csswg-drafts/css3-speech/Overview.html at f874f0313b7201eef1a70a371536c9bc8c2ffae1 · xfq/csswg-drafts · GitHub

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<!-- !DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd" -->

<html lang=en>
 <head>
  <title>CSS Speech Module, Level 3</title>
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    src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->

   <h1 id=top>CSS Speech Module</h1>

   <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 13 February
    2012</h2>

   <dl id=versions>
    <dt>This version:

    <dd>
     <!--<a href="http://www.w3.org/TR/2012/WD-css3-speech-20120213/">http://www.w3.org/TR/2012/ED-css3-speech-20120213/</a>-->
     <a
     href="http://dev.w3.org/csswg/css3-speech">http://dev.w3.org/csswg/css3-speech</a>


    <dt>Latest version:

    <dd> <a
     href="http://www.w3.org/TR/css3-speech">http://www.w3.org/TR/css3-speech/</a>


    <dt>Previous versions:

    <dd> <a
     href="http://www.w3.org/TR/2011/WD-css3-speech-20110818/">http://www.w3.org/TR/2011/WD-css3-speech-20110818/</a>

   </dl>

   <dl id=editors-list>
    <dt>Editor:

    <dd><a href="mailto:dweck@daisy.org">Daniel Weck</a> (<a
     href="http://www.daisy.org">DAISY Consortium</a>)

    <dt>Former editors:

    <dd><a href="mailto:dsr@w3.org">Dave Raggett</a> (<a
     href="http://www.w3.org/">W3C</a>/<a
     href="http://www.canon.com/">Canon</a>)

    <dd><a href="mailto:daniel@glazman.org">Daniel Glazman</a> (<a
     href="http://www.disruptive-innovations.com/">Disruptive
     Innovations</a>)

    <dd><a href="mailto:csant@opera.com">Claudio Santambrogio</a> (<a
     href="http://www.opera.com/">Opera Software</a>)
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2012 <a
    href="http://www.w3.org/"><acronym
    title="World Wide Web Consortium">W3C</acronym></a><sup>&reg;</sup> (<a
    href="http://www.csail.mit.edu/"><acronym
    title="Massachusetts Institute of Technology">MIT</acronym></a>, <a
    href="http://www.ercim.eu/"><acronym
    title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>,
    <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
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    use</a> rules apply.</p>
   <!--end-copyright--> <!-- hr title="Separator for header"  --></div>

  <h2 class="no-num no-toc" id=abstract>Abstract</h2>

  <p>CSS (Cascading Style Sheets) is a language that describes the rendering
   of markup documents (e.g. HTML, XML) on various supports, such as screen,
   paper, speech, etc. The Speech module defines aural CSS properties that
   enable authors to declaratively control the rendering of documents via
   speech synthesis, and using optional audio cues. Some of the features in
   this specification are conceptually close to functionality described in
   the Speech Synthesis Markup Language (SSML) Version 1.1 <a href="#SSML"
   rel=biblioentry>[SSML]<!--{{!SSML}}--></a>. However, the specificities of
   the CSS model mean that compatibility with SSML in terms of syntax and/or
   semantics is partially enabled. Note that this standard was developed in
   cooperation with the <a href="http://www.w3.org/Voice/">Voice Browser
   Activity</a>.

  <h2 class="no-num no-toc" id=status>Status of this document</h2>
  <!--begin-status-->

  <p>This is a public copy of the editors' draft. It is provided for
   discussion only and may change at any moment. Its publication here does
   not imply endorsement of its contents by W3C. Don't cite this document
   other than as work in progress.

  <p>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a
   href="mailto:www-style@w3.org?Subject=%5Bcss3-speech%5D%20PUT%20SUBJECT%20HERE">
   www-style@w3.org</a> (see <a
   href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
   discussion of this specification. When sending e-mail, please put the text
   &#8220;css3-speech&#8221; in the subject, preferably like this:
   &#8220;[<!---->css3-speech<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

