tri 2 saw modulation

[List, Chris]

Ahhh, yes, you're correct. Sorry - another example of me coming into
work, getting amped up on coffee, eager to start clicking on my
keyboard, and replying too quickly without thinking :).

x-fading would lead to a "break" in the lines of the rising and falling
wave, as the tri is mixed with the saw. What you want, however, is to
simply change the slope, but keep the two lines straight. Huh, never
appriciated the difference before.

- CList

> -----Original Message-----
> From:	Haible Juergen [SMTP:Juergen.Haible at nbgm.siemens.de]
> Sent:	Monday, October 27, 1997 9:52 AM
> To:	'synth-diy'; List, Chris
> Subject:	AW: tri 2 saw modulation
> 
> 
> 	>	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.
>