OwnAudioSharp.Mt3 4.0.2

OwnAudioSharp.Mt3

MT3 music transcription as an INoteTranscriber for OwnAudioSharp.

Where the built-in BasicPitch transcriber hears pitches, MT3 hears instruments: it is a sequence-to-sequence transformer that emits MIDI-like events with a program number attached, so a bass line and a piano voicing stay separate instead of collapsing into one smear on the chromagram. For chord detection that separation is usually worth more than the extra pitch accuracy.

This is a separate package on purpose. ONNX Runtime is linked into the native library, which costs about 26 MB per platform — no reason to put that in everyone's OwnAudioSharp download.

Install

dotnet add package OwnAudioSharp.Mt3

Desktop only: win-x64, win-arm64, linux-x64, linux-arm64, osx-arm64.

Intel Macs are not covered — the ort crate ships no prebuilt ONNX Runtime for x86_64-apple-darwin, and building the runtime from source in CI is not worth it for a platform Apple stopped selling in 2023. Everything else in OwnAudioSharp still runs there.

Getting the model files

The weights are not in the package — they are about 290 MB. Download the four files below into one folder and give that folder's path to Mt3ModelPaths.FromDirectory(). Keep the names as they are; that is what the helper looks for.

mkdir -p ~/models/mt3 && cd ~/models/mt3

BASE=https://huggingface.co/ModernMube/HTDemucs_onnx/resolve/main/mt3-onnx
for f in mt3_encoder.onnx mt3_decoder_init.onnx mt3_decoder_step.onnx vocab.json; do
  curl -L -o "$f" "$BASE/$f?download=true"
done

All four have to come from the same export run. Mixing them produces confident nonsense rather than an error, so don't assemble a folder from different sources.

Exporting your own from a different MT3-family checkpoint works too — the scripts are in tools/mt3/ in the repository. Mind the checkpoint's licence if you do: YourMT3, the usual source of PyTorch MT3 weights, is GPL-3.0. This package contains none of its code and only reads exported weights at runtime, but what you may ship with those weights is between you and that licence.

Use

// The folder you downloaded the four files into
using var transcriber = new Mt3Transcriber(Mt3ModelPaths.FromDirectory("/Users/me/models/mt3"));

var (chords, key, bpm) = ChordDetect.DetectFromFile("song.wav", transcriber);

Or straight to notes:

var notes = transcriber.Transcribe(samples, transcriber.PreferredSampleRate,
    p => Console.Write($"\r{p:P0}"));

foreach (var n in notes.Where(n => !n.IsDrum))
    Console.WriteLine($"{n.StartTime:F2}s pitch {n.Pitch} program {n.Program}");

Expect it to be slow

MT3 decodes autoregressively — up to a thousand tokens per two seconds of audio. Even with the KV cache the native side keeps, a full song is minutes of CPU work, not seconds. Run it offline, off the UI thread, and use the progress callback. If you need something interactive, BasicPitchTranscriber is still there and still the default.

What runs where

Everything below the C# surface is Rust: the ONNX sessions, the greedy decode loop, the MT3 event codec and the note state machine all live in ownaudio_mt3_ffi. The managed side only marshals a float buffer in and a note array out.

Building the native library locally

The published package carries prebuilt natives, but they are not committed to the repository — 26 MB per platform would bloat the history on every rebuild. Working from a checkout, build the one for your machine and drop it where the loader looks:

cd OwnAudio/Source/Mt3/rust-mt3
cargo build --release -p ownaudio-mt3-ffi

RID=osx-arm64   # or win-x64, linux-x64, linux-arm64, win-arm64
mkdir -p ../runtimes/$RID/native
cp target/release/libownaudio_mt3_ffi.dylib ../runtimes/$RID/native/

runtimes/ is gitignored, so this stays local. The token codec and the note state machine build without ONNX Runtime at all — cargo test -p ownaudio-mt3-core --no-default-features runs their tests in a couple of seconds and needs no native download.