  <p>This document was produced by the <a href="/Style/CSS/members">CSS
   Working Group</a> (part of the <a href="/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
   Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
   rel=disclosure>public list of any patent disclosures</a> made in
   connection with the deliverables of the group; that page also includes
   instructions for disclosing a patent. An individual who has actual
   knowledge of a patent which the individual believes contains <a
   href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
   W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p><span class=issue>TODO: this statement is now obsolete, as this document
   is being updated to include the resolutions from the disposition of
   comments of the Last Call Working Draft.</span>This is a “last call”
   working draft, i.e., the working group expects this to be the last call
   for comments before the document becomes a W3C Candidate Recommendation.
   The <strong>deadline for comments</strong> is <strong>30 September
   2011</strong>.</p>
  <!-- div class="issue">
      <p>The following issues need to be discussed and require working group resolutions:</p>
      <ul>
        <li>
          <a href="#issue-xxx">xxx</a>
        </li>
      </ul>
      <p>The CSS WG maintains a separate <a href="http://www.w3.org/Style/CSS/Tracker/products/29"
          >list of issues</a> for this module.</p>
          </div -->

  <p>The following features are at-risk and may be dropped at the end of the
   CR period if there has not been enough interest from implementers:
   &lsquo;<a href="#voice-balance"><code
   class=property>voice-balance</code></a>&rsquo;, &lsquo;<a
   href="#voice-duration"><code
   class=property>voice-duration</code></a>&rsquo;, &lsquo;<a
   href="#voice-pitch"><code class=property>voice-pitch</code></a>&rsquo;,
   &lsquo;<a href="#voice-range"><code
   class=property>voice-range</code></a>&rsquo;, and &lsquo;<a
   href="#voice-stress"><code class=property>voice-stress</code></a>&rsquo;.

  <p>The CSS Speech module is a community effort and if you would like to
   help with implementation and driving the specification forward along the
   W3C Recommendation track, please contact <a href="#editors-list">the
   editors</a>.

  <h2 class="no-num no-toc" id=contents>Table of contents</h2>
  <!--begin-toc-->

  <ul class=toc>
   <li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
    <ul class=toc>
     <li><a href="#design-goals"><span class=secno>1.1. </span>Design goals,
      motivations</a>

     <li><a href="#css21-rel"><span class=secno>1.2. </span>Relationship with
      CSS2.1</a>
    </ul>

   <li><a href="#values"><span class=secno>2. </span>CSS values</a>

   <li><a href="#example"><span class=secno>3. </span>Example</a>

   <li><a href="#aural-model"><span class=secno>4. </span>The aural
    formatting model</a>

   <li><a href="#mixing-props"><span class=secno>5. </span>Mixing
    properties</a>
    <ul class=toc>
     <li><a href="#mixing-props-voice-volume"><span class=secno>5.1.
      </span>The &lsquo;<code class=property>voice-volume</code>&rsquo;
      property</a>

     <li><a href="#mixing-props-voice-balance"><span class=secno>5.2.
      </span>The &lsquo;<code class=property>voice-balance</code>&rsquo;
      property</a>
    </ul>

   <li><a href="#speaking-props"><span class=secno>6. </span>Speaking
    properties</a>
    <ul class=toc>
     <li><a href="#speaking-props-speak"><span class=secno>6.1. </span>The
      &lsquo;<code class=property>speak</code>&rsquo; property</a>

     <li><a href="#speaking-props-speak-as"><span class=secno>6.2. </span>The
      &lsquo;<code class=property>speak-as</code>&rsquo; property</a>
    </ul>

   <li><a href="#pause-props"><span class=secno>7. </span>Pause properties
    </a>
    <ul class=toc>
     <li><a href="#pause-props-pause-before-after"><span class=secno>7.1.
      </span>The &lsquo;<code class=property>pause-before</code>&rsquo; and
      &lsquo;<code class=property>pause-after</code>&rsquo; properties</a>

     <li><a href="#pause-props-pause"><span class=secno>7.2. </span>The
      &lsquo;<code class=property>pause</code>&rsquo; shorthand property</a>

     <li><a href="#collapsing"><span class=secno>7.3. </span>Collapsing
      pauses</a>
    </ul>

   <li><a href="#rest-props"><span class=secno>8. </span>Rest properties</a>
    <ul class=toc>
     <li><a href="#rest-props-rest-before-after"><span class=secno>8.1.
      </span>The &lsquo;<code class=property>rest-before</code>&rsquo; and
      &lsquo;<code class=property>rest-after</code>&rsquo; properties</a>

     <li><a href="#rest-props-rest"><span class=secno>8.2. </span>The
      &lsquo;<code class=property>rest</code>&rsquo; shorthand property</a>
    </ul>

   <li><a href="#cue-props"><span class=secno>9. </span>Cue properties</a>
    <ul class=toc>
     <li><a href="#cue-props-cue-before-after"><span class=secno>9.1.
      </span>The &lsquo;<code class=property>cue-before</code>&rsquo; and
      &lsquo;<code class=property>cue-after</code>&rsquo; properties</a>

