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Gradium Phonon: 64 MB open TTS that runs on your phone

Gradium released Phonon, a text-to-speech family whose smallest 31M-parameter model is 64 MB, Apache 2.0 licensed and runs in a browser, on a phone or on a laptop CPU.

By: Mikola Máté Patrik · · 2 min read

Gradium logo on a black background with a wave of blue particles
Image: Gradium · Press kit · Official product page social image (og:image) · Source

What changed?

Gradium published Phonon (Gradium), an open-weight text-to-speech family built to run on the device itself, with no cloud round trip. A flow-matching transformer predicts audio latents one 80 ms frame at a time, and Kyutai's Mimi codec turns them into 24 kHz audio. It builds on Kyutai's Pocket TTS and streams audio while it is still generating.

Three models

  • Phonon 31m: English, 64 MB (8-bit quantized), Apache 2.0, commercial use allowed.
  • Phonon 40m: one model per language for English, French, German, Spanish and Portuguese, 74.6 MB, CC BY-NC 4.0.
  • Phonon 40m multilingual: all five languages in one model, also CC BY-NC; commercial use requires a deal with Gradium.

How to use it

The runtime is written in Rust and needs no PyTorch (GitHub). There is a CLI (uvx ptts), a Python package, an npm package for the browser, a Swift package and an Android library with Snapdragon NPU support. As a server it exposes an OpenAI-compatible speech API and also plugs into Home Assistant. Gradium says it runs up to 22x faster than real time on an iPhone 16 Pro, and even a Raspberry Pi 5 can handle it.

Why it matters

For offline assistants, screen readers, games and voice agents, cloud TTS latency and cost are often the bottleneck. A 64 MB model that is free for commercial use moves speech onto the device, and the text never leaves it. Hungarian is not included; Gradium builds custom language models on request.

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