the synth is working and giving its very particular "unicorn rainbow poop".
I've been playing with the synth for few days. The result is what I expected.
Still some work to do on the output circuit. It seems that I can't keep the bias on the op-amp right, bloody trimmers. Also don't trust On resistor of the CD4053 to be negligeable, it is not. I needed to add trimmers to compensate.
I added random voltages using PWM from the STM32F0. Those need to be very well filtered otherwise the noise will be noticeable on the screen. However the more low the cutt off is, the less high the change of voltage can be. At the moment the PWM outputs are off. I'm thinking about using the three jacks for a mixer/blender
The SMT32 is also creating the sync signals. On modern tv screen with VGA input it seems to be fine, the TV can lock on the signals beautifully. However on old monitor it can take quite some time to lock. I need to work on it as well.
5 VCOs is maybe a bit too much. A mixer / blender could be handy for background pattern creating. I'm also on the impression that the synth lacks someking of external input in spite of the enveloppe filter.
Being able to send and manipulate an external VGA signal (RGB) and being able to mix it with the synth could be quite a thing. Next project ?
Nonetheless, I need to tweak the details and make it really finished !