Notes for MoctaveCat20
980801 Sam Streeper

This is preliminary documentation for my first patch for my new
Nord Modular.  This patch mimics the architecture of the old Octave
Cat analogue synth, my favorite monosynth.  As much as possible,
I also tried to duplicate the sound and feel of that synth.

(later notes 990516 I'm placing this text file in a zip archive
of several MoctaveCat patches that I use.  These patches show some
of the capabilities of this synth (the "synth" here being the
virtual octave cat.)  Some of these patches are actually doable
on a real Cat and on rather a few of them I took some Modular
liberties.  My "reference" synth here is the MoctaveCat20 which
I haven't changed since I first released it.  The only real bug
I've found compared to the real Cat is that the square LFO that
modulates the oscs should be unipolar.  In some of these patches
I fixed that by adding a level-shifter module, but generally I
ignore this oversight.  Anyway, if you want to build "authentic"
Cat patches, MoctaveCat20 is a better start than some of the others
where I may have retuned oscillators within VCO1 (say) or added
modulations or somesuch.  Enjoy!)

--------------------------------------
Editor Conventions
--------------------------------------
I named all the interesting modules in this synth.  If there are
no symbols preceding the name, either I am indifferent about editing
the parameters in this module in the editor, or the values are
important enough that I exported their control to an external
controller.  If I put a '+' symbol before the name, that indicates
a module that has values that I want to encourage myself to tweak
in the editor, like significant or interesting modulations or
routings that I didn't export.  In the case of the switches a '+'
usually indicates that I intend to try different routings, but the
mix levels in the switch may be specifically set and I generally
won't mess with those.  If a module name is preceded by a '!', that
indicates a structural element or logic setup that is carefully
set (a necessary routing or a test) that I generally don't intend
to mess with.  In other words, these tweaking these modules is more
likely to break the patch than to provide interesting modulations.
I don't think I have any such discouraged modules in this Moctave
Cat.  In this document if I refer to the Cat I mean the Octave Cat,
not the Moctave Cat.

--------------------------------------
Transient Generator section
--------------------------------------

The ADSR amounts are controlled by the sliders on my QS8 keyboard
(my master controller), which are set up as controllers 12,
13, 91, and 93 respectively.

The Attack/Release (AR) sliders are not exported.  They can be set
in the "+AR" module, which looks suspiciously like an ADSR envelope.

The Cat's "ADSR repeat" function is not exported.  It can be set
in the editor between off and gated in the "+ADSR repeat" module.
The additional non-gated auto-repeat functionality of the Cat can 
be achieved by decoupling the envelopes' gates from the voice gate 
and holding it high.

The Cat's sample and hold switch is not exported, but it can be
set in the editor with the "+S&H source" module.  It samples either
VCO1 or noise, and is triggered by the LFO.


--------------------------------------
Lower Left section
--------------------------------------

The duties of the pitch slider are relegated to the pitch
controller.  The Cat's octave shift button is not implemented as
I have a big keyboard.

The Cat's glide (portamento) slider is not implemented in the
Moctave Cat; it should be there but I haven't figured out the right
way to do it yet.

The LFO frequency is exported on knob 1.  The Cat's LFO supplies
a square wave (this is also used to trigger the ADSR repeat if
enabled) and a triangle.  In the Moctave, I substitued a sine wave
for the triangle, since it gives a better sounding pulse width
modulation and smoother vibrato and otherwise doesn't matter on my
Cat patches.  I think the original Cat designers would have used
a sine wave here if they could have generated it inexpensively.



--------------------------------------
VCO1 section
--------------------------------------

Coarse tuning is on knob 4, fine tuning is not exported but can
done in the editor by tuning the VCO1 master osc and the 3 slaves.
The slaves are not phased accurately to the real Cat which means
you can't really do that goofy Cat note-sequencing thing accurately,
but I've always thought that that's more interesting than useful.
As long as the phases are wrong, I slightly detuned the slaves
because I like that sound better.

