Watts

MBTA text-to-speech service

Prerequisites

• asdf
• direnv

Development

• Run asdf install from the repository root.
• mix deps.get to fetch dependencies.
• Copy .envrc.template to .envrc, then edit .envrc and make sure all required environment variables are set. When finished, run direnv allow to activate them.
• To start the server, run iex -S mix.

Note: There is no automatic code reloading in development. To pick up code changes while the server is running, use the recompile command in IEx.

API

The main API endpoint is /tts, which accepts JSON POST requests with the following parameters:

• text - The text to generate, in SSML format. Required.
• voice_id - The id of the Polly voice to use. Required.
• output_format - The format to return. Defaults to mp3

An x-api-key HTTP header must also be set to the value specified in the WATTS_API_KEY environment variable.

Example request, using curl and a local development instance of the app:

curl localhost:4005/tts \
  --output voice.mp3 \
  --header "x-api-key: your_api_key_here" \
  --json '{"voice_id": "Matthew", "text": "<speak>Your text here.</speak>"}'

Note to synthesize new (uncached) voice lines, the WATTS_ENABLE_POLLY environment variable must be true. This incurs a cost per character of text, so avoid large amounts of text or a large number of unique lines.