Scheduling playback of AudioWorkletNode

Question

I'm working with soundtouchjs-audio-worklet in order to change the tempo/pitch of audio files that have been read into a buffer. The library creates a worklet that can process these buffers, and gives me an AudioWorkletNode to control it. What I need to do is schedule the playback of multiple AudioWorkletNodes so that different audio files can be on a "timeline" of sorts.

I know the AudioBufferSourceNode has a when parameter in it's start() function that you can use to schedule the playback of the node. But AudioWorkletNode doesn't seem to be scheduleable; it just immediately begins playback upon connect()ing it.

I could use setTimeout() to delay calling connect(), but I don't think the timing will be accurate enough. Does anyone know of a way to schedule playback of an AudioWorkletNode? Or an accurate way to connect it at the exact right time?

Answer 1

There is no built-in way to schedule an AudioWorkletProcessor but it's possible to use the global currentTime variable to build it yourself. The processor would then look a bit like this.

class ScheduledProcessor extends AudioWorkletProcessor {
    constructor() {
        super();

        this.port.onmessage = (event) => this.startTime = event.data;
        this.startTime = Number.POSITIVE_INFINITY;
    }

    process() {
        if (currentTime < this.startTime) {
            return true;
        }

        // Now it's time to start the processing.
    }
}

registerProcessor('scheduled-processor', ScheduledProcessor);

It can then be "scheduled" to start when currentTime is 15 like this:

const scheduledAudioNode = new AudioWorkletNode(
    audioContext,
    'scheduled-processor'
);

scheduledAudioNode.port.postMessage(15);