audio.js

/** @license 
  * Copyright 2023 Google LLC.
  * SPDX-License-Identifier: Apache-2.0 
  */

class AudioWorkletProcessor {};
class AudioStream extends AudioWorkletProcessor {
    constructor() {
        super();
        this.chunkSize = 1024;
        if (typeof registerProcessor === 'undefined') {
            return; // main thread
        }
        this.queue = [];
        this.pos = -1; // queue is empty, need fade-in
        this.frame = 0;
        this.port.onmessage = e=>this.queue.push(e.data);
        for (let i=0; i<2; ++i) {
            this.queue.push(new Float32Array(this.chunkSize*2));
            this._requestChunk();
        }
    }
    _requestChunk() {
        const buf = this.queue.shift();
        this.port.postMessage({frame:this.frame, sampleRate, buf}, [buf.buffer]);
        this.frame += this.chunkSize;
        this.pos = this.queue.length ? 0 : -1;
    }
    process(inputs, outputs, parameters) {
        if (!this.queue.length) return true;
        const src = this.queue[0];
        const [c0, c1]=outputs[0], n=c0.length;
        let v=1.0, dv=0.0;
        if (this.pos == -1) {
            this.pos = 0;
            v = 0.0; dv = 1.0/n;  // fade-in
        } else if (src.length - this.pos <= n*2 && this.queue.length==1) {
            v = 1.0; dv = -1.0/n; // fade-out
        }
        for (let i=0; i<n; ++i, this.pos += 2, v += dv) {
            c0[i] = src[this.pos]*v;
            c1[i] = src[this.pos+1]*v;
        }
        if (this.pos >= src.length) {
            this._requestChunk();
        }
        return true;
    }
    async start(callback) {
        this.audioContext = new AudioContext();
        const audioWorkletJS = AudioStream.toString() 
          + '\nregisterProcessor("worklet-processor", AudioStream);';
        const workletURL = URL.createObjectURL(
            new Blob([audioWorkletJS], {type: 'application/javascript'}));
        await this.audioContext.audioWorklet.addModule(workletURL);
        this.workletNode = new AudioWorkletNode(
           this.audioContext, 'worklet-processor',{outputChannelCount:[2]});
        const submit = buf=>this.workletNode.port.postMessage(buf, [buf.buffer]);
        this.workletNode.port.onmessage = msg=>callback(msg.data, submit);
        this.workletNode.connect(this.audioContext.destination);
        console.log('audio stream started');
    }
    stop() {
        this.audioContext.close();
    }
};