Supertonic TTS Is Now Available on OfflineTTS

Today we are adding Supertonic TTS to OfflineTTS.

Update — July 22, 2026: OfflineTTS now exposes all 31 languages supported by Supertonic 3 in the browser. The five-language list below describes the first public integration released in May.

This is a new browser-based text-to-speech engine for people who want local speech generation without API keys, accounts, or server-side audio processing. It joins Kokoro, Piper, and Kitten as another option in the OfflineTTS engine selector.

The first public version of our Supertonic integration focuses on five languages:

• English
• Spanish
• Portuguese
• French
• Korean

It also includes 10 built-in voices, WebGPU acceleration when available, WASM fallback for broader browser support, WAV/MP3 export, and browser caching for the model assets after the first load.

Why Add Supertonic?

OfflineTTS has always been built around a simple idea: text-to-speech should be easy to try, private by default, and useful without a cloud API.

Supertonic fits that direction well. Its browser path uses ONNX Runtime Web, which means the model can run directly inside a modern browser tab. You open the tool, load the model, type text, and generate audio locally.

That makes Supertonic useful for:

• Private scripts and drafts that should not be sent to a TTS API
• Multilingual narration in the currently supported languages
• Fast experiments with built-in voices, speed, and denoising settings
• Browser-based workflows where installing a desktop app would be friction
• Comparing local TTS engines side by side before choosing one for production

What Is Available Today

The new Supertonic TTS app includes the core controls you need for practical generation:

The current Supertonic 3 integration exposes all 31 officially supported languages. The selector is generated from one shared language registry, so the standalone Supertonic app and ebook-to-audio workflow stay consistent.

How Steps Work

The Steps setting controls how many denoising iterations the engine runs during generation.

Think of it as a quality and speed trade-off:

The default is 8 because it is a practical starting point. If you are testing wording, stay low. If you are exporting final narration, try 12-20 and compare the result.

How Speed Works

The Speed setting controls speech rate.

1.0 is the neutral baseline. Values below 1.0 slow the voice down. Values above 1.0 make it faster.

For most text, the useful range is narrow:

If the output feels too compressed, reduce speed before increasing steps. Pacing problems are often easier to fix with speed than with more denoising.

Browser Caching

Supertonic uses several model files: configuration JSON, a unicode indexer, ONNX sessions for duration prediction, text encoding, denoising, and vocoding, plus voice style JSON.

The first load downloads those assets from the configured model host. After that, OfflineTTS stores them in the browser’s IndexedDB cache, the same cache layer used by the other local engines.

That means repeat visits do not need to re-download the same model files unless the model version changes or browser storage is cleared.

Privacy Model

The Supertonic engine runs text-to-speech inference in the browser.

Your text is normalized, tagged with the selected language, converted into model inputs, and synthesized locally. The generated audio is played in the page and can be exported as WAV or MP3.

There is no account, no API key, and no server-side audio generation in this flow.

How It Compares to Other OfflineTTS Engines

Supertonic is not replacing Kokoro, Piper, or Kitten. It gives you another trade-off profile.

If you are producing English narration and want the most polished voice selection, start with Kokoro. If you want fast CPU-only English generation, try Piper. If you want a tiny model, try Kitten. If you want local generation across 31 languages with Supertonic’s style controls, try the new engine.

Editorial Review of the Current Supertonic Integration

The launch description was reviewed on August 1, 2026 against the live controls and linked Supertonic documentation. The page currently exposes the language, built-in voice, speed, generation steps, backend selection, caching, audio download, and subtitle workflow described above. Future upstream capabilities are not presented as already available in OfflineTTS.

For a release check, generate the same short passage at 8 and 16 steps, then repeat with the network disconnected after caching. Record the browser console, selected backend, render time, and resulting duration. Test at least one non-English language with a fluent reviewer; language support does not guarantee correct pronunciation of every name, number, or mixed-language sentence.

Try It

Open Supertonic TTS on OfflineTTS, load the model, select a language and built-in voice, then start with:

• Steps: 8
• Speed: 1.0 to 1.05
• Backend: Auto

For final audio, compare 8 steps against 12-20 steps and keep the speed close to natural speech.

This launch is the first step for Supertonic on OfflineTTS. The immediate goal is to make the browser experience stable, transparent, and useful for real text-to-speech work.