     <li><a href="#cue-props-cue"><span class=secno>9.2. </span>The
      &lsquo;<code class=property>cue</code>&rsquo; shorthand property</a>
    </ul>

   <li><a href="#voice-char-props"><span class=secno>10. </span>Voice
    characteristic properties</a>
    <ul class=toc>
     <li><a href="#voice-props-voice-family"><span class=secno>10.1.
      </span>The &lsquo;<code class=property>voice-family</code>&rsquo;
      property</a>

     <li><a href="#voice-props-voice-rate"><span class=secno>10.2. </span>The
      &lsquo;<code class=property>voice-rate</code>&rsquo; property</a>

     <li><a href="#voice-props-voice-pitch"><span class=secno>10.3.
      </span>The &lsquo;<code class=property>voice-pitch</code>&rsquo;
      property</a>

     <li><a href="#voice-props-voice-range"><span class=secno>10.4.
      </span>The &lsquo;<code class=property>voice-range</code>&rsquo;
      property</a>

     <li><a href="#voice-props-voice-stress"><span class=secno>10.5.
      </span>The &lsquo;<code class=property>voice-stress</code>&rsquo;
      property</a>
    </ul>

   <li><a href="#duration-props"><span class=secno>11. </span>Voice duration
    property</a>
    <ul class=toc>
     <li><a href="#mixing-props-voice-duration"><span class=secno>11.1.
      </span>The &lsquo;<code class=property>voice-duration</code>&rsquo;
      property</a>
    </ul>

   <li><a href="#lists"><span class=secno>12. </span>List items and counters
    styles</a>

   <li><a href="#content"><span class=secno>13. </span>Inserted and replaced
    content</a>

   <li><a href="#pronunciation"><span class=secno>14. </span> Pronunciation,
    phonemes </a>

   <li class=no-num><a href="#property-index">Appendix A &mdash; Property
    index</a>

   <li class=no-num><a href="#index">Appendix B &mdash; Index</a>

   <li class=no-num><a href="#definitions">Appendix C &mdash; Definitions</a>

    <ul class=toc>
     <li class=no-num><a href="#glossary">Glossary</a>

     <li class=no-num><a href="#conformance">Conformance</a>

     <li class=no-num><a href="#exit">CR exit criteria</a>
    </ul>

   <li class=no-num><a href="#ack">Appendix D &mdash; Acknowledgements</a>

   <li class=no-num><a href="#changes">Appendix E &mdash; Changes from
    previous draft</a>

   <li class=no-num><a href="#references">Appendix F &mdash; References</a>
    <ul class=toc>
     <li class=no-num><a href="#normative-references">Normative
      references</a>

     <li class=no-num><a href="#other-references">Other references</a>
    </ul>
  </ul>
  <!--end-toc-->

  <h2 id=introduction><span class=secno>1. </span>Introduction</h2>

  <p class=note>Note that this entire section is informative.

  <h3 id=design-goals><span class=secno>1.1. </span>Design goals, motivations</h3>

  <p>The aural rendering of a document combines speech synthesis (also known
   as "TTS", the acronym for "Text to Speech") and auditory icons (which we
   refer to as "audio cues" in this specification). The aural presentation of
   information is common amongst communities of users who are blind or
   visually-impaired. For instance, "screen readers" enable control of visual
   user-interfaces that would otherwise be inaccessible. There are other
   cases whereby listening to textual information (as opposed to reading it)
   is a necessity. Typical examples include in-car use of an e-book reader,
   industrial and medical documentation systems, home entertainment, helping
   users to learn reading, or supporting users who have reading difficulties
   (print disabilities).

  <p> When it comes to documents, the quality of the speech rendition depends
   on the structure and semantics authored within the content itself. The CSS
   Speech module provides properties that enable authors to declaratively
   control presentational aspects of the aural dimension (e.g. TTS voice,
   pitch, rate, and volume levels). These style sheet properties can be used
   together with visual properties (mixed media), or as a complete aural
   alternative to a visual presentation.

  <p> Content creators can conditionally include CSS properties dedicated to
   user-agents with text to speech synthesis capabilities, by specifying the
   "speech" media type via the <code>media</code> attribute of the
   <code>link</code> element, or with the <code>@media</code> at-rule, or
   within an <code>@import</code> statement. When doing so, the styles
   authored within the scope of such conditional statements are ignored by
   user-agents that do not support this module.

