frontend/src/worklets/pcm-worklet.js

/* pcm-worklet.js ---------------------------------------------------------
 * Down-samples the input to 16 kHz mono and posts Int16Array chunks back
 * to the main thread.  Buffer size is 128 frames to keep latency < 10 ms.
 */
class PCMWorklet extends AudioWorkletProcessor {
    constructor() {
      super();
      this._decimator = new Float32Array(0);
      this._ratio     = sampleRate / 16000;      // e.g. 48k / 16k = 3
      this._chunk     = 0;
    }
  
    process(inputs) {
      const input  = inputs[0][0];
      if (!input) return true;
  
      // Down-sample on the fly
      for (let i = 0; i < input.length; i += this._ratio) {
        this._decimator[this._chunk++] = input[i|0];
        if (this._chunk === 160) {               // 10 ms at 16 kHz
          // Convert to 16-bit PCM
          const i16 = new Int16Array(160);
          for (let j = 0; j < 160; ++j) {
            const s = Math.max(-1, Math.min(1, this._decimator[j]));
            i16[j]  = s < 0 ? s * 0x8000 : s * 0x7FFF;
          }
          this.port.postMessage(i16.buffer);
          this._chunk = 0;
        }
      }
      return true;
    }
  }
  registerProcessor("pcm-worklet", PCMWorklet);