Topic: General thoughts on a native Arduino engine
Thinking on what and how to do an Arduino specific 1-bit engine.
Feature set does not seem too important. It could be something like a scaled up Tritone or Phaser, with more channels, with ability to play tone or noise. It needs to support things that are difficult in ZX engines, such as clean vibrato, portamento, and maybe 'instruments' (fx tables) in general.
There is a major issue that stops me at the moment, the authoring tools. We can't really make music for engines with any complex instruments system using an XM converter. We can't really emulate Arduino to hear authentic sound while making music - well, we could make a whole dedicated tracker with AVR emulator, but that's a ton of work just to start. An VSTi can be made that will generate more or less similar sound, but this would kind of defeat the whole point, because such VSTi would be self-sufficient and the sound won't fully match.
One idea is to make MIDI interface, just as people asked, and control the Arduino with a MIDI-enabled DAW, to be able to use existing tools for composing and hear fully authentic sound in the process. This, however, requires to have the hardware and two MIDI interfaces (on the PC and Arduino's MIDI Shield, which is barely obtainable here and costs more than Arduino, by the way).
Yet another idea is a VSTi again, but this time one that would just emulate AVR and loaded firmware with the engine. Again, lots of work, but at least the whole Arduino thing won't be redundant in this case.
Any MIDI-related approach also makes it difficult to create instruments, it has to be done with SysEx and a custom tool. Or just limit a composer to use pre-defined patches.