  <h3 id=css21-rel><span class=secno>1.2. </span>Relationship with CSS2.1</h3>

  <p> The CSS Speech module is a re-work of the informative CSS2.1 Aural
   appendix, within which the "aural" media type was described, but also
   deprecated (in favor of the "speech" media type). Although the <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> specification
   reserves the "speech" media type, it doesn't actually define the
   corresponding properties. This module describes the CSS properties that
   apply to the "speech" media type, and defines a new "box" model
   specifically for the aural dimension.

  <h2 id=values><span class=secno>2. </span>CSS values</h2>

  <p>This specification follows the <a
   href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
   definition conventions</a> from <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
   this specification are defined in CSS Value and Units Level 3 <a
   href="#CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>.

  <p>In addition to the property-specific values listed in their definitions,
   all properties defined in this specification also accept the <a
   href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
   keyword as their property value. For readability it has not been repeated
   explicitly.

  <h2 id=example><span class=secno>3. </span>Example</h2>

  <div class=example>
   <p>This example shows how authors can tell the speech synthesizer to speak
    HTML headings with a voice called "paul", using "moderate" emphasis
    (which is more than normal) and how to insert an audio cue (pre-recorded
    audio clip located at the given URL) before the start of TTS rendering
    for each heading. In a stereo-capable sound system, paragraphs marked
    with the CSS class "heidi" are rendered on the left audio channel (and
    with a female voice, etc.), whilst the class "peter" corresponds to the
    right channel (and to a male voice, etc.). The volume level of text spans
    marked with the class "special" is lower than normal, and a prosodic
    boundary is created by introducing a strong pause after it is spoken
    (note how the <code>span</code> inherits the voice-family from its parent
    paragraph).</p>

   <pre>
h1, h2, h3, h4, h5, h6
{
  voice-family: paul;
  voice-stress: moderate;
  cue-before: url(../audio/ping.wav);
  voice-volume: medium 6dB;
}
p.heidi
{
  voice-family: female;
  voice-balance: left;
  voice-pitch: high;
  voice-volume: -6dB;
}
p.peter
{
  voice-family: male;
  voice-balance: right;
  voice-rate: fast;
}
span.special
{
  voice-volume: soft;
  pause-after: strong;
}

...

&lt;h1&gt;I am Paul, and I speak headings.&lt;/h1&gt;
&lt;p class="heidi"&gt;Hello, I am Heidi.&lt;/p&gt;
&lt;p class="peter"&gt;
  &lt;span class="special"&gt;Can you hear me ?&lt;/span&gt;
  I am Peter.
&lt;/p&gt;</pre>
  </div>

  <h2 id=aural-model><span class=secno>4. </span>The aural formatting model</h2>

  <p>The CSS formatting model for aural media is based on a sequence of
   sounds and silences that occur within a nested context similar to the <a
   href="#box-model-def">visual box model</a>, which we name the <dfn
   id=aural-box-model>aural "box" model</dfn>. The aural "canvas" consists of
   a two-channel (stereo) space and of a temporal dimension, within which
   synthetic speech and audio cues coexist. The selected element is
   surrounded by &lsquo;<a href="#rest"><code
   class=property>rest</code></a>&rsquo;, &lsquo;<a href="#cue"><code
   class=property>cue</code></a>&rsquo; and &lsquo;<a href="#pause"><code
   class=property>pause</code></a>&rsquo; properties (from the innermost to
   the outermost position). These can be seen as aural equivalents to
   &lsquo;<code class=property>padding</code>&rsquo;, &lsquo;<code
   class=property>border</code>&rsquo; and &lsquo;<code
   class=property>margin</code>&rsquo;, respectively. When used, the
   &lsquo;<code class=css>:before</code>&rsquo; and &lsquo;<code
   class=css>:after</code>&rsquo; pseudo-elements <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> get inserted between the
   element's contents and the &lsquo;<a href="#rest"><code
   class=property>rest</code></a>&rsquo;.

