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RP2A03 Logo

RP2A03 Synth is a work-in-progress NES synthesizer plugin for modern DAWs.

It emulates the NES RP2A03 APU at the register level rather than using samples or approximated chiptune waveforms. It also supports Konami VRC6 and Sunsoft 5B expansion audio.

A FamiTracker-style sequencer, software LFOs, portamento, MIDI, and host automation turn the hardware emulation into a practical software synth.

Alpha software: expect bugs and incomplete features.

Synth Showcase

"SUNNY" — Snail House cover by recme

Sunny.mp4

"The Brilliant Truth" — Mina the Hollower cover by Purpbatboi

demo2.mp4

Features

  • Register-level 2A03 emulation

    • Pulse 1/2
    • Triangle
    • Noise
    • Envelope, sweep, length/linear counters, and frame counter

  • VRC6 pulse and sawtooth

  • Sunsoft 5B PSG

  • Band-limited output via blip_buf

  • FamiTracker-style sequences:

    • Volume
    • Arpeggio
    • Pitch
    • Hi-pitch
    • Duty
    • Loop and release points

  • Up to 8-voice polyphony

  • Software vibrato and tremolo

  • Portamento

  • MIDI pitch wheel and pitch-bend sensitivity

  • DAW host automation

  • Adjustable sequence rate from 1–600 Hz

Parameters

The following parameters are available to the plugin. Delay and fade durations are measured in engine frames; at the default 60 Hz step time, one frame is approximately 16.7 ms.

Parameter Range / default Function
Sequence Index 0–127 / 0 Selects the shared sequence slot used by the envelope lanes.
Vibrato Depth 0–15 / 0 Sets the pitch modulation depth.
Vibrato Speed 0–63 / 4 Sets the vibrato phase advance per engine frame.
Vibrato Delay 0–127 frames / 0 Delays vibrato after a fresh note trigger.
Tremolo Depth 0–15 / 0 Sets the volume modulation depth.
Tremolo Speed 0–63 / 4 Sets the tremolo phase advance per engine frame.
Tremolo Delay 0–127 frames / 0 Delays tremolo after a fresh note trigger.
Delay Speed 0–127 frames / 0 Shared linear fade-in duration after either delay. Zero starts modulation at full depth.
HW Volume 0–15 / 15 Sets the channel's hardware volume ceiling.
Pitch -64–63 / 0 Applies a fine pitch period offset.
Hi-Pitch -64–63 / 0 Applies a coarse high-byte pitch period offset.
Pitch Slide -8192–8191 / 0 Applies the host-controlled pitch-wheel offset.
Pitch Slide Range 0–24 semitones / 2 Sets the pitch-wheel range used by MIDI RPN 0.
Step Time 1–600 Hz / 60 Sets the sequence and LFO engine frame rate.
Polyphony Off / Off Enables multi-voice playback.
Max Voices 1–8 / 8 Limits the number of simultaneous voices when polyphony is enabled.
Portamento Off / Off Enables glide between successive notes.
Portamento Speed 0–127 / 0 Sets the speed of enabled portamento.

Supported Chips

Chip Waveforms Examples
Ricoh 2A03 Pulse, Triangle, Noise Most NES games
Konami VRC6 Pulse, Sawtooth Castlevania III, Madara, Esper Dream 2
Sunsoft 5B PSG tone + noise Gimmick!, Batman: Return of the Joker

The 2A03 DPCM channel is not implemented because it is a sample-playback channel rather than a synthesis channel.

5B note

The 5B implementation includes nine selectable tone-duty widths based on the AY8930. This feature was not present on the original Sunsoft 5B, so the S5B mode is intentionally a hybrid.

The 5B hardware envelope is not implemented.

Accuracy

This is a synthesizer inspired by hardware emulation, not a cycle-perfect NES emulator.

The core models the important APU behavior at the register/channel level, but intentionally removes or adds some hardware limitations:

  • Polyphony is expanded to 8 voices.
  • Triangle volume is controllable.
  • Mixing uses conventional linear voice summing.
  • Software LFOs and portamento are added.
  • Sequence rate is adjustable rather than fixed to 60 Hz.
  • Voice allocation uses short ramps to prevent clicks.
  • Pitch controls extend beyond raw hardware period registers.
  • The 5B adds AY8930-style duty control.

