chrome/common/extensions/docs/templates/intros/tts.html

<h2 id="overview">Overview</h2>

<p>Chrome provides native support for speech on Windows (using SAPI
5), Mac OS X, and Chrome OS, using speech synthesis capabilities
provided by the operating system. On all platforms, the user can
install extensions that register themselves as alternative speech
engines.</p>

<h2 id="generating_speech">Generating speech</h2>

<p>Call <code>speak()</code> from your extension or
packaged app to speak. For example:</p>

<pre>chrome.tts.speak('Hello, world.');</pre>

<p>To stop speaking immediately, just call <code>stop()</code>:

<pre>chrome.tts.stop();</pre>

<p>You can provide options that control various properties of the speech,
such as its rate, pitch, and more. For example:</p>

<pre>chrome.tts.speak('Hello, world.', {'rate': 2.0});</pre>

<p>It's also a good idea to specify the language so that a synthesizer
supporting that language (and regional dialect, if applicable) is chosen.</p>

<pre>chrome.tts.speak(
    'Hello, world.', {'lang': 'en-US', 'rate': 2.0});</pre>

<p>By default, each call to <code>speak()</code> interrupts any
ongoing speech and speaks immediately. To determine if a call would be
interrupting anything, you can call <code>isSpeaking()</code>.  In
addition, you can use the <code>enqueue</code> option to cause this
utterance to be added to a queue of utterances that will be spoken
when the current utterance has finished.</p>

<pre>chrome.tts.speak(
    'Speak this first.');
chrome.tts.speak(
    'Speak this next, when the first sentence is done.', {'enqueue': true});
</pre>

<p>A complete description of all options can be found in the
$ref:tts.speak below.
Not all speech engines will support all options.</p>

<p>To catch errors and make sure you're calling <code>speak()</code>
correctly, pass a callback function that takes no arguments. Inside
the callback, check
$ref:runtime.lastError
to see if there were any errors.</p>

<pre>chrome.tts.speak(
    utterance,
    options,
    function() {
      if (chrome.runtime.lastError) {
        console.log('Error: ' + chrome.runtime.lastError.message);
      }
    });</pre>

<p>The callback returns right away, before the engine has started
generating speech. The purpose of the callback is to alert you to
syntax errors in your use of the TTS API, not to catch all possible
errors that might occur in the process of synthesizing and outputting
speech. To catch these errors too, you need to use an event listener,
described below.</p>

<h2 id="events">Listening to events</h2>

<p>To get more real-time information about the status of synthesized speech,
pass an event listener in the options to <code>speak()</code>, like this:</p>

<pre>chrome.tts.speak(
    utterance,
    {
      onEvent: function(event) {
        console.log('Event ' + event.type ' at position ' + event.charIndex);
        if (event.type == 'error') {
          console.log('Error: ' + event.errorMessage);
        }
      }
    },
    callback);</pre>

<p>Each event includes an event type, the character index of the current
speech relative to the utterance, and for error events, an optional
error message. The event types are:</p>

<ul>
  <li><code>'start'</code>: The engine has started speaking the utterance.
  <li><code>'word'</code>: A word boundary was reached. Use
          <code>event.charIndex</code> to determine the current speech
          position.
  <li><code>'sentence'</code>: A sentence boundary was reached. Use
          <code>event.charIndex</code> to determine the current speech
          position.
  <li><code>'marker'</code>: An SSML marker was reached. Use
          <code>event.charIndex</code> to determine the current speech
          position.
  <li><code>'end'</code>: The engine has finished speaking the utterance.
  <li><code>'interrupted'</code>: This utterance was interrupted by another
          call to <code>speak()</code> or <code>stop()</code> and did not
          finish.
  <li><code>'cancelled'</code>: This utterance was queued, but then
          cancelled by another call to <code>speak()</code> or
          <code>stop()</code> and never began to speak at all.
  <li><code>'error'</code>: An engine-specific error occurred and
          this utterance cannot be spoken.
          Check <code>event.errorMessage</code> for details.
</ul>

<p>Four of the event types&mdash;<code>'end'</code>, <code>'interrupted'</code>,
<code>'cancelled'</code>, and <code>'error'</code>&mdash;are <i>final</i>.
After one of those events is received, this utterance will no longer
speak and no new events from this utterance will be received.</p>

<p>Some voices may not support all event types, and some voices may not
send any events at all. If you do not want to use a voice unless it sends
certain events, pass the events you require in the
<code>requiredEventTypes</code> member of the options object, or use
<code>getVoices()</code> to choose a voice that meets your requirements.
Both are documented below.</p>

<h2 id="ssml">SSML markup</h2>

<p>Utterances used in this API may include markup using the
<a href="http://www.w3.org/TR/speech-synthesis">Speech Synthesis Markup
Language (SSML)</a>. If you use SSML, the first argument to
<code>speak()</code> should be a complete SSML document with an XML
header and a top-level <code>&lt;speak&gt;</code> tag, not a document
fragment.</p>

<p>For example:</p>

<pre>chrome.tts.speak(
    '&lt;?xml version="1.0"?&gt;' +
    '&lt;speak&gt;' +
    '  The &lt;emphasis&gt;second&lt;/emphasis&gt; ' +
    '  word of this sentence was emphasized.' +
    '&lt;/speak&gt;');</pre>

<p>Not all speech engines will support all SSML tags, and some may not support
SSML at all, but all engines are required to ignore any SSML they don't
support and to still speak the underlying text.</p>

<h2 id="choosing_voice">Choosing a voice</h2>

<p>By default, Chrome chooses the most appropriate voice for each
utterance you want to speak, based on the language and gender. On most
Windows, Mac OS X, and Chrome OS systems, speech synthesis provided by
the operating system should be able to speak any text in at least one
language. Some users may have a variety of voices available, though,
from their operating system and from speech engines implemented by other
Chrome extensions. In those cases, you can implement custom code to choose
the appropriate voice, or to present the user with a list of choices.</p>

<p>To get a list of all voices, call <code>getVoices()</code> and pass it
a function that receives an array of <code>TtsVoice</code> objects as its
argument:</p>

<pre>chrome.tts.getVoices(
    function(voices) {
      for (var i = 0; i < voices.length; i++) {
        console.log('Voice ' + i + ':');
        console.log('  name: ' + voices[i].voiceName);
        console.log('  lang: ' + voices[i].lang);
        console.log('  gender: ' + voices[i].gender);
        console.log('  extension id: ' + voices[i].extensionId);
        console.log('  event types: ' + voices[i].eventTypes);
      }
    });</pre>