PreenFM2 meets SID

What is it? A (emulated) SID synth playable via MIDI running on the PreenFM2 hardware platform.

Coming from BBC, Amstrad CPC and Atari ST I never really got into the details of the SID chip...

The midibox SID projects were tempting me but I also had a spare PreenFM2 with an older PCB revision lying around -- that and a few "I wonder ifs..." and this happened.

Basic features include:

• Polyphonic or unison mode
• All SID parameters should be editable
• Basic wavetables for waveform and pitch (although currently no editor)
• 3 LFOs with shape, rate, reset-on-key-pressed
• Modulation targets with selectable LFO and depth: osc frequency, PWM, filter frequency, resonance
• Glide (unison only)
• Switch between 6581 and 8580 emulation
• Runs at 44.1Khz on the 168MHz stm32f405 (see below)

Bonus features:

• ASID support over MIDI (switches on automatically on matching sysex)
• There's a SID file player that can play from a memory buffer. There's currently no (easy) way to get files in though.

Caveats

• It's only been run on my PreenFM2 R4 PCB with a LCD, i.e. "works for me".
• While it can optionally be built to hopefully support the original bootloader, this does cost the first 256K of flash. And I generally just upload via SWD or in gdb anyway.
• It wasn't necessary to implement a MIDI parser but I had some knowledge gaps to fill.
• DIN MIDI only.
• The UI is ad hoc, so it's mostly functional but not "designed".
• Some parameter ranges are just "first guess" so probably need some tweaking.
• Similarly modulation is still at the "basically works" stage but needs to be made more consistent.
• "tests" are mostly some plausibility checks.
• Bugs.

In summary it's more of a technical exercise with a (or yet another?) uninspired repo name, rather than anything approaching a "product" or even a proper "alt firmware" :)

SID Emulation

• Based on the "ancient" reSID version 0.16. This still seems "Good Enough" and while the following versions are much improved the footprint is bigger. Part of that c/should be constexpr-able with some effort but 16MB tables are out for sure :)
• Modifications to reSID were made to get it compiling and (eventually) strip out some unused functionality. Ideally they won't conflict too much with the upstream and are isolated commits.
• There are of course other SID implementations, e.g. I particularly like chips. Similarly, there are forks/mirrors of reSID, e.g. libsidplayfp.
• In order to simplify things, the code writes a batch registers at once, then calls clock later to generate a block of samples.
• While this works, it isn't how hardware with a bus functions. Ideally (?) the register write/clock might be suitably interleaved (difficulty: output sample interpolation).
• This is maybe a "to investigate"?
• There's a caching layer to only write dirty registers through to the emulation.

So yes, this project is a super convoluted way of writing a few 8-bit registers :)

General Miscellany

• The "ADSR delay bug" is emulated. This could be made optional.

TODO-dilly-do

(In no particular order)

• clang-tidy, sanitizers (pending clang support in stm32x)
• Support 2 chips/instances of reSID::SID. May require smaller blocksize and/or sample rate adjustments but we're at ca. 50% load with one instance, so it "should work". Overclocking is also an option.
• Bump modulator update rate (currently ca. 1.3Khz). May also benefit from smaller block size.
• Arpeggiator
• Wavetable editor (and/or sysex uploads)
• Web editor?
• Patches
• Sysex
• CC
• USB
• Support preenfm2 bootloader?

Derp

• To float or not to float seems to be a hard question :)
• In any case, the mixed case seems to be the worst option because rounding is expensive. So this code tends towards fixed-point ops. Which, given the topic seems mostly appropriate anyway...