audio-capture-worklet.js

/**
 * audio-capture-worklet.js
 * ------------------------
 * Runs in the AudioWorklet thread (dedicated audio thread, zero GC pauses).
 *
 * Responsibilities
 * ----------------
 * 1. Receive float32 PCM frames from the mic at AudioContext.sampleRate
 *    (typically 44100 or 48000 Hz).
 * 2. Downsample to TARGET_SR = 16000 Hz using averaged decimation
 *    (basic anti-aliasing — good enough for speech).
 * 3. Accumulate until CHUNK_SAMPLES (320 samples = 20 ms at 16 kHz) are
 *    ready, then convert to Int16 and post to the main thread.
 *
 * Messages posted to main thread
 * --------------------------------
 *   { type: 'audio', pcm: ArrayBuffer }   — 640 bytes = 320 int16 samples
 */

const TARGET_SR      = 16_000;
const CHUNK_SAMPLES  = 320;   // 20 ms at 16 kHz → 640 bytes as Int16

class AudioCaptureProcessor extends AudioWorkletProcessor {
  constructor () {
    super();
    this._buf    = [];          // Downsampled float32 accumulator
    this._active = false;

    this.port.onmessage = ({ data }) => {
      if (data.type === 'start') this._active = true;
      if (data.type === 'stop')  this._active = false;
    };
  }

  process (inputs) {
    if (!this._active) return true;

    const channel = inputs[0]?.[0];
    if (!channel || channel.length === 0) return true;

    // ── Downsample: average samples within each output window ───────── //
    const ratio      = sampleRate / TARGET_SR;   // e.g. 48000/16000 = 3.0
    const outCount   = Math.floor(channel.length / ratio);

    for (let i = 0; i < outCount; i++) {
      const srcStart = i * ratio;
      const srcEnd   = srcStart + ratio;
      let   sum      = 0;
      let   n        = 0;
      for (let j = Math.floor(srcStart); j < Math.min(Math.ceil(srcEnd), channel.length); j++) {
        sum += channel[j];
        n++;
      }
      this._buf.push(n > 0 ? sum / n : 0);
    }

    // ── Emit CHUNK_SAMPLES at a time ─────────────────────────────────── //
    while (this._buf.length >= CHUNK_SAMPLES) {
      const slice  = this._buf.splice(0, CHUNK_SAMPLES);
      const int16  = new Int16Array(CHUNK_SAMPLES);
      for (let i = 0; i < CHUNK_SAMPLES; i++) {
        int16[i] = Math.max(-32_768, Math.min(32_767, Math.round(slice[i] * 32_767)));
      }
      this.port.postMessage({ type: 'audio', pcm: int16.buffer }, [int16.buffer]);
    }

    return true;  // keep processor alive
  }
}

registerProcessor('audio-capture-processor', AudioCaptureProcessor);