Browser                        Your backend                  LansonAI
  │                                │                            │
  ├── request session token ─────→ POST session-token ───────→ rt_... token
  │←── rt_... token ──────────────│                            │
  │                                │                            │
  ├── open WebSocket ─────────────────────────────────────────→ session.created
  │   ?access_token=rt_...                                       │
  │                                │                            │
  ├── capture mic (16kHz mono) ───│                            │
  ├── send PCM16 frames ──────────────────────────────────────→ VAD + STT
   │←── conversation.item.input_audio_transcription.completed ───────────────────────────│
  │                                │                            │
  └── display captions              │                            │

// backend route
app.post('/session-token', async (req, res) => {
  const resp = await fetch('https://audio.lansonai.com/v1/audio/transcriptions/session-token', {
    method: 'POST',
    headers: { Authorization: `Bearer ${process.env.LANSON_AUDIO_API_KEY}` },
  });
  res.json(await resp.json());
});

const { token } = await fetch('/session-token', { method: 'POST' }).then(r => r.json());
const ws = new WebSocket(
  `wss://audio.lansonai.com/v1/audio/transcriptions/stream?access_token=${token}`
);

const audioContext = new AudioContext({ sampleRate: 16000 });
const stream = await navigator.mediaDevices.getUserMedia({ audio: true });
const source = audioContext.createMediaStreamSource(stream);

// Use AudioWorklet to get PCM16LE frames
await audioContext.audioWorklet.addModule('pcm-processor.js');
const node = new AudioWorkletNode(audioContext, 'pcm-processor');
source.connect(node);

// pcm-processor.js posts PCM16LE buffers to main thread
node.port.onmessage = (e) => {
  // Send as binary frame
  ws.send(e.data); // ArrayBuffer of PCM16LE data
};

class PcmProcessor extends AudioWorkletProcessor {
  process(inputs) {
    const input = inputs[0][0]; // mono, 16kHz
    if (input) {
      const pcm16 = new Int16Array(input.length);
      for (let i = 0; i < input.length; i++) {
        const s = Math.max(-1, Math.min(1, input[i]));
        pcm16[i] = s < 0 ? s * 0x8000 : s * 0x7fff;
      }
      this.port.postMessage(pcm16.buffer);
    }
    return true;
  }
}
registerProcessor('pcm-processor', PcmProcessor);

ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  if (data.type === 'conversation.item.input_audio_transcription.completed') {
    const div = document.getElementById('captions');
    const p = document.createElement('p');
    p.textContent = data.text;
    div.appendChild(p);
  }
};