SoundFont Kit

A high-performance, pure Dart reader and playback engine for SoundFont files (SF2, SF3, and SFZ), powered by the flutter_soloud low-latency audio engine.

Features

  • See web example
  • See piano 3D web example which uses flutter_scene
  • Multi-Format Support:
    • SF2 (SoundFont 2.04) — Full RIFF/sfbk parser (presets, instruments, zones, generators, modulators, and 16-bit PCM samples).
    • SF3 (SoundFont 3.0) — OGG Vorbis compressed sample streams with sub-chunk header extraction.
    • SFZ (SFZ Instrument Definitions) — Parses opcode definitions and links to external .wav, .flac, or .ogg audio files.
    • Compressed Archives — Transparently loads compressed archives (.zip, .gz, .bz2, .tar, .tgz, .tbz2) containing SoundFonts and loose sample hierarchies.
  • Flexible Data Sources:
    • Load from Flutter Assets (SoundFontFile.fromAsset).
    • Load from Local Disk (SoundFontFile.fromFile).
    • Load from In-Memory Byte Buffers (SoundFontFile.fromBytes).
    • Load from Remote HTTP URLs (SoundFontFile.fromUrl).
  • High-Performance Audio Playback with flutter_soloud:
    • Sample-Accurate Scheduling using playScheduled, fadeScheduled, and stopScheduled on the native engine clock.
    • Real-Time Synthesis: Pitch calculation, root key tracking, key-to-pitch scaling, MIDI velocity sensitivity, attenuation, panning, and loop points.
    • Stereo Channel Joining: Automatically interleaves paired Left and Right 16-bit PCM channels into true 2-channel stereo streams with centered mixer balance.
    • Dynamic Sustain & Release Control:
      • Native Sustain Multiplier (sustainMultiplier): Scales the instrument's authentic SoundFont envelope (0.0x to 10.0x, with 1.0x = exact SoundFont value).
      • Fallback Sustain Time (sustainTime): Configures manual release fade-out duration (e.g. 0.05s to 5.0s) for instruments or samples lacking a native release envelope.
      • Instant Release (0.0x): Cut off sound immediately upon key release.
    • Polyphonic Voice Lifecycle: Manage note-on/note-off, chords, and polyphonic voice pools.
  • Sample Preloading:
    • Preload all samples into memory upfront (preloadAll) for instant zero-latency playback, or stream them on-demand.
    • Granular preloading per preset (preloadPreset) or per instrument (preloadInstrument) with real-time progress callbacks.
  • Global Audio DSP Filters:
    • Built-in manager (SoundFontGlobalFilters) for flutter_soloud effects: Freeverb, Echo/Delay, Biquad Resonant Filter, Flanger, Bass Boost, Lo-Fi, and Wave Shaper.

Getting Started

Add soundfont_kit and flutter_soloud to your pubspec.yaml:

dependencies:
  flutter:
    sdk: flutter
  soundfont_kit: ^1.0.0
  flutter_soloud: ^3.0.0

Initialize the flutter_soloud audio engine prior to playback (e.g. in your main() function):

import 'package:flutter/material.dart';
import 'package:flutter_soloud/flutter_soloud.dart';
import 'package:soundfont_kit/soundfont_kit.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  // Initialize the SoLoud audio engine
  await SoLoud.instance.init();
  SoLoud.instance.setMaxActiveVoiceCount(128);

  runApp(const MyApp());
}

AI Agent Skills

soundfont_kit includes bundled Agent Skills (SKILL.md instruction files) to help AI coding agents (Claude, Cursor, Gemini, GitHub Copilot, Cline, Codex, OpenCode, etc.) generate correct, high-performance code for all features and functionalities of this package.

Install or update the skills in your project by running:

dart run soundfont_kit:skills

To check whether installed skills are up to date without modifying any files:

dart run soundfont_kit:skills --check

Usage Guide

1. Loading a SoundFont File

You can load a SoundFont from various sources. The format (sf2, sf3, sfz, or compressed archive) is automatically detected:

// From Flutter assets:
final sf = await SoundFontFile.fromAsset('assets/soundfonts/GeneralUser.sf2');

// From local file on disk:
final sf = await SoundFontFile.fromFile('/path/to/piano.sf3');

// From a zipped SFZ archive containing samples:
final sf = await SoundFontFile.fromFile('/path/to/celesta_sfz.zip');

// From in-memory bytes (e.g. from file picker or network):
final sf = await SoundFontFile.fromBytes(bytes, basePath: 'my_font.sf2');

// From remote HTTP URL:
final sf = await SoundFontFile.fromUrl('https://example.com/soundfonts/instrument.sf2');

2. Inspecting SoundFont Metadata

Access all parsed presets, instruments, zones, and audio samples:

print('SoundFont Name: ${sf.name}');
print('Format: ${sf.format.name}');
print('Total Presets: ${sf.presets.length}');
print('Total Instruments: ${sf.instruments.length}');
print('Total Samples: ${sf.samples.length}');

// Iterate over presets
for (final preset in sf.presets) {
  print('Preset [${preset.bank}:${preset.program}] ${preset.name}');
}

3. Playing Presets and Instruments

Create a SoundFontPlayer to play presets, instruments, or raw samples:

// Create the player instance
final player = sf.createPlayer(
  options: const SoundFontPlayerOptions(
    joinStereoChannels: true,       // Join paired L/R mono samples to stereo
    cacheAudioSources: true,        // Preserve audio buffers in RAM
    useScheduledPlayback: true,     // Use sample-accurate engine clock
    sustainMultiplier: 1.0,         // 1.0x = authentic SoundFont envelope
    sustainTime: 0.20,              // 200ms fallback for items without release
  ),
);