  <p> The following diagram illustrates the equivalence between properties of
   the visual and aural box models, applied to the selected &lt;element&gt;:

  <p> <img alt="A graph depicting the aural 'box' model." id=aural-box
   src=aural-box.png>

  <h2 id=mixing-props><span class=secno>5. </span>Mixing properties</h2>

  <h3 id=mixing-props-voice-volume><span class=secno>5.1. </span>The
   &lsquo;<a href="#voice-volume"><code
   class=property>voice-volume</code></a>&rsquo; property</h3>

  <table class=propdef summary="name: syntax">
   <tbody>
    <tr>
     <td>Name:

     <td> <dfn id=voice-volume>voice-volume</dfn>

    <tr>
     <td> <em>Value:</em>

     <td>silent | [[x-soft | soft | medium | loud | x-loud] ||
      &lt;decibel&gt;]

    <tr>
     <td> <em>Initial:</em>

     <td>medium

    <tr>
     <td> <em>Applies&nbsp;to:</em>

     <td>all elements

    <tr>
     <td> <em>Inherited:</em>

     <td>yes

    <tr>
     <td> <em>Percentages:</em>

     <td>N/A

    <tr>
     <td> <em>Media:</em>

     <td>speech

    <tr>
     <td> <em>Computed value:</em>

     <td>a keyword value, and optionally also a decibel offset (if not zero)
  </table>

  <p>The &lsquo;<a href="#voice-volume"><code
   class=property>voice-volume</code></a>&rsquo; property allows authors to
   control the amplitude of the audio waveform generated by the speech
   synthesiser, and is also used to adjust the relative volume level of <a
   href="#cue-props">audio cues</a> within the <a href="#aural-model">audio
   "box" model</a>.

  <p class=note> Note that the functionality provided by this property is
   related to the <a
   href="http://www.w3.org/TR/speech-synthesis11/#edef_prosody"><code>volume</code>
   attribute of the <code>prosody</code> element</a> from the SSML markup
   language <a href="#SSML" rel=biblioentry>[SSML]<!--{{!SSML}}--></a>.

  <dl><!-- dt>
        <strong>normal</strong>
      </dt>
      <dd>
        <p> Corresponds to +0.0dB, which means that there is no modification of volume level. This
          value overrides the inherited value.</p>
      </dd -->

   <dt> <strong>silent</strong>

   <dd>
    <p> Specifies that no sound is generated (the text is read "silently").
     Corresponds to negative infinity in dB units.</p>

    <p class=note> Note that there is a difference between an element whose
     &lsquo;<a href="#voice-volume"><code
     class=property>voice-volume</code></a>&rsquo; property has a value of
     &lsquo;<code class=property>silent</code>&rsquo;, and an element whose
     &lsquo;<a href="#speak"><code class=property>speak</code></a>&rsquo;
     property has the value &lsquo;<code class=property>none</code>&rsquo;.
     With the former, the selected element takes up the same time as if it
     was spoken, including any pause before and after the element, but no
     sound is generated (descendants can override the &lsquo;<a
     href="#voice-volume"><code class=property>voice-volume</code></a>&rsquo;
     value and may therefore generate audio output). With the latter, the
     selected element is not rendered in the aural dimension and no time is
     allocated for playback (descendants can override the &lsquo;<a
     href="#speak"><code class=property>speak</code></a>&rsquo; value and may
     therefore generate audio output).</p>

   <dt><strong>x-soft</strong>, <strong>soft</strong>,
    <strong>medium</strong>, <strong>loud</strong>, <strong>x-loud</strong>

   <dd>
    <p> This sequence of keywords corresponds to monotonically non-decreasing
     volume levels, mapped to implementation-dependent values (i.e. inferred
     by the user-agent) that meet the user's requirements in terms of
     perceived sound loudness . The keyword &lsquo;<code
     class=property>x-soft</code>&rsquo; maps to the user's <em>minimum
     audible</em> volume level, &lsquo;<code
     class=property>x-loud</code>&rsquo; maps to the user's <em>maximum
     tolerable</em> volume level, &lsquo;<code
     class=property>medium</code>&rsquo; maps to the user's
     <em>preferred</em> volume level, &lsquo;<code
     class=property>soft</code>&rsquo; and &lsquo;<code
     class=property>loud</code>&rsquo; map to intermediary values.</p>

   <dt> <strong>&lt;decibel&gt;</strong>

   <dd>
    <p>A <a href="#number-def">number</a> immediately followed by "dB"
     (decibel unit). This represents a change (positive or negative) relative
     to the given keyword value (see enumeration above), or to the default
     value for the root element, or otherwise to the inherited volume level
     (which may itself be a combination of a keyword value and of a decibel
     offset, in which case the decibel values are combined additively). When
     the inherited volume level is &lsquo;<code
     class=property>silent</code>&rsquo;, this &lsquo;<a
     href="#voice-volume"><code class=property>voice-volume</code></a>&rsquo;
     resolves to &lsquo;<code class=property>silent</code>&rsquo; too,
     regardless of the specified &lt;decibel&gt; value. Decibels represent
     the ratio of the squares of the new signal amplitude (a1) and the
     current amplitude (a0), as per the following logarithmic equation:
     volume(dB) = 20 log10 (a1 / a0)</p>

