eSpeak-ng

eSpeak-ng is an open-source speech synthesizer that runs entirely on your machine. It produces robotic, formant-based speech – not the most natural-sounding output, but it is free, fast, and works offline, making it ideal for rapid iteration on a language’s sound.

NOTE

If you just want the configuration fields, skip to Configuration.

Setup

Enable the feature flag when building:

cargo build --features voice-espeak

The espeak-ng system library must be installed on your machine. On Debian and Ubuntu, the package is espeak-ng-data; on other distributions, look for espeak-ng in your package manager.

You can verify the installation by running conlang config voice scan, which lists all voices that eSpeak-ng can find.

Configuration

Each eSpeak voice entry requires "driver": "espeak" and accepts the following optional fields:

A minimal entry:

{
  "driver": "espeak"
}

This uses the English voice at the default rate, pitch, and volume.

A more complete entry:

{
  "driver": "espeak",
  "voice": "mb-de5",
  "rate": 120,
  "pitch": 50,
  "volume": 100
}

MBROLA Voices

Voice codes starting with mb- (like mb-de5 or mb-en1) use the MBROLA speech synthesis system through eSpeak-ng. MBROLA voices produce smoother, more natural output than eSpeak-ng’s default formant synthesis, but they require separate data files to be installed on the system.

Run conlang config voice scan to see which MBROLA voices are available. The scan distinguishes between:

• Data installed – the voice is ready to use
• Definition exists, data missing – eSpeak-ng knows about the voice, but the data package is not installed

Install the data package for the voice you want (e.g., mbrola-de5 on Debian/Ubuntu) and it will appear in the “data installed” list on the next scan.

IPA Conversion

The toolkit generates IPA strings, but eSpeak-ng uses its own phoneme notation internally. When you speak through an eSpeak voice, the toolkit automatically converts IPA to eSpeak’s Kirshenbaum-based phoneme format.

This mapping covers the full consonant and vowel inventory described in Phonemes, including compound segments like affricates (/t͡ʃ/, /d͡ʒ/) and common diacritics (aspiration, nasalization). Some less common diacritics and tone markers have partial or approximate support – if a particular sound does not come through as expected, try a nearby phoneme or switch to AWS Polly, which accepts IPA directly.