Wiardgroup

> P.S. I also tried to control that feedback with a VCA, but then 
> changes in tune occur (why?).

Hello Olivier,

What you are describing is called "dynamic depth FM". The 
Classic VCO IS optimized for dynamic depth FM. In the case of 
feeding back an internal waveform to the FM input, this is called 
"self modulation".

Bernie Hutchins, the editor of the "Electronotes" newsletter, 
wrote a paper where he proved mathematicly that dynamic depth 
FM into an exponential input will always shift the base pitch of the 
oscillator.

In order to have dynamic depth FM without shifting the basic pitch 
of the oscillator, an AC coupled linear FM input is required. This 
is exactly the design of the LIN input on the Classic VCO. One of 
the purposes of the VCA in the module is to provide dynamic 
depth control for this FM input. Used with an external oscillator, 
the FM modulation can be varied dynamically with the minimum 
amount of pitch shift.

The reason that this does not work with an internal waveform is 
because it is synchronized. As you feed back the sawtooth signal 
into the FM input, you are telling to oscillator to go faster the 
further along the curve you are. This is what produces the 
bending of the line. (The same technique is used to bend the 
linear Envelator segments into exponential shapes by feeding 
back to the AMOD or DMOD inputs.)

Mathematicly, this synchronized "speed up" signal ends up like a  
constant offset changing the period (pitch) of the waveform, and 
is not blocked by the AC coupling. This is why the pitch changes 
when the feedback depth is varied.

You can partially compensate for this by taking a negative going 
version of the feedback depth modulation signal, and using it to 
push the pitch back down.

Try this patch: 
Classic VCO - Sawtooth to VCA IN1, VCA out to LIN (FM) input.
Envelator - OUT+ to VCA ENV input, OUT- to Classic VCO FC1 
input. Adjust depth of FC1 control to compensate for pitch shift.

This may or may not produce acceptable pitch compensation for 
a keyboard type patch. It may also produce interesting swept 
sounds if the control positions are exagerated.