For bit-accurate tracker playback, use FamiStudio, Furnace, or Dn-FamiTracker.

Building

Requirements

  • Rust stable
  • Rust 2024 edition support

The core is pure Rust; no C/C++ toolchain is required for the normal builds.

Build

cargo build --release

Run tests:

cargo test --workspace

Build the plugin bundles:

cargo xtask bundle rp2a03_niceplug --release

Output:

target/release/
target/bundled/

Platform Builds

Windows

Uses the default OpenGL backend.

cargo xtask bundle rp2a03_niceplug --release

Install:

Format Location
.vst3 %COMMONPROGRAMFILES%\VST3\
.clap %COMMONPROGRAMFILES%\CLAP\

Linux

Linux builds use wgpu/Vulkan.

Install the development dependencies:

sudo apt-get install -y \
  libasound2-dev libgl1-mesa-dev libx11-dev libxcursor-dev \
  libxrandr-dev libxi-dev libxkbcommon-dev libxkbcommon-x11-dev libxcb1-dev

Build:

cargo xtask bundle rp2a03_niceplug --release \
  --no-default-features --features wgpu

Install:

Format Location
.vst3 ~/.vst3/
.clap ~/.clap/

macOS

macOS uses wgpu/Metal.

For a universal Intel + Apple Silicon build:

rustup target add x86_64-apple-darwin aarch64-apple-darwin

cargo xtask bundle-universal rp2a03_niceplug --release \
  --no-default-features --features wgpu

The normal bundle command can be used for a single architecture.

AUv2

Build the AU wrapper after building the CLAP:

cmake -B build/auv2 -S packaging/auv2
cmake --build build/auv2 --config Release

Output:

build/auv2/RP2A03 Synth.component

macOS VST3

The release build wraps the CLAP using clap-wrapper:

cmake -B build/vst3-macos -S packaging/vst3-macos
cmake --build build/vst3-macos --config Release

Install:

Format Location
.vst3 /Library/Audio/Plug-Ins/VST3/
.clap /Library/Audio/Plug-Ins/CLAP/
.component /Library/Audio/Plug-Ins/Components/

Because development builds are ad-hoc signed, downloaded macOS releases may require:

xattr -dr com.apple.quarantine "/Library/Audio/Plug-Ins/VST3/RP2A03 Synth.vst3"

Project Structure

RP2A03-SYNTH/
├── rp2a03_core/       # APU and expansion-chip emulation
├── rp2a03_common/     # MIDI and shared GUI logic
├── rp2a03_niceplug/   # CLAP/VST3 plugin
├── xtask/              # Build and packaging tools
├── packaging/          # AUv2/VST3 wrappers
├── readme_assets/
├── Cargo.toml
└── README.md

The emulation core is independent of the plugin/UI and can be tested separately.

Release Builds

Pushing a v*.*.* tag builds all supported platforms and produces:

RP2A03_synth/
├── Windows/   # x86_64, OpenGL
├── Linux/     # x86_64, wgpu/Vulkan
└── Mac/       # x86_64 + arm64, wgpu/Metal

A manual GitHub Actions run also produces the build artifact without creating a release.

Crash Reporting

If the plugin crashes or hangs your DAW, attach the crash log to the issue.

Platform Crash log
Windows %LOCALAPPDATA%\rp2a03_synth\crash.log
macOS ~/Library/Application Support/rp2a03_synth/crash.log
Linux $XDG_DATA_HOME/rp2a03_synth/crash.log

Set NICE_LOG to override the log location.

Feature Status

Feature Status
2A03 Pulse ✅ Done
2A03 Triangle ✅ Done
2A03 Noise ✅ Done
VRC6 Pulse ✅ Done
VRC6 Sawtooth ✅ Done
Sunsoft 5B PSG ✅ Done
FDS Wavetable Planned
Namco 163 Planned
2A03 DPCM Not planned

Credits

This project builds on research and open-source work from:

See the upstream projects for their respective licenses.

License

Original code written specifically for this project is released under the WTFPL v2.

Adapted code remains under its original upstream license.

AI Disclosure

Development has used Anthropic Claude Code.

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A Modern NES plugin for DAWs

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