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add support for mulaw encoding within websocket transport #670

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Sep 9, 2025
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add support for mulaw encoding within websocket transport #670

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50 changes: 45 additions & 5 deletions fern/calls/websocket-transport.mdx
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,8 @@ Vapi's WebSocket transport enables real-time, bidirectional audio communication

To initiate a call using WebSocket transport:

### PCM Format (16-bit, default)

```bash
curl 'https://api.vapi.ai/call' \
-H 'authorization: Bearer YOUR_API_KEY' \
Expand All @@ -35,6 +37,25 @@ curl 'https://api.vapi.ai/call' \
}'
```

### Mu-Law Format

```bash
curl 'https://api.vapi.ai/call' \
-H 'authorization: Bearer YOUR_API_KEY' \
-H 'content-type: application/json' \
--data-raw '{
"assistantId": "YOUR_ASSISTANT_ID",
"transport": {
"provider": "vapi.websocket",
"audioFormat": {
"format": "mulaw",
"container": "raw",
"sampleRate": 8000
}
}
}'
```

### Sample API Response

```json
Expand All @@ -61,13 +82,25 @@ When creating a WebSocket call, the audio format can be customized:
| Parameter | Description | Default |
|-------------|-------------------------|---------------------|
| `format` | Audio encoding format | `pcm_s16le` (16-bit PCM) |
| `container` | Audio container format | `raw` (Raw PCM) |
| `sampleRate`| Sample rate in Hz | `16000` (16kHz) |
| `container` | Audio container format | `raw` (Raw audio) |
| `sampleRate`| Sample rate in Hz | `16000` for PCM, `8000` for Mu-Law |

### Supported Audio Formats

Vapi supports the following audio formats:

Currently, Vapi supports only raw PCM (`pcm_s16le` with `raw` container). Additional formats may be supported in future updates.
- **`pcm_s16le`**: 16-bit PCM, signed little-endian (default)
- **`mulaw`**: Mu-Law encoded audio (ITU-T G.711 standard)

Both formats use the `raw` container format for direct audio streaming.

### Format Selection Guidelines

- **PCM (`pcm_s16le`)**: Higher quality audio, larger bandwidth usage. Ideal for high-quality applications.
- **Mu-Law (`mulaw`)**: Lower bandwidth, telephony-standard encoding. Ideal for telephony integrations and bandwidth-constrained environments.

<Note>
Vapi automatically converts sample rates as needed. You can stream audio at 8kHz, 44.1kHz, etc., and Vapi will handle conversions seamlessly.
Vapi automatically converts sample rates as needed. You can stream audio at 8kHz, 44.1kHz, etc., and Vapi will handle conversions seamlessly. The system also handles format conversions internally when needed.
</Note>

## Connecting to the WebSocket
Expand All @@ -86,9 +119,16 @@ socket.onerror = (error) => console.error("WebSocket error:", error);

The WebSocket supports two types of messages:

- **Binary audio data** (PCM, 16-bit signed little-endian)
- **Binary audio data** (format depends on your configuration: PCM or Mu-Law)
- **Text-based JSON control messages**

### Audio Data Format

The binary audio data format depends on your `audioFormat` configuration:

- **PCM (`pcm_s16le`)**: 16-bit signed little-endian samples
- **Mu-Law (`mulaw`)**: 8-bit Mu-Law encoded samples (ITU-T G.711)

### Sending Audio Data

```javascript
Expand Down
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