Tone.js Schedule Callback on Every Transport Beat

This article explains how to schedule a callback function to execute on every beat of the Tone.js transport. You will learn how to use the scheduleRepeat method to trigger events at regular musical intervals, enabling precise, time-synced audio events in your web applications.

To schedule a function to run repeatedly on every beat, you use the Tone.Transport.scheduleRepeat method. This method takes a callback function and a time interval as its primary arguments. In standard musical notation, a single beat in 4/4 time is represented as a quarter note, which is denoted as "4n" in Tone.js time syntax.

Here is how to set up and start the repeating callback:

import * as Tone from 'tone';

// 1. Define the callback function
const onBeat = (time) => {
    // Insert your audio event here
    console.log("Beat triggered at transport time:", time);
};

// 2. Schedule the callback to run every quarter note ("4n")
const eventId = Tone.Transport.scheduleRepeat(onBeat, "4n");

// 3. Start the Transport to begin playback
Tone.Transport.start();

Key Considerations

• The time Parameter: The callback function automatically receives a time parameter. To ensure sample-accurate, drift-free audio playback, always pass this time argument to your synth triggers (for example, synth.triggerAttackRelease("C4", "8n", time)).
• Adjusting the Interval: The second argument of scheduleRepeat defines the frequency of the callback. If you want to trigger events on eighth notes instead of quarter notes, change "4n" to "8n". For a callback on every measure, use "1m".
• Stopping the Event: The scheduleRepeat method returns a unique ID number. If you need to stop the callback from firing without stopping the entire Transport timeline, you can cancel it by calling Tone.Transport.clear(eventId).