> ## Documentation Index
> Fetch the complete documentation index at: https://doc.rapida.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Text-to-Speech — Overview

> TTS transformer interface, factory function, and supported providers in assistant-api.

The `assistant-api` decouples speech synthesis from provider-specific logic through the same **transformer layer** as STT. Each TTS provider implements `Transformers[LLMPacket]` and is resolved at call time by the factory.

***

## Transformer Interface

```go theme={null}
// api/assistant-api/internal/type/transformer.go
type Transformers[IN any] interface {
    Initialize() error
    Transform(ctx context.Context, in IN) error
    Close(ctx context.Context) error
}

// Type alias for TTS
type TextToSpeechTransformer = Transformers[LLMPacket]
```

### LLMPacket Types

The `LLMPacket` interface is satisfied by three packet types that TTS providers must handle:

| Packet Type              | Description                        | Action                          |
| ------------------------ | ---------------------------------- | ------------------------------- |
| `LLMResponseDeltaPacket` | A text token from the LLM stream   | Send token to TTS for synthesis |
| `LLMResponseDonePacket`  | End of LLM response                | Flush/finalise synthesis        |
| `InterruptionPacket`     | User started speaking — cancel TTS | Clear/cancel queued audio       |

```go theme={null}
// Example from Deepgram TTS:
func (d *deepgramTTS) Transform(ctx context.Context, in LLMPacket) error {
    switch pkt := in.(type) {
    case InterruptionPacket:
        return d.conn.WriteJSON(map[string]string{"type": "Clear"})
    case LLMResponseDeltaPacket:
        return d.conn.WriteJSON(map[string]string{"type": "Speak", "text": pkt.Text})
    case LLMResponseDonePacket:
        return d.conn.WriteJSON(map[string]string{"type": "Flush"})
    }
    return nil
}
```

***

## Factory Function

```go theme={null}
// api/assistant-api/internal/transformer/transformer.go
func GetTextToSpeechTransformer(
    ctx        context.Context,
    logger     commons.Logger,
    provider   string,                     // AudioTransformer constant string
    credential *protos.VaultCredential,    // decrypted vault credential
    onPacket   func(AudioPacket),          // callback invoked with each synthesised audio chunk
    opts       utils.Option,
) (TextToSpeechTransformer, error)
```

***

## Supported TTS Providers

| Provider             | Identifier              | Streaming | Notes                                                    |
| -------------------- | ----------------------- | --------- | -------------------------------------------------------- |
| **Deepgram Aura**    | `deepgram`              | ✅         | Low-latency WebSocket, `wss://api.deepgram.com/v1/speak` |
| **ElevenLabs**       | `elevenlabs`            | ✅         | High-fidelity voice cloning                              |
| **Cartesia**         | `cartesia`              | ✅         | Ultra-low latency streaming                              |
| **Google Cloud TTS** | `google-speech-service` | ✅         | WaveNet / Neural2, 100+ voices                           |
| **Azure Cognitive**  | `azure-speech-service`  | ✅         | Neural voices, 140+ languages                            |
| **Sarvam AI**        | `sarvamai`              | ✅         | Indian languages                                         |
| **Resemble AI**      | `resemble`              | ✅         | Voice cloning                                            |
| **OpenAI TTS**       | `openai`                | ✅         | `tts-1`, `tts-1-hd`                                      |
| **AWS Polly**        | `aws`                   | ✅         | Neural voices                                            |

***

## Provider Pages

<CardGroup cols={3}>
  <Card title="Deepgram Aura" icon="audio-waveform" href="/opensource/services/assistant-api/tts/deepgram">
    Low-latency WebSocket synthesis
  </Card>

  <Card title="ElevenLabs" icon="volume" href="/opensource/services/assistant-api/tts/elevenlabs">
    High-fidelity voice cloning
  </Card>

  <Card title="Cartesia" icon="zap" href="/opensource/services/assistant-api/tts/cartesia">
    Ultra-low latency
  </Card>

  <Card title="Google Cloud" icon="globe" href="/opensource/services/assistant-api/tts/google">
    WaveNet / Neural2
  </Card>

  <Card title="Azure" icon="cloud" href="/opensource/services/assistant-api/tts/azure">
    Neural voices, 140+ languages
  </Card>

  <Card title="Sarvam AI" icon="languages" href="/opensource/services/assistant-api/tts/sarvamai">
    Indian languages
  </Card>

  <Card title="Configure Your Own" icon="code" href="/opensource/services/assistant-api/tts/custom">
    Implement the Transformers interface
  </Card>
</CardGroup>