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<h1 class="head">Speech Synthesis Markup Requirements<br>
for Voice Markup Languages</h1>

<h3 class="notoc">W3C Working Draft <i>23 December 1999</i></h3>

<dl>
<dt>This version:</dt>

<dd><a href="http://www.w3.org/TR/1999/WD-voice-tts-reqs-19991223">
http://www.w3.org/TR/1999/WD-voice-tts-reqs-19991223</a></dd>

<dt>Latest version:</dt>

<dd><a href=
"http://www.w3.org/TR/voice-tts-reqs">
http://www.w3.org/TR/voice-tts-reqs</a></dd>

<dt>Previous version:</dt>

<dd><a href=
"http://www.w3.org/Voice/Group/1999/tts-reqs-19991118.html">
http://www.w3.org/Voice/Group/1999/tts-reqs-19991118</a></dd>

<dt>Editor:</dt>

<dd>Andrew Hunt</dd>
</dl>

<p class="copyright"><a href=
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Copyright</a> &#169; 1999 <a href="http://www.w3.org/">
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<h2 class="notoc">Abstract</h2>

<p>The W3C Voice Browser working group aims to develop
specifications to enable access to the Web using spoken
interaction. This document is part of a set of requirements
studies for voice browsers, and provides details of the
requirements for markup used for speech synthesis.</p>

<h2>Status of this document</h2>

<p>This document describes the requirements for markup used for
speech synthesis, as a precursor to starting work on
specifications. Related requirement drafts are linked from the <a
href="/TR/1999/WD-voice-intro-19991223">introduction</a>. The
requirements are being released as working drafts but are not
intended to become proposed recommendations.</p>

<p>This specification is a Working Draft of the Voice Browser working
group for review by W3C members and other interested parties.  This is
the first public version of this document. It is a draft document and
may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use W3C Working Drafts as reference
material or to cite them as other than "work in progress".</p>

<p>Publication as a Working Draft does not imply endorsement by
the W3C membership, nor of members of the Voice Browser working
groups. This is still a draft document and may be updated,
replaced or obsoleted by other documents at any time. It is
inappropriate to cite W3C Working Drafts as other than "work in
progress."</p>

<p>This document has been produced as part of the <a href=
"http://www.w3.org/Voice/">W3C Voice Browser Activity</a>,
following the procedures set out for the <a href=
"http://www.w3.org/Consortium/Process/">W3C Process</a>. The
authors of this document are members of the <a href=
"http://www.w3.org/Voice/Group">Voice Browser Working Group</a>.
This document is for public review. Comments should be sent to
the public mailing list &lt;<a href=
"mailto:www-voice@w3.org">www-voice@w3.org</a>&gt; (<a href=
"http://www.w3.org/Archives/Public/www-voice/">archive</a>) by
14th January 2000.</p>

<p>A list of current W3C Recommendations and other technical
documents can be found at <a href="http://www.w3.org/TR">
http://www.w3.org/TR</a>.</p>

<h2>0. Introduction</h2>

<p>The main goal of this subgroup is to establish a prioritized
list of requirements for speech synthesis markup which any
proposed markup language should address. This document addresses
both procedure and requirements for the specification
development. The requirements are addressed in separate sections
on <a href="#design">Design Criteria</a>, <a href=
"#architecture">Architecture and Integration</a>, <a href=
"#text-content">Text Content</a>, <a href="#rendering">
Speech-Specific Rendering</a>, and <a href="#miscellaneous">
Miscellaneous</a> followed by links to <a href=
"#further-reading">Further Reading Material</a>.</p>

<h3> 0.1 Process</h3>

<p>The specification development process will consist of the
following steps:</p>

<ol>
<li>Collect requirements on speech synthesis markup and
prioritize those requirements.</li>

<li>Distribute requirements to, and take feedback from, relevant
groups working on specific markup languages for speech
synthesis.</li>

<li>Develop a specification based on the requirements for
delivery to the W3C Voice Browser Working Group.</li>
</ol>

<!--
<h3> 0.2 Terminology</h3>

There is some variance in the use of terminology in the speech synthesis
community.  The following definitions establish a common understanding
for this document.

