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Amazing platform here for miniMO! great job.
i'd love to introduce a kinda similar project, also based on the ATTINY85 as a modular and easy reprogrammable synth platform. the 8Bit Mixtape NEO: http://neo.8bitmixtape.cc/
we have developed a bootloader for the attiny, that makes it ultra simple to upload different synth codes to the attiny, using an audio file.
see more on this repo: https://github.com/ChrisMicro/TinyAudioBoot
now looking at some of your codes, and checking how to make them compatible to our schematics and use of pins / timers.
keep up the great work!