    <p class=note> Note that -6.0dB is approximately half the amplitude of
     the audio signal, and +6.0dB is approximately twice the amplitude.</p>
  </dl>

  <p class=note>Note that the actual perceived volume levels depend on
   various factors, such as the listening environment and personal user
   preferences. The effective volume variation between &lsquo;<code
   class=property>x-soft</code>&rsquo; and &lsquo;<code
   class=property>x-loud</code>&rsquo; represents the dynamic range (in terms
   of loudness) of the speech output. Typically, this range would be
   compressed in a noisy context, i.e. the perceived loudness corresponding
   to &lsquo;<code class=property>x-soft</code>&rsquo; would effectively be
   closer to &lsquo;<code class=property>x-loud</code>&rsquo; than it would
   be in a quiet environment. There may also be situations where both
   class=property>x-loud</code>&rsquo; would map to low volume levels, such
   as in listening environments requiring discretion (e.g. library,

  <h3 id=mixing-props-voice-balance><span class=secno>5.2. </span>The
   &lsquo;<a href="#voice-balance"><code
   class=property>voice-balance</code></a>&rsquo; property</h3>

  <table class=propdef summary="name: syntax">
   <tbody>
    <tr>
     <td>Name:

     <td> <dfn id=voice-balance>voice-balance</dfn>

    <tr>
     <td> <em>Value:</em>

     <td>&lt;number&gt; | left | center | right | leftwards | rightwards

    <tr>
     <td> <em>Initial:</em>

     <td>center

    <tr>
     <td> <em>Applies&nbsp;to:</em>

     <td>all elements

    <tr>
     <td> <em>Inherited:</em>

     <td>yes

    <tr>
     <td> <em>Percentages:</em>

     <td>N/A

    <tr>
     <td> <em>Media:</em>

     <td>speech

    <tr>
     <td> <em>Computed value:</em>

     <td>the specified value resolved to a &lt;number&gt; between
      &lsquo;<code class=css>-100</code>&rsquo; and &lsquo;<code
      class=css>100</code>&rsquo; (inclusive)
  </table>

  <p> The &lsquo;<a href="#voice-balance"><code
   class=property>voice-balance</code></a>&rsquo; property controls the
   spatial distribution of audio output across a lateral sound stage: one
   extremity is on the left, the other extremity is on the right hand side,
   relative to the listener's position. Authors can specify intermediary
   steps between left and right extremities, to represent the audio
   separation along the resulting left-right axis.

  <p class=note> Note that the functionality provided by this property has no
   match in the SSML markup language <a href="#SSML"
   rel=biblioentry>[SSML]<!--{{!SSML}}--></a>.

  <dl>
   <dt> <strong>&lt;number&gt;</strong>

   <dd>
    <p>A <a href="#number-def">number</a> between &lsquo;<code
     class=css>-100</code>&rsquo; and &lsquo;<code
     class=css>100</code>&rsquo; (inclusive). Values smaller than
     &lsquo;<code class=css>-100</code>&rsquo; are clamped to &lsquo;<code
     class=css>-100</code>&rsquo;. Values greater than &lsquo;<code
     class=css>100</code>&rsquo; are clamped to &lsquo;<code
     class=css>100</code>&rsquo;. The value &lsquo;<code
     class=css>-100</code>&rsquo; represents the left side, and the value
     &lsquo;<code class=css>100</code>&rsquo; represents the right side. The
     value &lsquo;<code class=css>0</code>&rsquo; represents the center point
     whereby there is no discernible audio separation between left and right
     sides (in a stereo sound system, this corresponds to equal distribution
     of audio signals between left and right speakers).</p>

   <dt> <strong>left</strong>

   <dd>
    <p>Same as &lsquo;<code class=css>-100</code>&rsquo;.</p>

   <dt> <strong>center</strong>

   <dd>
    <p>Same as &lsquo;<code class=css>0</code>&rsquo;.</p>

   <dt> <strong>right</strong>

   <dd>
    <p>Same as &lsquo;<code class=css>100</code>&rsquo;.</p>

   <dt> <strong>leftwards</strong>

   <dd>
    <p>Moves the sound to the left, by subtracting 20 from the inherited
     &lsquo;<a href="#voice-balance"><code
     class=property>voice-balance</code></a>&rsquo; value, and by clamping
     the resulting number to &lsquo;<code class=css>-100</code>&rsquo;.</p>