<br>
<br>
<table border cellpadding=3 >
<tr>
<td><b>Voice Browser</b></td>
<td> A device which interprets a (voice) markup language and is capable of
generating voice output and/or interpreting voice input, and possibly other
input/output modalities.
</tr>

<tr>
<td><b>Speech Synthesis</b></td>
<td> The process of automatic generation of speech output from data input
which may include plain text, formatted text or binary objects.  </td>
</tr>

<tr>
<td><b>Text-To-Speech</b></td>
<td> The process of automatic generation of speech output from text
or annotated text input. </td>
</tr>

</table>
--><!--
<p>
The requirements for speech synthesis markup are annotated with the
following priorities.  If a feature is deferred from the initial
specification to a future release, consideration may be given to
leaving open a path for future incorporation of the feature.
--><!--
<br>
<br>
<table border cellpadding=3 >
<tr>
<td><b>Must have</b></td>
<td> The first official specification must define the feature.
</tr>

<tr>
<td><b>Should have</b></td>
<td> The first official specification should define the feature
if feasible but may defer it until a future release.  </td>
</tr>

<tr>
<td><b>Nice to have</b></td>
<td> The first official specification may define the feature
if time permits, however, its priority is low. </td>
</tr>

<tr>
<td><b>Future revision</b></td>
<td> It is not intended that the first official specification
include the feature.  </td>
</tr>

</table>
--><a name="design"></a>
<p></p>

<h2>1. Design Criteria</h2>

<p>The markup language for speech synthesis will be developed
within the following broad design criteria. They are ordered from
higher to lower priority. In the event that two goals conflict,
the higher priority goal takes precedence. Specific technical
requirements are addressed in the following sections.</p>

<ol>
<li>The markup language for speech synthesis will enable
consistent control of voice output by speech synthesizers for use
in voice browsing and in other contexts. Consistent rendering of
speech synthesis markup should be possible across multiple
platforms and multiple speech synthesis implementations.</li>

<li>The markup language for speech synthesis will be an XML
Application and shall be interoperable with relevant W3C
specifications (see the <a href="#html-req"> interoperability
requirements</a> for details).</li>

<li>The markup language for speech synthesis should be
appropriate for speech output from a wide range of computer
applications with varying speech content (see the <a href=
"#app-req">application requirements</a> for details).</li>

<li>The markup language for speech synthesis will be
internationalized to enable speech output of a large number of
languages (see the <a href="#lang-req"> mono-lingual</a> and <a
href="#multi-lang-req">multi-lingual</a> requirements).</li>

<li>It should be easy to automatically <span class="diff">
generate, author by hand</span> and process documents using the
markup language for speech synthesis.</li>

<li>All features of the markup language for speech synthesis
should be implementable with existing, generally available
technology. Anticipated capabilities should be considered to
ensure future extensibility (but are not required to be covered
in the specification).</li>

<li>The number of optional features in the markup language for
speech synthesis will be kept to an absolute minimum, ideally
zero. For optional features, it is highly desirable that a
reasonable rendering behavior be available when not implemented
fully by a speech synthesizer. (See also the <a href=
"#compliance-req">compliance requirement</a>.)</li>

<li>Documents written in the markup language for speech synthesis
should be human-legible and reasonably clear and the
specification should avoid unnecessary terseness.</li>

<li>The element set should avoid unnecessary differences with
HTML, XHTML, ACSS and other relevant specifications. (See also
the <a href="#html-req"> interoperability requirements</a>.)</li>

<li>The markup language for speech synthesis specification should
be prepared quickly, where appropriate deriving from existing,
applied specifications.</li>
</ol>

