[sdiy] Additive VCO

[Richard Wentk]

At 00:25 08/03/2004 +0100, M.A. Koot wrote:
>Hi everyone,
>
>I'm trying to design an additive-VCO, there aren't too many around, and
>that's why I would like to at least try it myself.
>My initial idea is by having one expo-converter, having the output devided
>over several Opamp-adders (to create the harmonic-offset). Those Opamp-adders
>have 2 inputs each then, one with the CVin, and one with the offset voltage.
>Each of those opamp outputs then go to seperate Sine-VCO's, one sine for
>each opamp-output.

This is very likely best done digitally. An analogue approach is very 
expensive in components and assembly time and also hard to control.

You really need 8 sines with individual envelopes to get anything 
worthwhile from additive, and that's a very bare minimum. (The DX7 additive 
algorithm had six oscillators, and that wasn't very exciting.) Even with 
more overtones, if the relative volumes are static you just organ-type 
tones and they really aren't all that interesting. (And way less musically 
useful and fun to play with than - say - a good 8-bit wavetable DCO.)

One exception is the 'cloud oscillator' where you add four closely pitched 
oscillators and add slight random detuning. (Which you'll probably get some 
element of anyway, from an analogue design.)

But overall I'd strongly suggest analogue is not a good way to go here.

It would be worth looking at the OSCAR which used a non-real-time sine 
summing technique to create a static waveshape with arbitary harmonic 
content, which was then 'played' by a simple DCO.

>Then finaly mixing all the signals together, and that should be it.
>The things I'm troubling with though, is how many harmonics (i.e.
>Sine-VCO's) should I have at least to create a decent Additive VCO? I was 
>thinking of
>about 16 Sine-VCO's.
>Another thing that popped up is: How "correct" do they have to be?

There's no reason not to do additive with other waveshapes. You get a 
different effect, of course, but that's part of the point. If the cost is 
low (as it would be with digital), there's little reason not to include the 
option.

Richard