   <dt> <strong>rightwards</strong>

   <dd>
    <p>Moves the sound to the right, by adding 20 to the inherited &lsquo;<a
     href="#voice-balance"><code
     class=property>voice-balance</code></a>&rsquo; value, and by clamping
     the resulting number to &lsquo;<code class=css>100</code>&rsquo;.</p>
  </dl>

  <p> User agents may be connected to different kinds of sound systems,
   featuring varying audio mixing capabilities. The expected behavior for
   mono, stereo, and surround sound systems is defined as follows:

  <ul>
   <li> When user-agents produce audio via a mono-aural sound system (i.e.
    single-speaker setup), the &lsquo;<a href="#voice-balance"><code
    class=property>voice-balance</code></a>&rsquo; property has no effect.
   <li> When user-agents produce audio through a stereo sound system (e.g.
    two speakers, a pair of headphones), the left-right distribution of audio
    signals can precisely match the authored values for the &lsquo;<a
    href="#voice-balance"><code
    class=property>voice-balance</code></a>&rsquo; property.

   <li> When user-agents are capable of mixing audio signals through more
    than 2 channels (e.g. 5-speakers surround sound system, including a
    dedicated center channel), the physical distribution of audio signals
    resulting from the application of the &lsquo;<a
    href="#voice-balance"><code
    class=property>voice-balance</code></a>&rsquo; property should be
    performed so that the listener perceives sound as if it was coming from a
    basic stereo layout. For example, the center channel as well as the
    left/right speakers may be used altogether in order to emulate the
    behavior of the &lsquo;<code class=property>center</code>&rsquo; value.
  </ul>

  <p> Future revisions of the CSS Speech module may include support for
   three-dimensional audio, which would effectively enable authors to specify
   "azimuth" and "elevation" values. In the future, content authored using
   the current specification may therefore be consumed by user-agents which
   are compliant with the version of CSS Speech that supports
   three-dimensional audio. In order to prepare for this possibility, the
   values enabled by the current &lsquo;<a href="#voice-balance"><code
   class=property>voice-balance</code></a>&rsquo; property are designed to
   remain compatible with "azimuth" angles. More precisely, the mapping
   between the current left-right audio axis (lateral sound stage) and the
   envisioned 360 degrees plane around the listener's position is defined as
   follows:

  <ul>
   <li>The value &lsquo;<code class=css>0</code>&rsquo; maps to zero degrees
    (&lsquo;<code class=property>center</code>&rsquo;). This is in "front" of
    the listener, not from "behind".

   <li>The value &lsquo;<code class=css>-100</code>&rsquo; maps to -40
    degrees (&lsquo;<code class=property>left</code>&rsquo;). Negative angles
    are in the counter-clockwise direction (the audio stage is seen from the
    top).

   <li>The value &lsquo;<code class=css>100</code>&rsquo; maps to 40 degrees
    (&lsquo;<code class=property>right</code>&rsquo;). Positive angles are in
    the clockwise direction (the audio stage is seen from the top).

   <li>Intermediary values on the scale from &lsquo;<code
    class=css>-100</code>&rsquo; to &lsquo;<code class=css>100</code>&rsquo;
    map to the angles between -40 and 40 degrees in a numerically
    linearly-proportional manner. For example, &lsquo;<code
    class=css>-50</code>&rsquo; maps to -20 degrees.
  </ul>

  <p class=note> Note that sound systems may be configured by users in such a
   way that it would interfere with the left-right audio distribution
   specified by document authors. Typically, the various "surround" modes
   available in modern sound systems (including systems based on basic stereo
   speakers) tend to greatly alter the perceived spatial arrangement of audio
   signals. The illusion of a three-dimensional sound stage is often achieved
   using a combination of phase shifting, digital delay, volume control
   (channel mixing), and other techniques. Some users may even configure
   their system to "downgrade" any rendered sound to a single mono channel,
   in which case the effect of the &lsquo;<a href="#voice-balance"><code
   class=property>voice-balance</code></a>&rsquo; property would obviously
   not be perceivable at all. The rendering fidelity of authored content is
   therefore dependent on such user customizations, and the &lsquo;<a
   href="#voice-balance"><code class=property>voice-balance</code></a>&rsquo;
   property merely specifies the desired end-result.