<a name="architecture"></a>
<p></p>

<h2>2. Architecture and Integration</h2>

<a name="html-req"></a>
<p></p>

<h3>2.1 Speech generation from HTML, XHTML, DOM, XSL, ACSS etc.
(must have)</h3>

<p>It must be practical to generate speech synthesis output from
a wide range of existing document representations. Most
importantly speech output from HTML, HTML plus ACSS/CSS, XHTML,
XML plus XSL, and DOM must be possible. <a name="app-req">
</a></p>

<h3>2.2 Speech generation from application (must have)</h3>

<p>It must be practical <span class="diff">for a wide range of
applications to automatically generate speech synthesis
output</span>. Key examples include voice browsers, email
readers, web browsers, and accessibility applications.</p>

<h3>2.3 Integration with other Voice Markup (must have)</h3>

<p>The speech synthesis markup must be interoperable with other
relevant specifications developed by the W3C Voice Browser
Working Group. It must be possible to embed speech synthesis
markup into the dialog markup for prompt generation and other
spoken output. It must be possible to utilize pronunciations
defined in a standard pronunciation format. It must be possible
to utilize speech synthesis markup for universal access. (See
also <a href="#event-generation">5.2 Event generation</a>.) <a
name="mono-modal-req"></a></p>

<h3>2.4 Mono-modal output (must have)</h3>

<p>The speech synthesis markup must be appropriate in the context
of an audio-output-only (mono-modal) user interaction. <a name=
"multi-modal-req"></a></p>

<h3>2.5 Multi-modal output (must have)</h3>

<p>The speech synthesis markup must be appropriate in the context
of an multi-modal system output, most importantly, in combination
with visual output. Where appropriate, synchronization of speech
and other output should be supported with SMIL or a related
standard. (Also see <a href="#event-generation">5.2 Event
generation</a>.) <a name="text-content"></a></p>

<h2>3. Text Content</h2>

<a name="struct-req"></a>
<p></p>

<h3>3.1 Document Structure (must have)</h3>

<p>The speech synthesis markup must support the ability to
indicate document structure in a way that is instructive to a
speech synthesizer for rendering the document. <span class=
"diff">The specification must provide a well-defined set of
document structure elements</span>. At a minimum, it must be
possible to mark paragraph and sentence structures. <span class=
"diff">Dialog types and other structural elements with
distinctive spoken style will be considered in the specification
process</span>. <a name="lang-req"></a></p>

<h3>3.2 Mono-lingual document (must have)</h3>

<p>The speech synthesis markup must support the ability to
incorporate and render text of a single language in a single
document and to mark the language content appropriately. <a name=
"multi-lang-req"></a></p>

<h3>3.3 Multi-lingual document (should have)</h3>

<p>The speech synthesis markup may support the ability to
incorporate and render text of more than one language in a single
document where those languages are supported by the speech
synthesizer. The levels in document structure in which language
change is permitted would be determined during the specification
process as the definition of the speech synthesis document
structure emerges.</p>

<h3>3.4 Phonemic pronunciations (must have)</h3>

<p>The speech synthesis markup must provide the ability to
specify pronunciation entities as sequences of phonemes. Phonetic
pronunciation models may also be considered.</p>

<h3>3.5 Reference to externally defined pronunciations (should
have)</h3>

<p>The speech synthesis markup may support the ability to
reference extenally defined pronunciation or lexicon documents.
In particular, if the Voice Browser Working Group defines a
lexicon format it must be possible to reference it from the
speech synthesis markup. [In the absence of a Working Group
proposal, there are no obvious candidates for standard externally
referencable lexicon formats.]</p>

<h3>3.6 Out-of-vocabulary handling (Nice to have)</h3>

<p>The speech synthesis markup may support a mechanism to request
particular handling of out-of-vocabulary text or other
unpronunciable text. [This may instead be an API design issue and
out of the scope of these Speech Synthesis Markup
Requirements.]</p>

<h3>3.7 Acoustic-phonetic sequences (should have)</h3>

<p>The speech synthesis markup may provide a mechanism to exactly
specify the desired acoustic-phonetic rendering of a given text
segment. This may be accomplished with a sequence of high-level
phonetic and phonemic symbols, accompanied by a detailed acoustic
information for rendering the phonetic and phonemic symbols such
as duration, pitch movement, intensity, etc.</p>