The Cat's VCO1 keyboard control switch isn't fully implemented.
If you bump the Moctave polyphony down to 1 voice, then the Moctave
emultates the Cat's mono mode, and if you turn off keyboard tracking
in the "VCO1 pulse" module, it's like turning this switch off.  I
don't thing the Nord can really emulate the Cat's weird poly mode,
since that would require a bit of logic processing on the uncooked
keyboard status and the Nord really only forwards processed keys.

The VCO1 modulation routing switches are not exported, but they
can be set in the editor.  The first switch selects LFO sine (my
default), LFO square, or sample&hold as the modulator and is labeled
"+VCO1 mod1".  The second switch selects ADSR, the AR envelope, or
VCO2 (default) as the modulator, and this one is labeled "+VCO1
mod2".  The Cat has a single knob to select the amount of modulation
for each of the mod sources, and these knobs are in the "VCO1
modamt" module and exported as knobs 2 and 5.  The VCO1 mod amount
module also mixes in a bit of weather modulation since my VCO1 is
not entirely stable.  I did not have enough free modules to
accurately model the effects of weather on my Cat though!

The Cat allows you to either set the amount of VCO1 pulse width
modulation or to dial in the width of the pulse.  This functionality
is not exported but is found on the "+PulseMod switch" module.
Unlike most of my switches, I do need to set the knobs on this
module corresponding to the selected switch.

Finally, VCO1 supplies 4 waveforms: a suboctave square wave (1
octave down), a sawtooth, a triangle, and a pulse.  You set the
mix level for each of these waveforms on knobs 3, 6, 9, and 12.
These mix levels determine both the sound of VCO1 and the degree
of FM modulation of each VCO1 waveform on VCO2 if you have dialed
in FM on VCO2. (That's knob 11 and the default selection of switch
3 on the "VCO2 mod2" module for those of you who have read ahead...)
Similarly, the levels of the VCO2 waveforms will affect the amount
and sound of FM in this VCO1 section if you left the default routing
alone and dialed up knob 5.  By the way, if the knob assignments
aren't obvious (and I don't expect that they should be yet) there's
a simple organization to them that I will explain near the end of
this document.

--------------------------------------
VCO2 section
--------------------------------------

VCO2 coarse tuning is exported on knob 7.  VCO1 can be synced to
VCO2 with the switch in the "+Sync VCO1" module, though this doesn't
sound quite like the Cat's sync.
On the Cat, you can get a nice percussive sound by using AR as the
main envelope and using a quick ADSR envelope with 0 sustain to
modulate the synced pitch of VCO1. 

This section largely mirrors the VCO1 section, so "+VCO2 mod1"
selects LFO sine, LFO square, or S&H as a VCO2 modulator, and "+VCO2
mod2" selects ADSR, AR, or VCO1 as a modulator, and the mod levels
are set in "VCO2 modamt" and exported as knobs 8 and 11.

VCO2 supplies 3 waveforms: suboctave square, square, and saw, and
their mix levels are exported as knobs 10, 13, and 16.

--------------------------------------
Filter section
--------------------------------------

I use the Nord's filter D for the Cat filter, since this one is
closest to my Cat's sound.  By the way, my Cat is the original
model with discreet components, later Cats like the SRM have
completely different filters.  The Nord's D filter is the one that
comes closest to the Cat's high end sizzle, though its frequency
range doesn't go nearly high enough; The Cat filter has a magical
sound as it emerges from super-audible frequencies that no digital
filter that I have heard can match.  Also the D filter has the
closest behaviour in that magical hi-Q zone near oscillation, but
again a digital filter and midi controller really can't do justice
to an analogue filter and a pot.  Finally, at hi-Q, the D filter
thins the bass frequencies the least and again, this behaviour is
closer to the Cat sound than the other Nord filters.  There is a
bummer to filter D, though;  Its modulator input is control rate
rather than audio rate, which cruds up the filter if you attempt
to FM it, and filter FM is yet another key to the Cat sound.  I
don't know what to do about this yet.  Note that I left an E filter
in there as well; If you dial that one in with the "+Filter picker"
module you can get a decent Minimoog-like bass if you dial the
other parameters in right.

