It's alive! Last night I did the first test with a DCO, envelope and VCA. It worked quite well, some "thud" sounds at the start of the envelope, and a big issue with the DCO pitch that I later tracked down to it ignoring the low 8 bits of the pitch int, but it does indeed play notes :-D
Here is a video of the test: https://www.youtube.com/watch?v=0isvi7EzVX4
I have yet to figure out the exact pitch bug but I have to send six bytes instead of three and every 2nd byte is ignored. It may have been intentional and should be a fairly easy fix. Also, I should considering resetting SPI buffers etc on SS high.
I did run into some issues with high DCO update frequencies (>100 times per second) so that needs to be checked too.
The envelopes however, works a bit strange. I guess that exponential VCAs is the reason, I had to have a very high value for the sustain level to get any sound. I will study this closer.
Also, to speed things up in the CV department (now the CVs have a 1kHz refresh rate) I need to replace the clock and mux, here are some alternative clocks:
https://datasheet.lcsc.com/szlcsc/Texas-Instruments-TI-SN74HC4040DR_C6848.pdf
https://datasheet.lcsc.com/szlcsc/TOSHIBA-74VHC163FT_C150161.pdf
SN74HC393
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