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<DIV dir=ltr align=left><SPAN class=308501919-11062018><FONT color=#0000ff
size=2 face=Arial>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!)<BR></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=308501919-11062018><FONT color=#0000ff
size=2 face=Arial>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).</FONT></SPAN></DIV><BR>
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<FONT size=2 face=Tahoma><B>From:</B> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <B>On Behalf Of </B>Guy
McCusker<BR><B>Sent:</B> Monday, June 11, 2018 3:12 AM<BR><B>To:</B>
synth-diy@synth-diy org<BR><B>Subject:</B> Re: [sdiy] Filter
slopes<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV><BR></DIV>
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<DIV class=gmail_quote>
<DIV dir=auto>On Mon, 11 Jun 2018 at 10:35, Tom Wiltshire <<A
href="mailto:tom@electricdruid.net">tom@electricdruid.net</A>>
wrote:<BR></DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex; PADDING-LEFT: 1ex"
class=gmail_quote><BR>> On 11 Jun 2018, at 09:32, Neil Johnson <<A
href="mailto:neil.johnson71@gmail.com"
target=_blank>neil.johnson71@gmail.com</A>> wrote:<BR>> <BR>> For a
practical implementation hunt out the Oberheim multimode filter
circuit:<BR>> <A
href="http://www.synfo.nl/servicemanuals/Oberheim/OBERHEIM_MATRIX-12_SERVICE_MANUAL.pdf"
rel=noreferrer
target=_blank>http://www.synfo.nl/servicemanuals/Oberheim/OBERHEIM_MATRIX-12_SERVICE_MANUAL.pdf</A><BR>>
<BR>> See page 4.<BR><BR>While the Oberheim is definitely the original
source of this idea</BLOCKQUOTE>
<DIV dir=auto><BR></DIV>
<DIV dir=auto>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. </DIV>
<DIV dir=auto><BR></DIV>
<DIV dir=auto>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,</DIV>
<DIV dir=auto>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...</DIV>
<DIV dir=auto><BR></DIV>
<DIV dir=auto>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. </DIV></DIV></DIV></BLOCKQUOTE></BODY></HTML>