https://www.sanas.ai/worklets/pcm-capture-processor.js

1/**
2 * AudioWorkletProcessor that captures mic input and emits 20ms chunks
3 * of 16-bit PCM at 16kHz via postMessage.
4 *
5 * The AudioContext should be created at 16kHz sample rate so no
6 * resampling is needed inside the worklet.
7 *
8 * Each chunk = 320 samples (20ms at 16kHz) = 640 bytes of Int16 PCM.
9 */
10class PCMChunkProcessor extends AudioWorkletProcessor {
11  constructor() {
12    super();
13    this._buffer = new Float32Array(0);
14    this._chunkSize = 320; // 20ms at 16kHz
15  }
16
17  process(inputs) {
18    const input = inputs[0];
19    if (!input || !input[0] || input[0].length === 0) {
20      return true;
21    }
22
23    const channelData = input[0]; // mono
24
25    // Append to buffer
26    const newBuffer = new Float32Array(this._buffer.length + channelData.length);
27    newBuffer.set(this._buffer);
28    newBuffer.set(channelData, this._buffer.length);
29    this._buffer = newBuffer;
30
31    // Emit complete 20ms chunks
32    while (this._buffer.length >= this._chunkSize) {
33      const chunk = this._buffer.slice(0, this._chunkSize);
34      this._buffer = this._buffer.slice(this._chunkSize);
35
36      // Convert Float32 [-1, 1] to Int16
37      const int16 = new Int16Array(chunk.length);
38      for (let i = 0; i < chunk.length; i++) {
39        const s = Math.max(-1, Math.min(1, chunk[i]));
40        int16[i] = s < 0 ? s * 0x8000 : s * 0x7fff;
41      }
42
43      this.port.postMessage(int16.buffer, [int16.buffer]);
44    }
45
46    return true;
47  }
48}
49
50registerProcessor('pcm-capture-processor', PCMChunkProcessor);