chrome.ttsEngine

Description

Use the chrome.ttsEngine API to implement a text-to-speech(TTS) engine using an extension. If your extension registers using this API, it will receive events containing an utterance to be spoken and other parameters when any extension or Chrome App uses the tts API to generate speech. Your extension can then use any available web technology to synthesize and output the speech, and send events back to the calling function to report the status.

Permissions

ttsEngine

Concepts and usage

An extension can register itself as a speech engine. By doing so, it can intercept some or all calls to functions such as tts.speak() and tts.stop() and provide an alternate implementation. Extensions are free to use any available web technology to provide speech, including streaming audio from a server, HTML5 audio. An extension could even do something different with the utterances, like display closed captions in a popup or send them as log messages to a remote server.

To implement a TTS engine, an extension must declare the "ttsEngine" permission and then declare all voices it provides in the extension manifest, like this:

{
  "name": "My TTS Engine",
  "version": "1.0",
  "permissions": ["ttsEngine"],
  "tts_engine": {
    "voices": [
      {
        "voice_name": "Alice",
        "lang": "en-US",
        "event_types": ["start", "marker", "end"]
      },
      {
        "voice_name": "Pat",
        "lang": "en-US",
        "event_types": ["end"]
      }
    ]
  },
  "background": {
    "page": "background.html",
    "persistent": false
  }
}

An extension can specify any number of voices.

The voice_name parameter is required. The name should be descriptive enough that it identifies the name of the voice and the engine used. In the unlikely event that two extensions register voices with the same name, a client can specify the ID of the extension that should do the synthesis.

The lang parameter is optional, but highly recommended. Almost always, a voice can synthesize speech in just a single language. When an engine supports more than one language, it can easily register a separate voice for each language. Under rare circumstances where a single voice can handle more than one language, it's easiest to just list two separate voices and handle them using the same logic internally. However, if you want to create a voice that will handle utterances in any language, leave out the lang parameter from your extension's manifest.

Finally, the event_types parameter is required if the engine can send events to update the client on the progress of speech synthesis. At a minimum, supporting the 'end' event type to indicate when speech is finished is highly recommended, otherwise Chrome cannot schedule queued utterances.

Once loaded, an extension can replace the list of declared voices by calling chrome.ttsEngine.updateVoices. (Note that the parameters used in the programatic call to updateVoices are in camel case: e.g., voiceName, unlike the manifest file which uses voice_name.)

The possible event types that you can send correspond to the event types that the speak() method receives:

  • 'start': The engine has started speaking the utterance.
  • 'word': A word boundary was reached. Use event.charIndex to determine the current speech position.
  • 'sentence': A sentence boundary was reached. Use event.charIndex to determine the current speech position.
  • 'marker': An SSML marker was reached. Use event.charIndex to determine the current speech position.
  • 'end': The engine has finished speaking the utterance.
  • 'error': An engine-specific error occurred and this utterance cannot be spoken. Pass more information in event.errorMessage.

The 'interrupted' and 'cancelled' events are not sent by the speech engine; they are generated automatically by Chrome.

Text-to-speech clients can get the voice information from your extension's manifest by calling tts.getVoices, assuming you've registered speech event listeners as described below.

Handle speech events

To generate speech at the request of clients, your extension must register listeners for both onSpeak and onStop, like this:

const speakListener = (utterance, options, sendTtsEvent) => {
  sendTtsEvent({type: 'start', charIndex: 0})

  // (start speaking)

  sendTtsEvent({type: 'end', charIndex: utterance.length})
};

const stopListener = () => {
  // (stop all speech)
};

chrome.ttsEngine.onSpeak.addListener(speakListener);
chrome.ttsEngine.onStop.addListener(stopListener);

The decision of whether or not to send a given speech request to an extension is based solely on whether the extension supports the given voice parameters in its manifest and has registered listeners for onSpeak and onStop. In other words, there's no way for an extension to receive a speech request and dynamically decide whether to handle it.

Types

AudioBuffer

Chrome 92+

Parameters containing an audio buffer and associated data.

Properties

  • audioBuffer

    ArrayBuffer

    The audio buffer from the text-to-speech engine. It should have length exactly audioStreamOptions.bufferSize and encoded as mono, at audioStreamOptions.sampleRate, and as linear pcm, 32-bit signed float i.e. the Float32Array type in javascript.

  • charIndex

    number optional

    The character index associated with this audio buffer.

  • isLastBuffer

    boolean optional

    True if this audio buffer is the last for the text being spoken.

AudioStreamOptions

Chrome 92+

Contains the audio stream format expected to be produced by an engine.

Properties

  • bufferSize

    number

    The number of samples within an audio buffer.

  • sampleRate

    number

    The sample rate expected in an audio buffer.

LanguageInstallStatus

Pending

The install status of a voice.

Enum

"notInstalled"

"installing"

"installed"

"failed"

LanguageStatus

Pending

Install status of a language.

Properties

  • error

    string optional

    Detail about installation failures. Optionally populated if the language failed to install.

  • installStatus

    LanguageInstallStatus

    Installation status.

  • lang

    string

    Language string in the form of language code-region code, where the region may be omitted. Examples are en, en-AU, zh-CH.

SpeakOptions

Chrome 92+

Options specified to the tts.speak() method.

Properties

  • gender

    VoiceGender optional

    Deprecated since Chrome 92

    Gender is deprecated and will be ignored.

    Gender of voice for synthesized speech.

