PolyModular Standards

[Ingo Debus]

Haible Juergen wrote:
> If there is someone who does the keyboard scanning and/or Midi stuff with
> a uC, this is the best that could happen on that part.

I can offer the mircoprocessor part of my polyphonic synth here. It is a
pretty old design, based on a 6502, but it works. The software can be
modified easily, if necessary, since it's completely by me. (Hah! Who
said that a synth OS cannot be written by one person alone?! :-) But,
agreed, it was indeed a lot of work.)

Some details:

- Input is MIDI. A four-octave keyboard can be connected too, but it's
not velocity sensitive (the MIDI input is velocity sensitive of course).
I don't have the keyboard connected anymore, since it doesn't make any
sense for me. But the keyboard scanning code is still there, I was too
lazy to remove it.

- There are 32 control voltage outputs and some digital outputs. The
analog output interface was a kit from Doepfer, one DAC (8 bit), buffer
RAM, and analog multiplexers/s&hs/buffer amps. I intended to make it 6
voice, with 2 VCOs per voice, so 12 CV outputs are for VCO control and 6
CV outputs for VCF cutoff control (V/oct). The other CV outputs are used
to control other parameters, this might not be interesting for a modular
(I don't have any knobs on my synth except Volume and Tune). They could
be used for general MIDI controlled cv outputs instead. The digital
outputs are used for ADSR gate signals. Actually I have 5 voices in my
synth, but the software could be modified to handle any other number of
voices.

- User interface: 21 pushbuttons and a 6 digit 7-segment display (from
the Elektor Junior Computer). The thing can be controlled via Sysex too.
I made a SoundDiver adaptation for it.

- MIDI messages processed: Note On/Off, pitch bend, mono and poly
aftertouch, controller, program change, Sysex. Filter cutoff can be
affected by note number (i.e. keyboard tracking), velocity, one
controller and aftertouch. Aftertouch (especially polyphonic) for filter
cutoff is a great feature for analog filters!

- Voices can be stacked (i.e. more than one voice for one note). VCO and
VCF cutoff offsets and velocity/controller/AT sensitivity can be set
seperately in semitones then. A keyboard split point can be defined with
different VCO/VCF offsets below and above. Either keyboard zone can be
played polyphonic or monophonic.

- Presets can be stored in battery-buffered RAM and recalled via front
panel or program change messages.

- The software is in EPROM. However, for testing/debugging it can be
downloaded via MIDI into RAM too.

- Compared to today's all-in-one microcontrollers, it's rather high
component count. To use a modern micro, the code had to be re-written. I
will not do this!!! Does anybody know whether the 6502 family ICs (6502,
6520, 6522, 6532, 6850) are still available? In the Conrad catalog there
are only 6522 and 6850 listed. There are also some old RAM ICs in the
Doepfer interface, but I think these can be replaced with modern SRAMS
easily. Also, it might be a good idea not to use the original 8-bit-DAC,
but a 12-bit (or even 16 bit) instead.

I began with that project in the early 80s and made the last
modifications some months ago. There's still a lot that could be done.
I'd be happy to share it for the polyphonic modular. What do you think?
Juergen?

Ingo