Building a Sample-Based Beat Sequencer in Unity

Achieving sample-accurate timing in a game engine’s Update loop is notoriously difficult due to fluctuating frame rates. This project explores the architecture of a custom Beat Sequencer for Unity, designed to handle multi-track audio playback with high precision and minimal memory overhead.

The Architecture

The system is built on a three-tier architecture that separates the “pulse” from the “playback logic.”

1. The Pulse: BeatProvider

The core of the sequencer is the BeatProvider. Instead of relying purely on Time.deltaTime, it calculates intervals based on the BPM (Beats Per Minute) and exposes UnityEvents for different note resolutions (Whole, Half, Quarter, down to Thirty-Second notes).

void OnSettingChange() {
    float quarterBeatLength = 60.0f / bpm;
    _beats[2].SetInterval(quarterBeatLength); // Quarter Note
    _beats[5].SetInterval(quarterBeatLength / 8); // 32nd Note
}

2. The Orchestrator: TrackManager

The TrackManager listens to these beat events. It maintains a sequence of Note objects and decides which sound to play at each tick. It uses a simple but effective index-based system to stay in sync with the global beat.

3. The Engine: BeatPlayer & Object Pooling

Spawning and destroying AudioSource components every 16th note would cause massive Garbage Collection (GC) spikes. The BeatPlayer solves this by using Unity’s IObjectPool to reuse AudioHelper instances.

private AudioHelper CreatePooledItem() {
    var audioHelper = Instantiate<AudioHelper>(_audioHelperPrefab, transform, false);
    audioHelper.pool = AudioSourcePool;
    return audioHelper;
}

Technical Highlight: ADSR Envelopes

Beyond simple sample playback, I implemented a software-based ADSR (Attack, Decay, Sustain, Release) envelope. This allows for more expressive sounds by controlling the gain profile of a sample in real-time.

Crucially, the ADSR logic uses AudioSettings.dspTime. This is the most accurate clock available in Unity, decoupled from the frame rate, ensuring that the envelope curves remain smooth even if the game’s FPS drops.

if (isKeyDown && AudioSettings.dspTime > attackStartTime) {
    double deltaTime = AudioSettings.dspTime - attackStartTime;
    if (deltaTime < attackTime) {
        adsrGain = Mathf.Lerp(0, 1, (float)(deltaTime / attackTime));
    }
    // ... handle Decay and Sustain
}

Pitch Shifting via Frequency Mapping

The AudioHelper includes a lookup table for musical pitches. By adjusting the pitch property of the AudioSource based on a semi-tone ratio, the sequencer can play melodies using a single audio sample (e.g., a C4 bell sound re-pitched to play an A4).

if (AudioConstant.PitchTable.TryGetValue(pitch, out float pitchValue)) {
    audioSource.pitch = pitchValue;
}

Summary

This sequencer project serves as a robust foundation for music-based games or interactive audio tools. By combining Object Pooling for performance and DSP Timing for accuracy, it overcomes the traditional limitations of game engine audio systems.