AW: tri 2 saw modulation

[Haible Juergen]

	>	Another option;
	>	Simply create a saw-VCO. Create a tri converter using a
	>transistor and resistor. Make an exponential  VC-Crossfade
between the
	>two outputs using the technique in the data sheet for the
SSM2024 (see
	>the Analog Devices www page), or a linear x-fade using two
3080's such
	>as the one in the wave shaper described in both electronotes
PCC and the
	>Barry Klein book (I forget the name of the circuit but it
x-fades
	>between a FWR signal and a normal signal).
	>
	>	Both of these are circuits that can be built with 5
chips or
	>less.
	>
	>	Of course you can build this VC-xfade into the VCO, but
you
	>might want to make it a separate module :).
	>
	>	The coolest option (if you want to get fancy) would be
to build
	>some kind of trapezoid-function x-fade that would take 4 inputs
(saw,
	>sine, square, tri) and cross-fade between each as the input
voltage goes
	>from 0 to +6v, so that 0v would be saw, 1v would be saw + sine,
2v would
	>be sine, 3v would be sine + square, etc, etc... The tricky part
is
	>creating the CVs for the 4 VCAs. Juergen accomplished this
rather
	>elegantly using transistor tricks with his "Interpolating
Scanner"
	>design (a great design, BTW). 
	>
	>	- CList 

Thank you Chris for the kind words. Crossfading between various
waveforms has benefits on its own, BUT I think Don was suggesting 
something even more interesting: A kind of crossfading circuit with
special
cv generation that emulates a variable slope triangle signal similar
to the MS-20's LFO ! 
This is indeed rather hard to do, while achieving acuurate frequency
tracking
and steep edges in the almost-saw-wave case, all at the same time.
So this idea from Don is really worth trying out (as most of his ieas,
btw.).

BTW, words like "waveform control" or "VC waveform" are used for very
different things in different contexts:

RSF Kobol: 
This seems to be a _crossfade_ between different waveforms. (Don't
remember which waveforms.)
(with some CA3080s). (My Scanner can do similar things, but uses
other circuits.)

Korg MS-20:
This is a real change of _slopes_, while the frequency stays fairly
constant.
Drawback: shape range limited to approx. 1:100. Frequency range
limited. (because of opamp offset voltages, handy resistor values etc.)
Not voltage controlled (that's what we'd like to gain!), but shows the
way to go for a smooth transition.

Korg MS-10:
Similar to MS-20, but even more limited due to cheaper components.

EMS VCS-3:
Now THAT was a surprise! I saw long time users who still thought the
Tri/Saw control would do something similar as the MS-20's (And others
who thought the Sine shape control had something to do with rectifying)
!
Actually, something very different, easy to implement and producing
interesting results of their own: Standard Saw-to-Triangle converter
(1-transistor full wave rectifier) with an offset voltage added. Looks
funny on the scope, and produces funny modulations! (Sine shape 
control is just adding an offset voltage to the smooth diode clipping,
btw.)

Oberheim OB-1:
???
I'd love to see schematics of this. I wonder if it does a MS-20-like
shape
control (but I doubt it). Anybody looked at the output on the scope?
Or has schematcs?

JH.