// Play a preset by note number (MIDI Key: 60 = Middle C) and velocity (1..127)
final preset = sf.presets.first;
final voice = await player.playPreset(
  preset,
  key: 60,
  velocity: 100,
);

// Release the note after 1.5 seconds (triggers release envelope)
await Future.delayed(const Duration(milliseconds: 1500));
await voice.release();

4. Sustain and Release Modes

The player provides two complementary modes for controlling sustain/release:

// 1. Multiplier Mode (for instruments WITH native release envelopes)
// Scale the instrument's authentic SoundFont release time up or down:
player.sustainMultiplier = 1.0;  // 100% native SoundFont release
player.sustainMultiplier = 2.5;  // Extended sustain (like a sustain pedal)
player.sustainMultiplier = 0.0;  // Staccato (instant cutoff on key release)

// 2. Time Mode (for instruments/samples WITHOUT native release)
// Sets explicit fade-out duration in seconds:
player.sustainTime = 0.5;        // 500ms fade-out upon note release

5. Interactive Keyboard & Polyphony

Manage polyphonic notes and chords with noteOn and noteOff:

// Note ON: triggers preset playback and tracks active voices
final voice = await player.noteOn(
  preset: preset,
  key: 64, // E4
  velocity: 110,
);

// Note OFF: fades out smoothly using the voice's release envelope
await player.noteOff(64);

// Or stop all playing notes at once:
await player.allNotesOff();

6. Sample Preloading

To eliminate I/O and streaming overhead during real-time performance, you can preload samples into memory:

// Preload all samples in the soundfont with a live progress callback:
await player.preloadAll(
  onProgress: (progress, loaded, total) {
    print('Preloading: ${(progress * 100).toStringAsFixed(0)}% ($loaded/$total)');
  },
);

// Or preload only the samples needed for a specific preset:
await player.preloadPreset(preset);

7. Applying Global Audio DSP Filters

Use SoundFontGlobalFilters to apply and modulate audio effects on the master mix:

const filters = SoundFontGlobalFilters();

// Activate Freeverb (Reverb)
filters.toggle(SoundFontFilterType.freeverb, true);

// Adjust parameters:
final params = filters.getParameters(SoundFontFilterType.freeverb);
for (final p in params) {
  if (p.id == 'room_size') {
    p.setValue(0.8); // 80% room size
  }
}

// Available global filters:
// - SoundFontFilterType.freeverb
// - SoundFontFilterType.echo
// - SoundFontFilterType.biquad (Low-pass / resonant)
// - SoundFontFilterType.flanger
// - SoundFontFilterType.bassBoost
// - SoundFontFilterType.lofi
// - SoundFontFilterType.waveShaper (Drive / distortion)

Example App Capabilities

The included example/ directory contains a complete Flutter desktop, mobile, and web demonstration app featuring:

  1. Interactive SoundFont Inspector:
    • Tree inspector for Presets, Instruments, Generator Zones, and Raw Sample descriptors.
    • Raw binary header inspection (RIFF magic, OGG headers, FLAC metadata).
    • Embedded audio waveform visualization widget (SampleWaveform) with:
      • Real-time animated playhead tracking (red vertical line) driven by SoLoud.instance.getPosition().
      • Yellow vertical markers indicating loopStart and loopEnd boundary positions.
  2. Dynamic Docked Piano Keyboard:
    • Resizable docked bottom panel.
    • Chromatic multi-touch keyboard with vertical touch-velocity sensing (pressing higher on a key plays softer; pressing lower plays louder).
    • Visual MIDI key numbers and keybinding badges on both white and black keys.
    • Red indicator dots highlighting the root key and mapped ranges of the currently selected zone/sample.
    • Full computer keyboard control and navigation:
      • Space: Audition currently selected preset, instrument, or sample.
      • ▲ / ▼ (Up / Down): Select previous / next item in the active tab list.
      • ◄ / ► (Left / Right): Shift keyboard octave down / up.
      • Canc / Delete: Emergency stop for all playing voices.
      • A..K: 1-octave chromatic QWERTY keyboard mapping.
      • Top-right App Bar Info button with a built-in keyboard shortcuts guide dialog.
  3. Rotary Knobs Parameter Rack:
    • Vol: Master volume control.
    • Sus x (Sustain Multiplier): Active when the selected target has native sustain (0.0x to 10.0x).
    • Sus (Sustain Time): Active when the selected target has no native sustain (0.05s to 5.0s).
  4. Global Audio DSP Filters Rack:
    • Dedicated Filters button with an active filter counter badge.
    • Interactive configuration sheet to toggle and adjust multiple global effects in real time:
      • Freeverb (Room Size, Damp, Wet/Dry, Width)
      • Echo (Delay Time, Decay, Filter)
      • Biquad Resonant Filter (Low-pass / Band-pass frequency and resonance)
      • Flanger (Delay, Frequency, Depth)
      • Bass Boost (Boost dB)
      • Lo-Fi (Sample rate reduction & bit depth decimation)
      • Wave Shaper (Drive saturation & distortion)
  5. File & Folder Pickers:
    • Open individual SoundFont files (.sf2, .sf3, .sfz, .zip, .tar.gz, etc.).
    • Open entire uncompressed SFZ directory hierarchies with automatic nested sample resolution.
    • Optional preloading prompt with real-time percentage progress bar.

To run the example app:

cd example
flutter run

License

This project is open source and available under the terms of the MIT License.

Libraries

soundfont_kit