I’ve been a bit too busy to open the ETI up and keep working on it, but I’ve already started on some plans for the front-end.
The keyboard controller in this thing is primitive digital: shaky, slow (~20ms delay), complicated and probably totally broken. I’m planning on replacing it with a little Arduino rig – I found one in stock at gorillabuilderz.
It runs a 32-bit ARM chip, which is ridiculous overkill for what I want – mainly went with this one because it has lots of digital inputs (enough to just wire up every key on the keyboard) and a couple of 12-bit analog outputs (will use one for CV, maybe the other for an extra LFO?). The nice thing about this is it means I can do other fun stuff later, like build in a chip-tune style arpeggiator, or maybe a MIDI input. It also means I can remove the most complicated board on there – the keyboard controller takes up a lot of space and it’s covered in scary unavailable chips.
The actual keyboard mechanism is really basic: each key activates a switch. At the moment, it’s hooked up to a resistor network: 12 lines for note, 4 lines for octave, which means it can get into a really weird state if you hit multiple keys at once.
After a bit of reading, sorting this out looks dead simple: I need to rewire it so that when a key is down, it pulls a line to ground. That’ll let me rip out a huge amount of horrible wiring, and I doubt the Arduino will be any more delicate than the original digital circuit.
The other problem is that the voltages I need to generate are a bit weird. Trigger is -7V low, +7V high (!?), CV is 0-5V. The outputs I have will probably max out about 3.3V. So there’s probably going to be a little bit of buffer and amp circuitry in there as well.