OpenMK

A browser-based SA-synthesis electric piano. No plugins, no installs - just open and play.

▶ Play now

OpenMK runs the rdpiano emulator by Giulio Zausa, compiled to WebAssembly inside an AudioWorklet. rdpiano is a silicon-level emulation of the CPU-B board used in the Roland RD-1000, MKS-20, and Rhodes MK-80 digital pianos: the HD6301 microcontroller and the custom SA sound chip, both reverse engineered from decapped chips.

Companion project to OpenDX7.

Sounds

All 16 patches from both machines:

• MK-80 (Rhodes): Classic, Special, Blend, Contemporary, A. Piano 1/2, Clavi, Vibraphone
• MKS-20 (Roland): Piano 1/2/3, Harpsichord, Clavi, Vibraphone, E-Piano 1/2

Plus the Space D stereo chorus (a BBD approximation, same as the rdpiano plugin).

Playing

• MIDI: plug in a keyboard, it just works (Chrome/Edge)
• Computer keys: A-L rows play notes, Z-M row plays the lower octave
• On-screen: click the keyboard, drag the bend and mod wheels

ROMs

OpenMK ships no ROM data. The ROM images are fetched at runtime from the rdpiano repository and cached in your browser (IndexedDB). If the download fails, the app shows a drop zone where you can supply the files yourself.

Architecture

engine/               C++ sources (vendored librdpiano + Space D chorus + wrapper)
engine/build.sh       emscripten build -> js/rdpiano.wasm
js/ep-processor.js    AudioWorklet: hosts the WASM, resamples 20/32 kHz -> context rate
js/rom-loader.js      runtime ROM fetch + IndexedDB cache + drop-zone fallback
js/main.js            UI wiring, Web MIDI, QWERTY keys, demo player

The emulator produces mono samples at the patch's native rate (20 kHz or 32 kHz). The worklet renders exactly as many source samples as each 128-frame quantum consumes and linearly interpolates up to the context rate. The chorus runs inside the WASM at native rate, producing stereo, like the hardware.

Building the engine

Only needed if you change engine/:

brew install emscripten
./engine/build.sh

There is also a native test that renders a chord to WAV without a browser:

cd engine
c++ -O2 -std=c++17 -Ilibrdpiano/include -Ilsp native_test.cpp wrapper.cpp \
    librdpiano/src/mcu.cpp librdpiano/src/sound_chip.cpp lsp/spaced.cpp \
    -o native_test && ./native_test <roms_dir> out.wav

Running locally

Any static server works:

python3 -m http.server 8472

License

GPL-3.0 (see LICENSE). Copyright:

• Emulation core (engine/librdpiano, engine/lsp): Copyright (c) Giulio Zausa, from the rdpiano project, GPL-3.0.
• Web shell (everything else): Copyright (c) 2026 Keith Adler, GPL-3.0. Parts adapted from OpenDX7 by the same author.
• Demo MIDI files (midi/): Public Domain, from the Mutopia Project.

Roland and Rhodes are trademarks of their respective owners; this project is not affiliated with either.