Hi Jurgen,
the Vocoder project is very interesting. Can you include a switchable mode for the unvoiced (sybilance), allowing use of noise or an extra filter (HP) like in the SVC350? I found also interesting the use of digital noise for this function like in the Sennheiser but with variable rate.
I have had many brand and models of vocoders and I still have all the service manuals if You need them.
Ciao
Fabio