Anyway, take the 8255 which has 24 digital I/O ports. Use 8 for a bus. Use some for control and use some to turn 4051s on and off. 24 lines should be more than enough.
There is code to talk to the 8255 and it ends up only a few lines of assembly.
I'm not sure I quite follow...
You wouldn't need another eprom to store note tables. Write a program in CP/M and save it. You can save it on a ram disk or a real hard disk or a floppy disk or even an sd card. Write in Basic or C or Assembly or any other language that works on CP/M.
I am kinda tickled by the thought of having a 3.25" floppy drive on this thing...
I'm not sure about polyphony for the 8910 though a very simple answer would be to mix the four 8910 chips using an op amp mixer circuit (one op amp and 5 resistors).
I've seen polyphony done with analog synths before... cards of voices and the first note gets shipped to the first voice, the second note to the second voice, etc. I think I was just wondering if things would be fast enough to do that with the various channels of the 8910s without much lag. The code would have to go something like:
detect first note
set 8910 address for channel A coarse tune
set data into that register
Set address for channel A fine tune
set data for that register
repeat for following notes.
Now you could start to think about wavetable polyphony.
that is a whole other project... allowing the user to program a custom waveform into a ram chip and then pumping the data through an 8-bit D/A converter at whatever rate desired.
What is the ADC you are using? Does it need a negative voltage supply and can it swing rail to rail?
I'm planning on using an ADC0808CCN. Datasheet up on Futurlec...
http://www.futurlec.com/ADConv/ADC0808.shtml
I have a few Z80 chips and the two 8910s, but other than that I'm holding off on getting more things before I have a solid plan of attack.
-Ian