moont

Roland CM-32L synth emulator, based on Munt, written in Rust with no code dependencies.

The CM-32L is in the MT-32 family of synthesizers. This hardware predates General MIDI, but many early 1990s PC games directly support these synths.

moont aims to be sample-accurate with Munt. It accomplishes this across a variety of input comparison tests with no known incompatibilities. Inputs for which moont output does not match Munt should be reported as a bug.

Usage

Add the moont crate as dependency in Cargo.toml:

[dependencies]
moont = "0.9.2"

Loading ROMs

This repository does not distribute the Roland CM-32L ROMs. In the default build configuration, the only option is to load them at runtime:

use moont::{CM32L, Rom, Synth};

let rom = Rom::new(&control_rom, &pcm_rom)?;
let mut synth = CM32L::new(rom);

For situations where redistribution of the ROMs is not a concern, the bundle-rom feature flag avoids runtime loading:

use moont::{CM32L, Rom, Synth};

let mut synth = CM32L::new(Rom::bundled());

Place CM32L_CONTROL.ROM and CM32L_PCM.ROM in the rom/ directory inside the moont crate (i.e. moont/rom/), or set the MOONT_ROM_DIR environment variable to a directory containing them. The tools/download-roms.sh script downloads them to the default location.

This feature goes beyond bundling ROM data as binary blobs: at build time, the ROMs are validated, parsed, PCM-unscrambled and converted to Rust values. Rust literals are generated, compiled into the library, and Rom::bundled() returns a handle to this static generated data.

General MIDI Translation

When the input is General MIDI, use GmSynth to make it playable through the CM-32L engine. It does the following:

• Assigns parts 1-8 to MIDI channels 1-8 (rhythm on channel 10).

• Sets pitch bend range to 2 semitones (the GM default) on all parts.

• Translates program changes and drum notes to their CM-32L equivalents.

• Reverses pan (CC#10) to match GM stereo convention.

• Reapplies GM defaults after device reset.

  use moont::{GmSynth, Rom, Synth};

  let mut synth = GmSynth::new(Rom::new(&control_rom, &pcm_rom)?);

Rendering Audio

Use play_msg() or play_msg_at() to enqueue MIDI events, and render() to advance the time and output 32kHz stereo samples to a buffer.

use moont::{Frame, Synth};

// 0x90: Note On, Channel 1.
// 0x3c: Note C-3 (Middle C).
// 0x7f: Velocity 127 (Max).
synth.play_msg(0x7f3c91);

// 1 second of output.
let mut buf = vec![Frame(0, 0); 32000];
synth.render(&mut buf);

// 0x81: Note Off, Channel 1.
// 0x3c: Note C-3 (Middle C).
synth.play_msg(0x3c81);

Munt Feature Comparison

moont makes major scope limitations compared to mt32emu:

• No support for alternate configurations or other hardware models.
• No C/C++ API, just Rust library crate.
• No display functionality.
• Very limited runtime state introspection.

moont behaves like this libmt32emu configuration:

• CM-32L hardware model.
  • Compared to MT-32: 33 extra rhythm sounds.
  • None of the MT-32 "quirks".
• 32kHz output sample rate (matching CM-32L).
• 4096 MIDI queue size.
• "Coarse" output mode (no upsampling).
• "Nice" DAC input mode.
• "Nice" amplitude ramp mode.
• "Nice" panning mode.
• "Nice" partial mixing mode.

Related Crates

Testing

Since the ROMs are not distributed, most tests are excluded from the published crate. When built from source, the sample accurate comparison tests require a patched Munt fork at ./munt in the workspace root to make its behavior deterministic. It is built and linked as a static library automatically.

License

moont is a derivative work of Munt. Both are distributed under LGPL 2.1+.

Copyright (C) 2021-2026 Geoff Hill geoff@geoffhill.org

Copyright (C) 2003-2026 Dean Beeler, Jerome Fisher, Sergey V. Mikayev

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.