A fundamental problem of exponential VCOs?

[Gene & Debby Stopp]

I too find a nice detune to be quite pleasing and all too often
uncontrollable over a keyboard's range. Sometimes a machine will somehow
magically just do it as I'm playing with it (due to random variations in
trimming errors) but this certainly can't be relied upon if you want that
sound for a gig! Perfect trimming is of course the best way to maintain a
machine but then the beating effect you describe will happen.

Has anybody tried this - one exponential converter, driving two oscillator
circuits? In other words, split (or mirror) the exponential current into
two integrator/comparator circuits, and then alter some parameter to make
the oscillator sections slightly different. Some thoughts include:

* Alter the discharge trip point voltage slightly (for sawtooth-based
designs)
* Alter the schmitt trigger trip points (for tri-square designs)
* Vary the integrator cap values (they'll be different anyway, by some
amount)

Will this just cause the same effect as two detuned exponential VCO's? If
so, is there some other parameter I haven't listed above? I suppose that it
would become obvious given some thought, or after making some drawings on
paper, but I'd rather toss the question out to see what comes up... :)

- Gene

----------
> From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
> To: 'DIY' <synth-diy at horus.sara.nl>
> Subject: A fundamental problem of exponential VCOs?
> Date: Thursday, July 10, 1997 9:30 AM
> 
> Hi!
> 
> I spent some thoughts on beating between a pair of exponential
> (V/Octave) VCOs.
> 
> I found that I run into one problem quite often: When I detune the VCOs
> so much
> that I get a decent beat rate of 0.2 to 1 Hz (which I find very
> pleasant), the VCOs
> sound very detuned in higher octaves, *regardless* of the scale
> adjustment of each
> VCO.

<snip>