MOTM

EMS VCS3/Synthi 
LPF
http://www.ems-synthi.demon.co.uk/

Have only heard random clips here and there but this machine/filter 
is supposed to be one of the weirdest/most unique around.

Roland Juno/SH-101
HP/LPF
http://members.optushome.com.au/coolcolj/CCJ_Juno106.mp3

So nice and mellow - I'd really like to see one of these in MOTM 
form.

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

A few comments to your mail, Bruce as well as to George's original mail:

>Synth: Korg PS-3100
>Filter: three 6db/oct BP
>MOTM: 410

One MOTM-420 (per voice), for the VCF(s) in the PS-3100, plus one
MOTM-410 (for all voices in common).


>Synth: Oberheim Ob-Xa
>Filter: multimode 12/24db/oct LP w/res
>MOTM: is this simply a subset of the Matrix-12?

Just use the standard MOTM filters without overdrive:
MOTM-440 (bass enhance off) for 4-pole and the soon to come
SEM-style VCF for 2-pole. (While nothing in the world will replace
anything else to 100.0%, this will be very close. Oberheim tried
to emulate their own SEM VCF with the OB-X 2pole mode, after all.)


>Synth: Roland Jupiter 6
>Filter: multimode 24db/oct LP/HP or 12db/oct BP, w/res
>MOTM: future?

This is from memory (must check the schemos), but I think the JP-6
has two state variable filters in series. They don't have individual
control,
you just select the configuration and the (common) cutoff frequency.
So I'd say you can emulate it quite well with a pair of the announced
SEM-style filters.

Which brings me to the following (long quote included):

> 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.

No need for a CPU here. A pair of SEM-style VCFs and some (analogue)
processing of the CVs will do. What you need is to send one CV to
both cutoff modulation inputs, and a second CV to one input directly,
and to the other inverted. You have all degrees of freedom here - that's
part of what Modular is all about. BTW, that famous Serge filter with two
resonant peaks does nothing else (to my knowledge, at least).


>Synth: Yamaha CS-80
>Filter: 12db/oct LP w/res and 12db/oct HP w/res in series
>MOTM: is this akin to the Korg MS and therefore covered by the 420?

Not exactly. A better approximation will (surprise !) be done with the
SEM-style VCF, I mean with a pair of these. I have done some research on
the CS-80 VCF some months ago, and it's really just a pair of state
variable filters, *but* with both, cutoff and resonance ranges limited
to rather conservative values. So, don't turn the resonance all the way
up, and limit the cutoff range, and you should get a nice CS-80 VCF
emulation with two SEM-style VCFs in series.

JH.

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




Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/

I am very excited about this as well. I have been poking around with 
it off and on trying to get it to work as I wish on a breadboard, but 
so far it does not.

It will go into self-oscillation. The idea of the pair of 420's is not 
too far off, but the sound qualities are very different.  The filter 
has a very distinct "howl" to it when the Q gets up there.  I have an 
OSCar and it sound pretty fantastic on that. 

Earlier this year another list member came up with a circuit that 
could generate separation voltages.  The OSCar filter as I had been 
trying to get work as CV for:

Cutoff
Resonance
Separation
Overdrive

The original OSCar circuit will overdrive pretty easily, and it has 
some nice distortion that adds to the characteristic howl.

In addition there would need to be switches for

Slope:
12 db
18 db
24 db

Mode:
Lowpass
Bandpass
Highpass

So that is the though anyhow, but so far it is not behaving itself in 
the lab.  I am a software guy, not a hardware guy, and I am doing this 
mostly to amuse myself.  Some of the difficulties include the fact 
that the OSCar schematics are hand drawn, missing some part 
descriptions, and (I believe) wrong in some parts.  So one day it will 
probably work, till then it is just a science experiment.

Bruce

A few corrections or clarifications interspersed below:
>>>>>
Synth: Moog Minimoog/Modular
Filter: 904A 24db/oct LP w/res
MOTM: 440
<<<<

The 440 is the equivalent of the Prophet Rev 1 and 2 filters, not a 
Moog ladder. Technically an OTA ladder instead of a transistor 
ladder. An Oakley Superladder or Multiladder is closer to the Moog. 
Paul will eventually release a transister ladder filter that would be 
more equivalent.
 
>>>>
Synth: Moog Rackmount Module
Filter: 12-stage phaser (today: Big Briar MF-103)
MOTM: future?
<<<<

A MOTM voltage controlled 12 stage phaser already exists on 
breadboard and it sounds great! There used to be some demo sound 
files on the SynthTech site somewhere. It will be released eventually.

>>>>
Synth: Oberheim 2/4/8 voice (SEM)
Filter: multimode 12db/oct LP/BP/HP (sweepable) or Notch, w/res
MOTM: I think I remember seeing this on a "future" list somewhere?
<<<<

Yes, this will probably be the next filter released.

>>>>
Synth: Oberheim Matrix-12/Xpander
Filter: multimode 6/12/18/24db/oct LP/BP/HP/Notch/PhaseShift w/res
MOTM: future?
<<<<

Paul has been talking about this one a long time, too. I expect to 
see it someday.

>>>>
Synth: Yamaha CS-80 
Filter: 12db/oct LP w/res and 12db/oct HP w/res in series
MOTM: is this akin to the Korg MS and therefore covered by the 420?
<<<<

No. The Yamaha has a smooth creamy sound - "like buttah". What a 
great filter set to have, though!

Moe