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# LiveKit: text to speech

## Overview

Sprag provides an OpenAI compatible text-to-speech API allowing you to use LiveKit's OpenAI plugin out of the box.
For TTS on LiveKit that means `openai.TTS` is usable by simply setting Sprag's base URL and invoking one of our models.

## Basic usage

Use `openai.TTS` in an `AgentSession`, or as a standalone speech service:

```python
import os

from livekit.agents import AgentSession
from livekit.plugins import openai

session = AgentSession(
    tts=openai.TTS(
        model="chorus",
        voice="wade",
        response_format="pcm",
        base_url="https://api.sprag.ai/v1",
        api_key=os.environ["SPRAG_API_KEY"],
    ),
    # ... stt, llm, etc.
)
```

## Configuration

### Constructor arguments

| Argument | Type | Default | Description |
| --- | --- | --- | --- |
| `model` | `str` | -- | Required parameter when used with Sprag |
| `voice` | `str` | -- | A preset id for the model you pass. |
| `response_format` | `str` | -- | Required. `pcm` or `wav`. |
| `base_url` | `str` | -- | `https://api.sprag.ai/v1`. |
| `api_key` | `str` | `OPENAI_API_KEY` env var | Your Sprag API key. |

See [text-to-speech models](/docs/tts/models) for a list of available models.

### Audio format

We stream audio back as it is generated, in `pcm` or `wav`. Choose `pcm` when
the audio feeds straight into the session. It arrives as 24 kHz mono.

You can find more information about the choice between them in
[audio formats](/docs/tts/misc/audio-formats).

## Voices

We speak in whichever preset you pass in the `voice` field, and each model has
its own set.

```bash
curl "https://api.sprag.ai/v1/audio/voices?model=chorus" \
  -H "Authorization: Bearer $SPRAG_API_KEY"
```

You can find the full list in [voice presets](/docs/tts/voices/presets).

## Realtime speech sessions

Our speech models also run as a realtime session over a WebSocket, which
LiveKit reaches through `openai.realtime.RealtimeModel`. You send text as a
chat message and we stream the spoken audio back on the same connection.

```python
import os

from livekit.agents import llm
from livekit.plugins.openai.realtime import RealtimeModel

model = RealtimeModel(
    model="chorus",
    voice="wade",
    modalities=["audio"],
    base_url="wss://api.sprag.ai/v1/audio/speech",
    api_key=os.environ["SPRAG_API_KEY"],
)
session = model.session()

chat_ctx = llm.ChatContext.empty()
chat_ctx.add_message(role="user", content="Thanks for calling. How can I help?")
await session.update_chat_ctx(chat_ctx)

reply = await session.generate_reply()
async for message in reply.message_stream:
    async for frame in message.audio_stream:
        ...  # 24 kHz mono PCM frames
```

## Reference

- [LiveKit OpenAI TTS plugin](https://docs.livekit.io/agents/models/tts/openai/)
- [Sprag text-to-speech API](/docs/tts/api)
- [Voice presets](/docs/tts/voices/presets)