A powerful multipurpose SF2/DLS/MIDI TypeScript/JavaScript library for the browsers, based on spessasynth_core. This is a WebAudioAPI wrapper for the spessasynth_core library.

It allows you to:

• Play MIDI files using SF2/SF3/DLS files!
• Read and write MIDI files!
• Write SF2/SF3 files!
• Convert DLS to SF2! (and back!)
• and more!

TIP:

Looking for a bare JS version that works without WebAudioAPI? Try spessasynth_core!

Featuring robust MIDI analysis and editing, broadened MIDI exclusive support, and reworked, more powerful API!

• SpessaSynth - Online SF2/DLS MIDI Player
• signal - Online MIDI Editor

SpessaSynth Project index

• spessasynth_core - SF2/DLS/MIDI library
• spessasynth_lib (you are here) - spessasynth_core wrapper optimized for browsers and WebAudioAPI
• SpessaSynth - online/local MIDI player/editor application
• SpessaFont - online SF2/DLS editor

• Fully typed: Faster development and IDE auto-completion!
• Modular design: Easy integration into other projects (load what you need)
• Detailed documentation: With examples!
• Easy to Use: Basic setup is just two lines of code!
• No external dependencies: Just spessasynth_core!
• Export audio files using OfflineAudioContext!
• AudioWorklet synthesizer:
  • Runs in a separate thread for maximum performance!
  • Does not stop playing even when the main thread is frozen!
  • Supported by all modern browsers!
• Web Worker synthesizer:
  • Synthesize directly in the Web Worker!
  • Direct audio engine access!
  • Export audio automatically in the worker!
  • Avoids Chromium Audio Bugs!
• High-performance mode: Play Rush E! note: may kill your browser ;)

• FluidSynth - for the source code that helped implement functionality and fixes
• Polyphone - for the sound bank testing and editing tool
• Meltysynth - for the initial low-pass filter implementation
• RecordingBlogs - for detailed explanations on MIDI messages
• stbvorbis.js - for the Vorbis decoder
• fflate - for the MIT DEFLATE implementation
• tsup - for the TypeScript bundler
• foo_midi - for useful resources on XMF file format
• Falcosoft - for help with the RMIDI format
• Christian Collins - for various bug reports regarding the synthesizer
• And You! - for checking out this project. I hope you like it :)

import { WorkletSynthesizer } from "spessasynth_lib";

// SF2, SF3, SFOGG and DLS files are all supported!
const sfont = await (await fetch("soundfont.sf3")).arrayBuffer();
const ctx = new AudioContext();
// make sure you copied the worklet processor!
await ctx.audioWorklet.addModule("./spessasynth_processor.min.js");
const synth = new WorkletSynthesizer(ctx);
await synth.soundBankManager.addSoundBank(sfont, "main");
await synth.isReady;
document.getElementById("button").onclick = async () => {
    await ctx.resume();
    synth.programChange(0, 48); // strings ensemble
    synth.noteOn(0, 52, 127);
};

Copyright © 2026 Spessasus Licensed under the Apache-2.0 License.

This project is in no way endorsed or otherwise affiliated with the MIDI Manufacturers Association, Roland Corporation, Yamaha Corporation, Creative Technology Ltd. or E-mu Systems, Inc., or any other organization mentioned. SoundFont® is a registered trademark of Creative Technology Ltd. All other trademarks are the property of their respective owners.