MIDI BPM Detection

MIDI BPM Detection estimates tempo from incoming MIDI note-on events while you play. The goal is to let a musician record freely, infer the tempo from the performance in realtime, and feed that tempo back to the host DAW so the recording can fit a loop with less manual adjustment.

The detector compares intervals between recent notes, scores likely beat durations, and exposes both a single estimated BPM and a histogram that shows competing tempo candidates. The histogram is important: it makes the guess inspectable instead of hiding the model behind one number.

The screenshot below shows the plugin/demo UI with detection parameters and the realtime histogram. The strongest peak is the current most likely BPM.

Try the browser demo: https://valsteen.github.io/midi_bpm_detection/

What This Repository Contains

This is an experimental BPM detection monorepo. The Rust side contains three runtime modes:

• plugin: the CLAP/VST3 target intended to run inside a DAW. This is the production constraint.
• desktop: a native GUI app used for local iteration and native MIDI experiments.
• wasm: a browser demo that makes the detector easy to try and share.

The Kotlin side contains the companion Bitwig controller extension used by the production Bitwig tempo-control path. The extension creates the Bitwig remote connection, writes its port into the selected plugin's DAW Port parameter, and applies incoming BPM updates to Bitwig's transport tempo.

The project is still a work in progress. Tempo detection depends on play style and parameter tuning, and the host tempo feedback path is currently shaped around Bitwig integration.

The core BPM evaluation lives in the Rust BPM core crate; see Rust workspace architecture for the crate layout.

Quick Start

The full Bitwig tempo-control path needs both build roots:

• the Rust CLAP/VST3 plugin, built from rust/;
• the Kotlin Bitwig controller extension, built from extension/.

Prerequisites

• Rust via rustup.
• The stable Rust toolchain selected by rust/rust-toolchain.toml.
• Nightly rustfmt, because rust/rustfmt.toml uses nightly-only formatting options.
• A JDK available to run Gradle. The extension build targets JVM 17, and Gradle can auto-provision that toolchain.
• Bitwig Studio.

Install the Rust components:

cd rust
rustup component add clippy rustfmt rust-src
rustup toolchain install nightly --component rustfmt

Check the Rust toolchain:

./scripts/dev.sh doctor

Check the Gradle and Java setup from the Kotlin build root:

cd ../extension
./gradlew --version
./gradlew javaToolchains

Build The Plugin

cd rust
cargo xtask bundle midi-bpm-detector-plugin --release

The plugin bundles are written under:

rust/target/bundled/

The release bundle command currently creates:

rust/target/bundled/midi-bpm-detector-plugin.clap
rust/target/bundled/midi-bpm-detector-plugin.vst3

Build The Bitwig Extension

cd extension
./gradlew packageBitwigExtension

The Bitwig extension package is written to:

extension/extensions/beat-detection-controller/build/bitwig-extension/BeatDetectionExtension.bwextension

To copy it into the default Bitwig extensions folder for your user account:

./gradlew installBitwigExtension

The install task defaults to:

${HOME}/Documents/Bitwig Studio/Extensions

You can override the install location with -PbitwigExtensionsDir=..., BITWIG_EXTENSIONS_DIR, or an ignored local extension/gradle-local.properties file. See development commands for details.

Install In Bitwig

Bitwig scans plug-ins from the folders configured in Dashboard > Settings > Locations > Plug-in Locations. Copy or symlink the bundled CLAP/VST3 plug-in into one of those folders, or add the bundle folder to Bitwig's plug-in locations, then let Bitwig rescan. Bitwig's user guide documents the Dashboard settings and plug-in locations in The Dashboard and plug-in behavior in Plug-in Handling and Options.

Install the .bwextension package with ./gradlew installBitwigExtension or copy it into Bitwig's user extensions folder. In Bitwig, add the controller extension from Dashboard > Settings > Controllers > Add; it appears as the Beat Detection Bitwig Extension from Midi BPM Detection. Bitwig's user guide covers controller setup in MIDI Controllers. Bitwig Studio includes the local controller API guide and reference under Help > Documentation > Developer Resources. Bitwig also publishes official controller-extension examples at bitwig/bitwig-extensions.

Use In Bitwig

1. Load the Bitwig controller extension.
2. Add the BPM detector plug-in to a track that receives MIDI notes.
3. Select the plug-in device in Bitwig.
4. Enable the plug-in's Send tempo parameter.

The extension follows the currently selected device. When it recognizes this plug-in, it writes a localhost port into the plug-in's DAW Port parameter. The plug-in then sends detected BPM updates over that socket, and the extension applies them to Bitwig's transport tempo.

This Bitwig tempo-control path has been manually tested on macOS with Bitwig Studio 6.0.6.

Documentation

• Architecture: cross-build-root overview, runtime modes, and architecture boundaries.
• Rust workspace architecture: Rust crate map, crate groups, and Rust runtime constraints.
• Runtime lifecycle: bootstrap wiring and data flows between plugin, desktop, WASM, GUI, and BPM detection components.
• Plugin flow: host buffer processing, realtime handoff, background work, and tempo feedback.
• Bitwig tempo bridge: the narrow plugin-to-controller-extension contract used to set Bitwig tempo, including the selected-device parameter rendezvous.
• Native MIDI flow: desktop MIDI service, controller boundary, worker messages, and native MIDI output ownership.
• Algorithm archaeology: the original tempo-detection idea and why the histogram exists.
• Development commands: setup, formatting, checking, plugin bundling, and WASM demo commands.