WhisperLiveKit/whisperlivekit/web/recorder_worker.js

let sampleRate = 48000;
let targetSampleRate = 16000;

self.onmessage = function (e) {
  switch (e.data.command) {
    case 'init':
      init(e.data.config);
      break;
    case 'record':
      record(e.data.buffer);
      break;
  }
};

function init(config) {
  sampleRate = config.sampleRate;
  targetSampleRate = config.targetSampleRate || 16000;
}

function record(inputBuffer) {
  const buffer = new Float32Array(inputBuffer);
  const resampledBuffer = resample(buffer, sampleRate, targetSampleRate);
  const pcmBuffer = toPCM(resampledBuffer);
  self.postMessage({ buffer: pcmBuffer }, [pcmBuffer]);
}

function resample(buffer, from, to) {
    if (from === to) {
        return buffer;
    }
    const ratio = from / to;
    const newLength = Math.round(buffer.length / ratio);
    const result = new Float32Array(newLength);
    let offsetResult = 0;
    let offsetBuffer = 0;
    while (offsetResult < result.length) {
        const nextOffsetBuffer = Math.round((offsetResult + 1) * ratio);
        let accum = 0, count = 0;
        for (let i = offsetBuffer; i < nextOffsetBuffer && i < buffer.length; i++) {
            accum += buffer[i];
            count++;
        }
        result[offsetResult] = accum / count;
        offsetResult++;
        offsetBuffer = nextOffsetBuffer;
    }
    return result;
}

function toPCM(input) {
  const buffer = new ArrayBuffer(input.length * 2);
  const view = new DataView(buffer);
  for (let i = 0; i < input.length; i++) {
    const s = Math.max(-1, Math.min(1, input[i]));
    view.setInt16(i * 2, s < 0 ? s * 0x8000 : s * 0x7FFF, true);
  }
  return buffer;
}