  <p class=note> Note that many speech synthesizers only generate mono sound,
   and therefore do not intrinsically support the &lsquo;<a
   href="#voice-balance"><code class=property>voice-balance</code></a>&rsquo;
   property. The sound distribution along the left-right axis consequently
   occurs at post-synthesis stage (when the speech-enabled user-agent mixes
   the various audio sources authored within the document)

  <h2 id=speaking-props><span class=secno>6. </span>Speaking properties</h2>

  <h3 id=speaking-props-speak><span class=secno>6.1. </span>The &lsquo;<a
   href="#speak"><code class=property>speak</code></a>&rsquo; property</h3>

  <table class=propdef summary="name: syntax">
   <tbody>
    <tr>
     <td>Name:

     <td> <dfn id=speak>speak</dfn>

    <tr>
     <td> <em>Value:</em>

     <td>auto | none | normal

    <tr>
     <td> <em>Initial:</em>

     <td>auto

    <tr>
     <td> <em>Applies&nbsp;to:</em>

     <td>all elements

    <tr>
     <td> <em>Inherited:</em>

     <td>yes

    <tr>
     <td> <em>Percentages:</em>

     <td>N/A

    <tr>
     <td> <em>Media:</em>

     <td>speech

    <tr>
     <td> <em>Computed value:</em>

     <td>specified value
  </table>

  <p>The &lsquo;<a href="#speak"><code class=property>speak</code></a>&rsquo;
   property determines whether or not to render text aurally.

  <p class=note> Note that the functionality provided by this property has no
   match in the SSML markup language <a href="#SSML"
   rel=biblioentry>[SSML]<!--{{!SSML}}--></a>.

  <dl>
   <dt> <strong>auto</strong>

   <dd>
    <p>Resolves to a computed value of &lsquo;<code
     class=property>none</code>&rsquo; when <a
     href="#display-def">&lsquo;<code
     class=property>display</code>&rsquo;</a> is &lsquo;<code
     class=property>none</code>&rsquo;, otherwise resolves to a computed
     value of &lsquo;<code class=property>auto</code>&rsquo; which yields a
     used value of &lsquo;<code class=property>normal</code>&rsquo;.</p>

    <p class=note> Note that the &lsquo;<code
     class=property>none</code>&rsquo; value of the <a
     href="#display-def">&lsquo;<code
     class=property>display</code>&rsquo;</a> property cannot be overridden
     by descendants of the selected element, but the &lsquo;<code
     class=property>auto</code>&rsquo; value of &lsquo;<a href="#speak"><code
     class=property>speak</code></a>&rsquo; can however be overridden using
     either of &lsquo;<code class=property>none</code>&rsquo; or &lsquo;<code
     class=property>normal</code>&rsquo;.</p>

   <dt> <strong>none</strong>

   <dd>
    <p> This value causes an element (including pauses, cues, rests and
     actual content) to not be rendered (i.e., the element has no effect in
     the aural dimension).</p>

    <p class=note> Note that any of the descendants of the affected element
     are allowed to override this value, so descendants can actually take
     part in the aural rendering despite using &lsquo;<code
     class=property>none</code>&rsquo; at this level. However, the pauses,
     cues, and rests of the ancestor element remain "deactivated" in the
     aural dimension, and therefore do not contribute to the <a
     href="#collapsing">collapsing of pauses</a> or additive behavior of
     adjoining rests.</p>

   <dt> <strong>normal</strong>

   <dd>
    <p> The element is rendered aurally (regardless of its <a
     href="#display-def">&lsquo;<code
     class=property>display</code>&rsquo;</a> value and the <a
     href="#display-def">&lsquo;<code
     class=property>display</code>&rsquo;</a> and &lsquo;<a
     href="#speak"><code class=property>speak</code></a>&rsquo; values of its
     ancestors).</p>

    <p class=note> Note that using this value can result in the element being
     rendered in the aural dimension even though it would not be rendered on
     the visual canvas.</p>
  </dl>

  <h3 id=speaking-props-speak-as><span class=secno>6.2. </span>The &lsquo;<a
   href="#speak-as"><code class=property>speak-as</code></a>&rsquo; property</h3>

  <table class=propdef summary="name: syntax">
   <tbody>
    <tr>
     <td>Name:

     <td> <dfn id=speak-as>speak-as</dfn>

    <tr>
     <td> <em>Value:</em>

     <td>normal | spell-out || digits || [ literal-punctuation |
      no-punctuation ]

    <tr>
     <td> <em>Initial:</em>

     <td>normal

    <tr>
     <td> <em>Applies&nbsp;to:</em>

     <td>all elements