Coppersynth

A General MIDI sound module in safe Rust, with the habits of a Roland Sound Canvas: a sixteen-part SoundFont synthesizer, an MT-32 translation layer in front, and an SC-55-shaped front panel model behind it. MIDI goes in as bytes, stereo samples come out, and a Sierra-era game that uploads sysex meant for an MT-32 "just works" with no "ROMs" at all.

Written for the Copperline Amiga emulator, which pins this crate and offers it as the built-in MIDI output; this repository is the upstream. For now it is purely a Copperline thing -- the API moves at Copperline's pace and promises nothing to anyone else -- though nothing in it knows what an Amiga is, and it may stand alone one day. A sibling of mt32-rs and FluxBridge.

Use

use coppersynth::engine::Engine;
use coppersynth::mt32::translator::Mt32Mode;

let mut engine = Engine::open_bundled(44_100, Mt32Mode::Auto)
    .expect("the bundled bank");

// Raw bytes, straight off the wire: running status, SysEx and all.
// MT-32 traffic is recognised and translated as it arrives.
for byte in midi_bytes {
    engine.write_byte(byte);
}

// Interleaved stereo frames at the rate the engine was opened with.
let mut frames = vec![(0.0f32, 0.0f32); 4096];
engine.render(&mut frames);

coppersynth::panel::FrontPanel is the front panel as a model: buttons go in, a composed LCD screen and lamp states come out. The host only draws; every character is decided here.

What's inside

• The sixteen-part layer: receive channels, mutes and solo, per-part level caps, key shifts, panel edits that hold against the wire, and live meters read straight from the voices.
• The MT-32-to-GM translation, inspired by Sierra's own driver data: custom timbre uploads matched by name, patch memory re-pointed, rhythm remapped, display text shown on the LCD.
• A synthesis core derived from RustySynth (under src/synth/), extended with SF2.01 modulator support, live introspection, master effect returns, and tolerant loading that repairs bruised soundfonts instead of refusing them.
• Deterministic: the same bytes render the same samples on every run.

Building and testing

cargo build --release
cargo test --release

The build script uses curl to fetch the bundled soundfont and git to stamp the release date; both ship with stock Windows 10+, macOS and Linux, and a build without them still builds (without the bundled bank, and with an undated splash). The first build downloads the bank (about 32 MB) into assets/; delete assets/GeneralUser-GS.sf2 to pick up a newer release. The replay-mt32 tool replays captured game MIDI through the translation layer and reports what an MT-32 would have understood.

The bundled bank

The default soundfont is GeneralUser GS by S. Christian Collins -- an instrument library in its own right, with the complete General MIDI sound set including the SFX bank and drum kits at a very reasonable size. It is fetched from its repository at build time, zipped and embedded, licence text alongside. Any other .sf2 (or a .zip holding one) loads at runtime through Engine::open.

Credits

• RustySynth and MeltySynth by Nobuaki Tanaka (MIT) -- the synthesis core this work stands on.
• Freeverb by Jezar at Dreampoint (public domain), via RustySynth.
• GeneralUser GS by S. Christian Collins -- a complete General MIDI library of his own making -- under its own licence (bundling and redistribution permitted; the licence text ships in the bundle).
• The Roland SC-55 owner's manual, as the behavioural reference for the front panel, and the MT-32 owner's manual for the translation layer's memory map.
• The demo songs: "Railgun Rain" by Ivan Stanton (northivanastan), public domain; "Title Screen", artist unknown, under a licence asking no credit.
• ScummVM's MT-32-to-GM driver and wildmidi, cross-referenced (with Sierra's own patch data) when the translation tables were assembled; the tables here were built independently from that comparison.

Licence and provenance

MIT, as a derivative work of RustySynth:

• The synthesis core under src/synth/ descends from RustySynth v1.3.6 by Nobuaki Tanaka, itself a port of his MeltySynth; LICENSE-rustysynth.txt carries the original MIT licence and copyright, which continue to apply to that code.
• Everything above the core -- the engine, the translation layer, the panel model -- is Copperline's own: LICENSE.txt, copyright Lee Hobson.