Piper

Local text to speech for @micdrop/server, driving the Piper binary as a subprocess.

Piper voices are small VITS models, a few dozen megabytes each, covering around forty languages including French. They generate much faster than real time even on a modest CPU, which makes Piper the option to reach for outside English. In English itself, Kokoro sounds better, and English is all it speaks in Micdrop.

Installation

Install the package:

npm install @micdrop/piper

Install the binary:

pip install piper-tts

Then download a voice. It comes as a pair of files that have to sit next to each other:

mkdir -p voices && cd voices
BASE=https://huggingface.co/rhasspy/piper-voices/resolve/main/fr/fr_FR/siwis/medium
curl -LO $BASE/fr_FR-siwis-medium.onnx
curl -LO $BASE/fr_FR-siwis-medium.onnx.json

Piper ships a download_voices module, but calling it means finding the exact interpreter pip installed it under, which is rarely the python3 in your path. Downloading the two files is shorter and gives the same result.

Usage with MicdropServer

import { PiperTTS } from '@micdrop/piper'
import { MicdropServer } from '@micdrop/server'

const tts = new PiperTTS({
  modelPath: './voices/fr_FR-siwis-medium.onnx',
})

// Use with MicdropServer
new MicdropServer(socket, {
  tts,
  // ... other options
})

Usage without MicdropServer

import { PiperTTS } from '@micdrop/piper'
import { Readable } from 'stream'

const tts = new PiperTTS({
  modelPath: './voices/fr_FR-siwis-medium.onnx',
})

// Audio is raw PCM, 16 bits, 16 kHz, mono
tts.on('Audio', (chunk) => console.log('Audio:', chunk.length, 'bytes'))
tts.on('Failed', (texts) => console.error('Failed:', texts))

tts.speak(Readable.from(['Hello! ', 'What can I do for you?']))

Events

Event	Payload	Description
`Audio`	`Buffer`	A chunk of audio, PCM 16 bits, 16 kHz, mono, ready to be played.
`Failed`	`string[]`	The Piper process exited with an error. The payload stays empty.

See the TTS interface for the full contract.

Options

Option	Type	Default	Description
`modelPath`	`string`	Required	Path to the `.onnx` voice file
`configPath`	`string`	`<modelPath>.json`	Voice configuration
`binaryPath`	`string`	`'piper'` in `PATH`	Piper executable
`speaker`	`number`	Optional	Speaker index, for voices with several speakers
`lengthScale`	`number`	Optional	Duration multiplier, above 1 to slow the voice down
`noiseScale`	`number`	Optional	Variability of the generated speech
`noiseWidth`	`number`	Optional	Variability of the phoneme durations
`sentenceSilence`	`number`	Optional	Silence added after each sentence, in seconds

Voices

Voices are named after their language, their speaker and their quality, such as fr_FR-siwis-medium or en_GB-alba-medium. The voice samples page lets you listen to them before downloading.

Each voice ships as <name>.onnx and <name>.onnx.json. The JSON file gives the sample rate the model generates at, and Micdrop reads that file at startup so the audio reaches the client at the rate it expects.

One process for the whole call

Loading a voice takes about a second, and synthesizing a sentence adds almost nothing on top of it. PiperTTS starts the process as soon as you construct it and keeps it alive across sentences, so the voice loads once per call.

An interruption kills the process, which cuts the audio at once, and starts its replacement right away. The new process loads its voice while the user is still speaking and is ready by the time the next answer begins.

The whole answer is usually ready before the user has finished hearing its first sentence.

Documentation

Read the guide on running Micdrop with local models to see how Piper fits with a local LLM and local speech to text. The comparison of local TTS engines in Node measures Piper against Kokoro and Pocket TTS on latency, memory and languages.

License

The Piper engine is released under GPL-3.0 since it embedded espeak-ng. The archived rhasspy/piper repository was MIT. Micdrop drives the binary as a separate process, and redistributing that binary yourself brings the GPL terms with it. Each voice carries its own license, some of them restrictive, so read the model card of the voice you pick.