Lin detune,MS20 Core,Polysix/Trident

[jh.]

> I dont agree, when i look at a Korg CCO i see a core
> wery similare to a Moog Rouge etc, core for example.
> The principle of "charge full" instead of the usuall
> short the capp is wery similare. But you are saying VCO
> and that is confusing!
[...]
> Still i dont agree in this reasoning about what is expo and lin
> CCO's to me they are all lin CCO cores but with different principles
> in the front end of the Core.I find it quite confusing when
> people refer to it as a expo or lin VCO, i think this issue has
> to be sorted out once for all!?

... and I had thought I was the one who helped sorting it out on
this mailing list (;->).
But seriously, it all depends on where you make your imaginary
"cuts" in the schemos to separate function blocks:

All VCOs have a CCO core, of course.
Most VCOs drive that core from an exponential current source,
so you have an inner block - linear CCO - and an outer block
- expo VCO. You could also define a different block, with the cut
at the expo converter's summing node, then you'd have an intermediate
exponential CCO - and the resistor to that summing node makes it
a exop VCO. It's all just a matter of definition. But one thing you cannot
do: find a cut where a partial block acts as a linear VCO (unless you're
using the reference current input - which is exactly what the MS-20 does.)

The Polysix and Trident oscillators are performing an exponential
conversion that includes an expo *voltage* node. Then this expo
*voltage* is converted to an expo current with a *linear* voltage
controlled current source. Which then feeds the CCO core. The big
difference to an ordinary expo VCO is that you *can* make a cut
here such that an inner block is a linear VCO. Sure, the innermost
block will always be a CCO, but next larger block (CCO + VC current
source) *is* a linear VCO. So it does exist (;->) - and if you have
offset voltage at that point you must compensate it (what Korg
called "Low trim") - and that's also a point where you can add an
offset voltage on purpose, which results in that infamous "linear
detuning".

All right now - I almost went on writing "but Korg has chosen to
do it at a different point, changing the trigger threshold of the
CCO core." But you have tricked me - it's true they do it that way,
and it's also beyond doubt that this results in "ordinary" detuning
(just as in every expo VCO), and *not* in linear detuning. (At least
not in what I called linear detuning when I coined that word.
Ok, I see the word was misleading from the start.)

Why is this ? You're changing the thresholds to a certain amount,
and depending on the VCO frequency it will take a different time
for the ramp to pass that additional voltage. Detuning is related
to the current frequency, and not constant.

So - nice idea, and I really believed it for a moment ! But in reality
the Trident's detuning is quite ordinary, and when there is an additional
linear detuning effect (and from the sound, I have no doubt about it),
then it originates in less than accurate "low trim" - i.e. some remaining
offset voltage in the linear VCO block. And in the fact that there is
something like that at all - it can not happen in VCOs that consist
of an exponential current source + linear CCO without anything
in between.

Which does not mean that a synth that would actually use lin detune
*only* would not make sense. I made some experiments with the JH-4
meanwhile, turning the ordinary detune as close to zero as possible,
and switching in Lin Detune does then result in a fixed beat rate over
the whole keyboard range. Is this desirable ? As an option, yes. But
it's quite a restriction, too. A bit like PWM which is often used to
sort of emulate a second VCO on single oscillator synths. IMO, for
most applications a mix of lin and expo detune is desirable. Just
like adding a bit (a quarter octave per volt) LFO tracking to PWM
maybe. Modulation rate increasing with higher notes, but not doubling
every octave. That's what I built this lin detune circuit for in the JH-4,
and that's what (involuntarily ? or rather on purpose - who knows ?)
happens in early Moog Modular VCOs, the Taurus, the Trident, the
CS-80: expo detune dominating, but a tiny bit of lin detuning (due to
inaccuracies) added to add charm in the lower range.

At least that's how I see it (again) now.

JH.