Thank you for your reply, Florian.
The reason I ask this is, that sometimes when you VC tune/pitch-V/Oct a VCO, the sound changes, so you can not play a decent melody.
You can notice it here in this video - not so clearly - from 6.16 and at 7.06 minute.
If the signal was distorted more and more , the changes in sound would be also more and more.
It's a pity, because the OptoFet filter has some very nice [sweetspot] distorted sounds.
https://youtu.be/Hsf7nTXDhYw t=6m16s