I have a separate project which is a TRS-80 Model I emulator. Back in the day, there was a voice synthesizer for it. I have been meaning to add that emulation, and was recently spurred into action by another hackadayer [Michael Wessel] who had produced a hardware version for those with physical machines. While in my case I am making an emulator, his input was still useful to me because I didn't have the phoneme list for the original synth. Anyway, I needed to do a bunch of exploratory work before producing the final code, and part of that involved hooking up a physical SP0256-AL2 to a BluePill so I could record phonemes. That activity seems potentially useful to others irrespective of my TRS-80 code, so I decided to make it a separate project.
For the first iteration I am simply hooking up a SP0256-AL2 to a BluePill as a USB-serial-to-SP0256 bridge. This was to setup for recording phonemes, since I am planning to play pre-recorded signals rather than recreate the LPC filter and whatnot. As such I consider this a 'simulator' rather than an 'emulator'.
It did then occur to me that if that works out, I might be able to put the simulator in toto into the Blue Pill, and not even require the physical '0256. We'll see how that goes -- the BluePill is strapped for space, and even at low bit rates the sound files will eat it up quickly. I will experiment with some compression -- probably ADPCM. We'll see how much the quality suffers.
I also have some Text-to-Speech code from years back, so I will also try my hand at porting that over, too.