pass in Electronotes 85 (1978). That predates the
Oberheim implementations that I'm aware of but there could be more. 

It might be worth adding that although the asymptotic slopes are always a
multiple of 6dB/Oct, the slope near to the cutoff frequencies can be
different, depending on the details of the integrator stages for instance,
and this may have an impact on the synthesis of acoustic sounds. Certainly
that was what the marketing copy of the Serge variable slope filter
implied...

There's quite a bit of material on variable slope filters and some stuff
about their use for acoustic sounds in Electronotes too, I think around
issues 109-120 but I don't have them to hand right now. 

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