MOTM

Bruce,

The OSCar filter description sounds fantastic!  Think of the possibilities
if this filter were redone in MOTM format.  Actually, it could get quite
complex if one were to try and make use of all of the possibilities.  There
could be a switch to set 2x2-pole, 1x3-pole or 1x4-pole.  Multimode settings
could be HP, LP or BP for each section.  CV in for separation.  Each section
has CV in for FC and Q.  Possibly offer multiple tap points (and use 830 to
mix).  Very interesting!

In the scenario you describe where there are two 12db sections, if one set
to HP and the other set to LP, it sounds a little like two 420s in serial
(one HP, the other LP) has that covered.  You have control of CV for the FC
and Q for each 420 (aka. "section"), essentially a very flexible BP.

At high Q levels, does this filter support going into self-oscillation?

George
----- Original Message -----
From: <bruce@...>
To: <motm@yahoogroups.com>
Sent: Friday, June 22, 2001 3:39 PM
Subject: [motm] Re: The Major Vintage Filters


Not misplaced at all.

One filter that I think is over looked in your list (it often is) is
the multi mode filter in the Oxford OSCar.  The OSCar was a hybrid
Digital / Analog synth from the mid 80's.  The waveforms were produced
by the Z80 CPU that ran the beastie, and the rest of the signal path
was analog.

OSCar was designed by Chris Hugget, the person who was behind the EDP
synths before.  The EDP Wasp used a pair of OTAs to produce a multi
mode 2 pole filter.  This filter exists as a module for some german
company today, as well as Jurgen's schematic as well.

The OSCar filter put two of these filters in series to create a 4 pole
multi mode filter, with a couple of interesting twists.

The filters funtioned as 2 12 db sections, and each section could have
both low pass and high pass output. Chris could have designed BP and
NF in there as Jurgen did, but wait for the interesting trick.

Because the whole thing was under CPU control, there were some
interesting manipulations that could happen in the circut.  First of
which is a knob called "Separation".  The 2 12db stages could have
their cutoff frequencies controlled independantly.  By moving this
knob the FC would be moved up or down (depending on stage and setting)
to widen out the resonance peak.  At high Q levels this was (and still
is for those of us with OSCars) lots of fun.

MODE        STAGE 1    STAGE 2     SEPARATION
Highpass    HP         HP          +Stage1 / - Stage2
Lowpass     LP         LP          +Stage1 / - Stage2
Bandpass    LP         HP          -Stage1 / + Stage2

Now, looking at the circuit, there are a lot more things that could
have been done.  Each stage is really 2 6db OTA based things, so it
should be possible to use 3 of the 4 to get an 18db response, or just
2 of them to do 12db.  Again because the CPU is supplying the CV for
cutoff and resonance, it should be possible to have nice separation
effects in each mode.

This circuit is based largely on the LM13600 OTA, but it should be
possible with the CA3080 as well (As Jurgen has done).

Great examples of the sound of this very unique filter:

Ultravox - Almost everything they did from Quartet on
J.M. Jarre - Oxygen and onward
Future Sound of London - ISDN, Dead Cities

Bruce

Further OSCar reading at:
http://www.sospubs.co.uk/sos/sep99/articles/oscar.htm




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