  • lang

    string optional

    The language to be used for synthesis, in the form language-region. Examples: 'en', 'en-US', 'en-GB', 'zh-CN'.

  • pitch

    number optional

    Speaking pitch between 0 and 2 inclusive, with 0 being lowest and 2 being highest. 1.0 corresponds to this voice's default pitch.

  • rate

    number optional

    Speaking rate relative to the default rate for this voice. 1.0 is the default rate, normally around 180 to 220 words per minute. 2.0 is twice as fast, and 0.5 is half as fast. This value is guaranteed to be between 0.1 and 10.0, inclusive. When a voice does not support this full range of rates, don't return an error. Instead, clip the rate to the range the voice supports.

  • voiceName

    string optional

    The name of the voice to use for synthesis.

  • volume

    number optional

    Speaking volume between 0 and 1 inclusive, with 0 being lowest and 1 being highest, with a default of 1.0.

TtsClient

Pending

Identifier for the client requesting status.

Properties

  • id

    string

    Client making a language management request. For an extension, this is the unique extension ID. For Chrome features, this is the human-readable name of the feature.

  • Type of requestor.

TtsClientSource

Pending

Type of requestor.

Enum

"chromefeature"

"extension"

VoiceGender

Chrome 54+ Deprecated since Chrome 70

Gender is deprecated and will be ignored.

Enum

"male"

"female"

Methods

updateLanguage()

Pending
chrome.ttsEngine.updateLanguage(
  status: LanguageStatus,
)

Called by an engine when a language install is attempted, and when a language is uninstalled. Also called in response to a status request from a client. When a voice is installed or uninstalled, the engine should also call ttsEngine.updateVoices to register the voice.

Parameters

updateVoices()

Chrome 66+ 
chrome.ttsEngine.updateVoices(
  voices: TtsVoice[],
)

Called by an engine to update its list of voices. This list overrides any voices declared in this extension's manifest.

Parameters

  • voices

    TtsVoice[]

    Array of tts.TtsVoice objects representing the available voices for speech synthesis.

Events

onInstallLanguageRequest

Pending
chrome.ttsEngine.onInstallLanguageRequest.addListener(
  callback: function,
)

Fired when a TTS client requests to install a new language. The engine should attempt to download and install the language, and call ttsEngine.updateLanguage with the result. On success, the engine should also call ttsEngine.updateVoices to register the newly available voices.

Parameters

  • callback

    function

    The callback parameter looks like: 

    (requestor: TtsClient, lang: string) => void

onLanguageStatusRequest

Pending
chrome.ttsEngine.onLanguageStatusRequest.addListener(
  callback: function,
)

Fired when a TTS client requests the install status of a language.

Parameters

  • callback

    function

    The callback parameter looks like: 

    (requestor: TtsClient, lang: string) => void

onPause

chrome.ttsEngine.onPause.addListener(
  callback: function,
)

Optional: if an engine supports the pause event, it should pause the current utterance being spoken, if any, until it receives a resume event or stop event. Note that a stop event should also clear the paused state.

Parameters

  • callback

    function

    The callback parameter looks like: 

    () => void

onResume

chrome.ttsEngine.onResume.addListener(
  callback: function,
)

Optional: if an engine supports the pause event, it should also support the resume event, to continue speaking the current utterance, if any. Note that a stop event should also clear the paused state.

Parameters

  • callback

    function

    The callback parameter looks like: 

    () => void

onSpeak

chrome.ttsEngine.onSpeak.addListener(
  callback: function,
)

Called when the user makes a call to tts.speak() and one of the voices from this extension's manifest is the first to match the options object.

Parameters

  • callback

    function

    The callback parameter looks like: 

    (utterance: string, options: SpeakOptions, sendTtsEvent: function) => void

    • utterance

      string

    • options

      SpeakOptions

    • sendTtsEvent

      function

      The sendTtsEvent parameter looks like: 

      (event: tts.TtsEvent) => void

      • event

        tts.TtsEvent

        The event from the text-to-speech engine indicating the status of this utterance.

onSpeakWithAudioStream

Chrome 92+ 
chrome.ttsEngine.onSpeakWithAudioStream.addListener(
  callback: function,
)

Called when the user makes a call to tts.speak() and one of the voices from this extension's manifest is the first to match the options object. Differs from ttsEngine.onSpeak in that Chrome provides audio playback services and handles dispatching tts events.

Parameters

  • callback

    function

    The callback parameter looks like: 

    (utterance: string, options: SpeakOptions, audioStreamOptions: AudioStreamOptions, sendTtsAudio: function, sendError: function) => void

    • utterance

      string

    • options

      SpeakOptions

    • audioStreamOptions

      AudioStreamOptions

    • sendTtsAudio

      function

      The sendTtsAudio parameter looks like: 

      (audioBufferParams: AudioBuffer) => void

      • audioBufferParams

        AudioBuffer

        Parameters containing an audio buffer and associated data.

    • sendError

      function

      Chrome 94+

      The sendError parameter looks like: 

      (errorMessage?: string) => void

      • errorMessage

        string optional

        A string describing the error.

onStop

chrome.ttsEngine.onStop.addListener(
  callback: function,
)

Fired when a call is made to tts.stop and this extension may be in the middle of speaking. If an extension receives a call to onStop and speech is already stopped, it should do nothing (not raise an error). If speech is in the paused state, this should cancel the paused state.

Parameters

  • callback

    function

    The callback parameter looks like: 

    () => void