church-translate/app/capture-worklet.js

/*
 * capture-worklet.js — pulls audio off the graph in fixed frames.
 *
 * Runs on the audio thread, so it does the least possible work: pick the
 * channels that carry a microphone, average them, fill a frame, hand it over.
 * All judgement about what is speech happens on the main thread where it
 * cannot cause a dropout.
 */

/* An empty pick means "mix everything", which is right for a two-channel
   console feed. A specific set is right for Dante or SoundGrid, where the
   device presents 64 channels and the voice is on one or two of them — mixing
   those would bury the speaker under 62 silent channels and make a perfectly
   good feed look like silence. */
function normalize(list) {
  if (!Array.isArray(list)) return null;
  const out = [];
  for (const v of list) {
    const n = Number(v);
    if (Number.isInteger(n) && n >= 0 && out.indexOf(n) === -1) out.push(n);
  }
  return out.length ? out.sort((a, b) => a - b) : null;
}

/* Per-channel levels, posted about ten times a second. They are what makes a
   64-channel device usable at all: without them, choosing the right channel is
   guesswork against a list of identical names, and the only feedback is
   whether the whole app appears broken. With them you watch for the one that
   moves when someone talks. */
const LEVEL_MS = 100;

class CaptureProcessor extends AudioWorkletProcessor {
  constructor(options) {
    super();
    const opts = (options && options.processorOptions) || {};
    this._buf = new Float32Array(opts.frameSize || 1024);
    this._n = 0;
    this._pick = normalize(opts.channels);
    this._reported = -1;

    this._sumSq = null;      // per-channel accumulator for the level meters
    this._sumN = 0;
    this._lastLevel = 0;

    /* Changing channel used to mean tearing the capture graph down and
       building it again, which is not something to do mid-service. A message
       is, so the picker now works while translation is running. */
    this.port.onmessage = (e) => {
      const d = e.data;
      if (d && d.t === 'channels') this._pick = normalize(d.channels);
    };
  }

  process(inputs) {
    const input = inputs[0];
    if (!input || input.length === 0) return true;

    const chans = input.length;
    const len = input[0].length;

    /* Tell the main thread how many channels actually arrived, once. */
    if (this._reported !== chans) {
      this._reported = chans;
      this._sumSq = new Float32Array(chans);
      this._sumN = 0;
      this.port.postMessage({ channels: chans });
    }

    /*
     * Resolve the pick ONCE per render quantum, not once per sample. A channel
     * index that no longer exists (the device changed underneath us) falls back
     * to mixing rather than reading past the end of the array.
     */
    let picks = null;
    if (this._pick) {
      picks = [];
      for (let k = 0; k < this._pick.length; k++) {
        if (this._pick[k] < chans) picks.push(this._pick[k]);
      }
      if (!picks.length) picks = null;
    }
    const np = picks ? picks.length : chans;

    for (let i = 0; i < len; i++) {
      let sum = 0;
      if (picks) for (let k = 0; k < np; k++) sum += input[picks[k]][i];
      else       for (let c = 0; c < chans; c++) sum += input[c][i];
      this._buf[this._n++] = sum / np;

      if (this._n === this._buf.length) {
        this.port.postMessage(this._buf.slice(0));
        this._n = 0;
      }
    }

    /* Levels for EVERY channel, including the ones not selected — the whole
       point is to find the one that is moving. */
    if (this._sumSq && this._sumSq.length === chans) {
      for (let c = 0; c < chans; c++) {
        const ch = input[c];
        let s = 0;
        for (let i = 0; i < len; i++) s += ch[i] * ch[i];
        this._sumSq[c] += s;
      }
      this._sumN += len;

      const now = currentTime * 1000;
      if (now - this._lastLevel >= LEVEL_MS && this._sumN > 0) {
        this._lastLevel = now;
        const rms = new Float32Array(chans);
        for (let c = 0; c < chans; c++) rms[c] = Math.sqrt(this._sumSq[c] / this._sumN);
        this.port.postMessage({ levels: rms });
        this._sumSq.fill(0);
        this._sumN = 0;
      }
    }
    return true;
  }
}

registerProcessor('capture', CaptureProcessor);