> ## 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.

# Speech-to-Text — Overview

> STT transformer interface, factory functions, and supported providers in assistant-api.

The `assistant-api` decouples audio transcription from provider-specific logic through a **transformer layer**. Every STT provider implements the same generic interface. The factory resolves the provider string at call time.

***

## Transformer Interface

Every STT provider implements `Transformers[UserAudioPacket]`:

```go theme={null}
// api/assistant-api/internal/type/transformer.go
type Transformers[IN any] interface {
    // Initialize sets up the provider connection (WebSocket, gRPC, HTTP client).
    // Called once per call session before audio begins.
    Initialize() error

    // Transform sends one audio packet to the provider.
    // Transcription results are delivered via the onPacket callback registered at construction.
    Transform(ctx context.Context, in IN) error

    // Close tears down the connection and releases resources.
    Close(ctx context.Context) error
}

// Type alias for STT
type SpeechToTextTransformer = Transformers[UserAudioPacket]
```

`UserAudioPacket.Audio` contains raw **PCM 16-bit mono 16kHz** bytes. All providers receive this same format — resampling from 8kHz telephony audio is handled upstream.

***

## Factory Function

```go theme={null}
// api/assistant-api/internal/transformer/transformer.go
func GetSpeechToTextTransformer(
    ctx        context.Context,
    logger     commons.Logger,
    provider   string,                     // AudioTransformer constant string
    credential *protos.VaultCredential,    // decrypted vault credential
    onPacket   func(STTPacket),            // callback invoked with each transcript result
    opts       utils.Option,               // assistant-level options (language, model, etc.)
) (SpeechToTextTransformer, error)
```

The factory switches on the `provider` string. To add a new provider, add a case to this switch.

***

## Provider Identifiers

```go theme={null}
// api/assistant-api/internal/transformer/transformer.go
const (
    DEEPGRAM              AudioTransformer = "deepgram"
    GOOGLE_SPEECH_SERVICE AudioTransformer = "google-speech-service"
    AZURE_SPEECH_SERVICE  AudioTransformer = "azure-speech-service"
    CARTESIA              AudioTransformer = "cartesia"
    REVAI                 AudioTransformer = "revai"
    SARVAM                AudioTransformer = "sarvamai"
    ELEVENLABS            AudioTransformer = "elevenlabs"
    ASSEMBLYAI            AudioTransformer = "assemblyai"
)
```

***

## Supported STT Providers

| Provider             | Identifier              | Streaming | Notes                          |
| -------------------- | ----------------------- | --------- | ------------------------------ |
| **Deepgram**         | `deepgram`              | ✅         | Nova-2 / Nova-3, WebSocket SDK |
| **Google Cloud STT** | `google-speech-service` | ✅         | 100+ languages                 |
| **Azure Cognitive**  | `azure-speech-service`  | ✅         | Neural Speech, 140+ languages  |
| **AssemblyAI**       | `assemblyai`            | ✅         | Speaker diarization            |
| **Rev.ai**           | `revai`                 | ✅         | Real-time                      |
| **Sarvam AI**        | `sarvamai`              | ✅         | Indian languages               |
| **AWS Transcribe**   | `aws`                   | ✅         | Real-time streaming            |
| **OpenAI Whisper**   | `openai`                | ✗         | Batch only                     |
| **Speechmatics**     | `speechmatics`          | ✅         | Real-time                      |

***

## Provider Pages

<CardGroup cols={3}>
  <Card title="Deepgram" icon="audio-waveform" href="/opensource/services/assistant-api/stt/deepgram">
    Nova-2/Nova-3, WebSocket streaming
  </Card>

  <Card title="Google Cloud" icon="globe" href="/opensource/services/assistant-api/stt/google">
    100+ languages, Neural Speech
  </Card>

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

  <Card title="AssemblyAI" icon="activity" href="/opensource/services/assistant-api/stt/assemblyai">
    Speaker diarization, real-time
  </Card>

  <Card title="Rev.ai" icon="audio-waveform" href="/opensource/services/assistant-api/stt/revai">
    Real-time streaming
  </Card>

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

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