The Cat switch for VCO1 audio out is not exported but can be set
by turning off knob 1 in the "+VCF Input Mix" module; this turns
off its audio without disabling its FM abilities.  Filter keyboard
tracking is not exported but can be set in the "VCF" module.

Like the VCOs, there are two switches in the filter section for
mod routings, and they are not exported.  "+Filter Mod1" selects
between LFO sine(default), LFO square, and S&H to modulate the
filter cutoff frequency, and "+Filter Mod2" selects between ADSR,
AR, and VCO1 (default) to do the same.  On the Cat, you can get
some great nasty but still musically useful sounds by FM'ing the
filter frequency.  A good modification for the Cat would be to use
an envelope to just FM the filter frequency on the attack.  The
filter modulation amounts are set in the "Filter Mods" module,
exported as knobs 14 and 17.  This module also includes a non-Cat
mod routing: the third mixer sets the amount that ADSR modulates
the cutoff frequency and this level is assigned to the main
modulation controller (my mod wheel).  This allows me to use both
FM and ADSR on the filter, which I always wanted on the Cat.

Finally, filter frequency and resonance (Q) are exported on knobs
15 and 18.


--------------------------------------
VCA section
--------------------------------------

Still reading?  Well we're almost done since the Cat is almost out
of knobs, and the Nord is completely out of knobs...

The switch for amplitude level is not exported, but it can be set
in the "+AEnv choose" module.  The selections are ADSR, AR, and
bypass, and by default I use the ADSR envelope to determine the
output amplitude.  On the Cat, you often use AR as the amp envelope
when you are using ADSR for effects like a filter sweep or transient
pitch weirdness.  Bypass mode makes for a great drone.

And for the final knob, I don't export the Cat's noise control but
you can set it with knob 3 in "+VCF Input Mix".  A worthwhile
modification to the Cat would be to route the noise through an
envelope, but that's not how the Cat does it.

--------------------------------------
Knob organization
--------------------------------------

There's a method to the knob organization that makes it reasonably
easy to remember, especially if you're used to the real Cat.  The
first vertical block of 6 knobs is loosely dedicated to VCO1, the
second block to VCO2, and the third block to the filter.  The middle
row is dedicated to modulators, so by default knobs 2 and 5 modulate
VCO1 by the LFO and FM respectively, and knobs 8 and 11 do the same
thing for VCO2, and knobs 14 and 17 do the same thing for the
filter.  The 4 knobs in the lower left select the output levels
for the 4 VCO1 waveforms, and the upper right 3 knobs do the same
thing for the 3 VCO2 waveforms.  That leaves us with 3 knobs in
the upper left which are LFO speed, VCO1 tuning and VCO2 tuning,
and 2 knobs in the lower right which are filter cutoff and
resonance, just where they are on the Cat.

--------------------------------------
Interesting modifications
--------------------------------------
A lot of the Cat's nasty sound comes from all those rough edges in
the waveforms.  You can smooth the Cat a good deal by changing the
suboscillators to sine waves.  This will lose a bunch of high
frequencies from the sound; it can sound very good but it won't
sound like a Cat.

Many of the weirder Cat sounds are had when you do bi-directional
FM, where VCO1 modulates VCO2 and vice-versa.  You can route a
signal for FM into either an oscillators pitch input or its FM
input.  These inputs have very different ranges and somewhat
different responses, so I let you pick whichever sound works better
for you; pick the ones you like in each of the two "+FM type"
modules.


-------------------------------------------

I hope you enjoy this synthesizer.  I've had my Nord Modular for
about a week now and this is my first "finished" patch.  Let me
know what you think; I love getting feedback on my sounds!

(later note: I've had my nord for quite a while now.  Of course I still
love this amazing machine.  And I still turn to the MoctaveCat all the
time when I need to dial in some vintage sound quickly and I don't
have proper time to construct a nord patch!)
cheers,
Sam Streeper
(sams@best.com) (<- This address will go away soon, I don't know yet 
what my next one will be, sorry!)

