S2S

A high-performance, lightweight API server written in Rust that provides local, privacy-conscious Speech-to-Text (STT) and Text-to-Speech (TTS) capabilities. By leveraging the sherpa-onnx framework, S2S offers efficient local inference with minimal latency, requiring no external cloud dependencies.

The project aims to provide a drop-in local alternative for speech processing, featuring an API structure inspired by industry standards.

Key Features

• Local Inference: All processing is done locally on your hardware.
• Request Tracing: Integrated logging providing real-time insights into IP addresses, status codes, and request latency.
• Automated Model Management: Built-in bootstrap logic to download necessary models automatically when using the --auto flag.
• Flexible Service Fallbacks: The server starts as long as at least one model is present. If only one model is loaded, requests to the missing service will return 404 Not Found.
• OpenAI-Compatible Voice Directory: Exposes a standard /v1/audio/voices list, allowing client integrations to discover voices dynamically.
• Broad STT Language Support: Supports 25+ languages including English, Spanish, German, French, Russian, and many more.
• Flexible TTS: Integration with the Kokoro model, supporting over 50 distinct voices across 9 major languages.
• Robust STT: Powered by the NVIDIA Parakeet TDT model for accurate transcriptions.

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Getting Started

Installation

Download the latest executable for your platform from the Releases page.

Running the Server

The application requires at least one of the two models to be present locally in order to run. Execute the binary to start the server:

./s2s

If neither model is found on your system, the server will inform you and exit. You can instruct the server to automatically download and configure the required ONNX models (~1GB total) by specifying the --auto flag:

./s2s --auto

Configuration Options

The server can be customized via command-line arguments:

Argument	Description	Default
--host	The IP address to bind the server to	127.0.0.1
--port	The port to listen on	35248
--kokoro	Path to the Kokoro TTS model directory	./models/kokoro...
--parakeet	Path to the Parakeet STT model directory	./models/sherpa...
--threads	Number of threads for inference	4
--auto	Automatically download missing models	false
--log	Path to a file for persistent logging	stderr

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API Reference

Note: If the TTS or STT model is missing at startup, the server still launches, but any requests to the missing endpoints will return 404 Not Found.

1. Speech-to-Text (STT)

Endpoint: POST /v1/audio/transcriptions

Transcribe an audio file to text. The endpoint expects a multipart/form-data request containing a WAV file. The model automatically detects the language from the supported list.

Request:

curl http://127.0.0.1:35248/v1/audio/transcriptions \
  -H "Content-Type: multipart/form-data" \
  -F "file=@audio.wav"

Response:

{
  "text": "Hello world, this is a local transcription."
}

2. Text-to-Speech (TTS)

Endpoint: POST /v1/audio/speech

Synthesize text into audio.

Request Body:

Field	Type	Description
input	String	The text to be synthesized
voice	String	(Optional) The voice ID (Default: af_alloy)

Example:

curl http://127.0.0.1:35248/v1/audio/speech \
  -H "Content-Type: application/json" \
  -d '{
    "input": "Hello, I am a locally hosted voice.",
    "voice": "af_bella"
  }' --output output.wav

3. Voice Discovery

Endpoint: GET /v1/audio/voices

Retrieve the list of available TTS voices sorted alphabetically.

Request:

curl http://127.0.0.1:35248/v1/audio/voices

Response:

{
  "voices": [
    { "id": "af_alloy", "name": "af_alloy" },
    { "id": "af_aoede", "name": "af_aoede" }
  ]
}

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Language & Voice Support

Speech-to-Text (STT) Languages

S2S supports transcription for the following languages:

Bulgarian (bg)	Croatian (hr)	Czech (cs)	Danish (da)	Dutch (nl)
English (en)	Estonian (et)	Finnish (fi)	French (fr)	German (de)
Greek (el)	Hungarian (hu)	Italian (it)	Latvian (lv)	Lithuanian (lt)
Maltese (mt)	Polish (pl)	Portuguese (pt)	Romanian (ro)	Slovak (sk)
Slovenian (sl)	Spanish (es)	Swedish (sv)	Russian (ru)	Ukrainian (uk)

Text-to-Speech (TTS) Voices

For TTS, the language is determined automatically based on the prefix of the selected voice.

Language	Voice Prefix	Examples
English (US)	af_, am_	af_alloy, af_sky, am_adam, am_echo
English (UK)	bf_, bm_	bf_alice, bm_daniel
Spanish	ef_, em_	ef_dora, em_alex
French	ff_	ff_siwis
Hindi	hf_, hm_	hf_alpha, hm_psi
Italian	if_, im_	if_sara, im_nicola
Japanese	jf_, jm_	jf_alpha, jm_kumo
Portuguese	pf_, pm_	pf_dora, pm_santa
Chinese	zf_, zm_	zf_xiaobei, zm_yunxi

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Requirements

• Operating System: Linux, macOS, or Windows.
• Audio Format: For STT, input must be in WAV format (16kHz mono recommended).
• Disk Space: Approximately 1.5GB for models and dependencies.

Acknowledgments

• sherpa-onnx for the underlying inference engine.
• Kokoro for the TTS weights.
• NVIDIA for the Parakeet TDT ASR models.