VCOs as modulation sources

[gstopp at fibermux.com]

     Audio-frequency modulation of one VCO by another in a modular system 
     is one of my favorite areas of sythesis. It's fun because the sounds 
     can be sooooooo complex, but at the same time it's frustrating because 
     repeatability and maintaining intonation are problems. The tiniest 
     analog imperfections and drifts cause huge changes from one moment to 
     the next.
     
     (This is exactly where the Chowning/Yamaha FM stuff has the advantage 
     over analog - but to me, that's like riding a mechanical horse rather 
     than a real live bucking bronco - *too* controlled).
     
     But - the best machines for these experiments are home-built modulars! 
     At least that's what I've found. The first modular I ever built, back 
     in high school in 1976, is the *best* FM thing I've ever played 
     with!!! The potential modulation depths are about twice as deep on 
     that machine than on any other commercial system I've got.
     
     One cool effect is to modulate one VCO with another, both at audio 
     frequencies, and dynamically change the modulation depth from zero to 
     maximum. This gets really intense only when the modulating waveform 
     has a substantial amplitude, like 10 volts pk-pk. As you open up the 
     mod pot you will hear wild sideband-like sweeps that repeatedly 
     transcend the audio range, all the time accompanied by complex 
     inharmonic overtones much like a ring modulator.
     
     My Moog modular is a 901-based system, and the 901 VCO outputs are 
     line level (just a volt or two), pretty lame for FM experiments. 
     Someday I plan to add non-inverting positive gain buffers to all the 
     VCO outputs....
     
     The ARP 2600 is better, but the sliders jerk a little while you change 
     FM depths and that messes up the sweeps. Rotary controls are better 
     for this because they're smoother for micro-changes in amount, and a 
     VCA would work, but the 2600 has only one VCA.
     
     I have a tiny notebook-sized modular made from Emu parts, which I made 
     just a couple years ago and haven't really gotten the chance to use 
     much, and I've noticed that it also has this great wide-sweep gnarly 
     sound capability.
     
     I really want to mess with this more - my stuff has been in storage 
     for so long, and now we've moved to a really huge house and I've got a 
     whole "wing" to dedicate to music stuff, and I'm gonna go crazy as 
     soon as we get all moved in. Can't happen soon enough....
     
     Anyway I just wanted to chime in about this subject. I haven't noticed 
     much conversation about it and I think that it's mostly unexplored 
     territory because mod depths on commercial machines just don't reach 
     far enough. I'm dying to dive into it again.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: VCOs as modulation sources
Author:  Christopher List <Christopher_List at sonymusic.com> at ccrelayout
Date:    7/12/96 11:22 AM


Hi DIYers - 
     
This is more a "Modular Heaven" topic than strictly DIY, but I thought this 
would be the best forum for it. 
     
Last night I got the EN "Timbre Modulator #2" mounted to the panel and installed
in my modular. This does 0 rectification through full wave rectification of the 
input signal (with a pot control). Then it does VC xfading of this signal with a
squared-up version of the original signal. It's also the second timbre modulator
in the Barry Klien book. 
     
Anyway, I set it up with a sawtooth wave in. I set the CV input to another VCO 
- in tune with the first one. This created some great waveforms depending on 
what wave shape the CVing VCO was set to and what I had the FWR amount set to. 
Really interesting sounds that went from "quantized" type sequencer-generated 
waveforms, to (as you might expect) FM type waves, to weird resonant waveforms. 
Kinda different from any sound I had ever gotten from the modular while still 
being very musically useful. 
     
It occurred to me that the VCO as modulator, in tune with the audio VCO, and 
controlled by the same CV is one of my favorite modular routings. Using it for 
things like audio frequency xfading, amplitude modulation, PWM, and of course FM
(of VCO's and filters), always seems to create really cool thick sounds. One of 
the nice things is that you can "pinch" the pitch of the modulating VCO with 
little cv's and get the sound to jump into weird side-band frequencies before 
settling back in tune - or run the pitch CV of one VCO through portamento, and 
the other one straight for a similar effect.
     
I was wondering if anyone had any other interesting uses for this type of 
modulation they might like to throw out for discussion...
     
- Chris