<h3>3.8 Special text constructs (must have)</h3>

<p>The speech synthesis markup must provide the ability to mark a
set of common text constructs that require special handling by
speech synthesizers. The list should include dates, times,
numbers, phone numbers, currency amounts, URLs, postal addresses
and measures. A mechanism should also be provided to indicate
locale or other information that enables a speech synthesizer to
incorporate dates and other locale-sensitive constructs.</p>

<h3>3.9 Spelling - literal output (must have)</h3>

<p>The speech synthesis markup must provide the ability to mark
regions of text for "spelled" or literal output, as appropriate
to the text language.</p>

<h3>3.10 Non-speech output (must have)</h3>

<p>The speech synthesis markup must provide the ability to
incorporate non-speech audio output. This may include references
to audio files (e.g. wave and MIDI files) that are linked inline.
This may also include generation of a set of defined audio
samples such as touch-tone or other commonly used prompt
sounds.</p>

<h3>3.11 Within document natural language generation (future
revision)</h3>

<p>The speech synthesis markup may provide a mechanism to allow
on-the-fly generation/modification of output text. For example,
based on dialog context or based on what a user said to a dialog
system, the speech output may choose the appropriate verb/noun to
echo the user's spoken words. Another example could be the use of
style sheets to apply style rules to control how things like
dates are transformed before being spoken. <a name="rendering">
</a></p>

<h2>4. Speech-Specific Rendering</h2>

<h3>4.1 Speaking voice control (must have)</h3>

<p>The speech synthesis markup must provide the ability to
indicate a speaking voice for a document or for regions of text
within a document. A set of common speaking voice (or speech
font) characteristics must be defined and <span class="diff">may
include gender, age, name and instance selection (where multiple
voices have common characteristics; e.g. two male voices)</span>.
<!-- Deleted 1999/11/09 --> <!--
<h3>4.2 Speaking voice characteristics (nice to have)</h3>

<p>The speech synthesis markup may provide a mechanism for
applying variants to a speaking voice such as breathiness or
richness.  It may be a significant challenge to identify and
define characteristics that are commonly agreed upon and can
be consistently implemented by different speech synthesis systems.
--></p>

<h3>4.2 Emphasis (must have)</h3>

<p>The speech synthesis markup must provide the ability to mark
words and other regions of text for spoken emphasis (also
referred to as prominence or stress).</p>

<h3>4.3 Intonation control (should have)</h3>

<p>The speech synthesis markup may provide the ability to mark
words and other regions of text with intonational characteristics
including boundary tones (rise or fall at sentence/phrase end)
and sentential intonation (movements across
phrases/sentences).</p>

<h3>4.4 Acoustic prosodics (must have)</h3>

<p>The speech synthesis markup must provide the ability to mark
regions of text with acoustic characteristics such as pitch,
pitch range, speaking rate and volume.</p>

<h3>4.5 Synchronized facial animation (nice to have)</h3>

<p>The speech synthesis markup may provide the ability to mark
text with features that enhance synchronized facial animation.
Features may include positions of physical facial features (e.g.
lip rounding, jaw position, eye brow movements), timing data, and
expressions (e.g. smile).</p>

<h3>4.6 Spatial audio (nice to have)</h3>

<p>The speech synthesis markup may provide a mechanism for
generating spatial audio (also known as 3D audio). For instance,
this could request that the voice output be in the upper-right
quadrant forward of the listener. It may also allow the voice
location to shift over time. <a name="miscellaneous"></a></p>

<h2>5. Miscellaneous</h2>

<a name="compliance-req"></a>
<p></p>

<h3>5.1 Compliance Definition (must have)</h3>

<p>The specification must address the issue of compliance by
defining the sets of features that must be implemented for a
system to be considered compliant with the specification. Where
appropriate, compliance criteria may be defined with variants for
different contexts or environments. <a name="event-generation">
</a></p>

<h3>5.2 Event generation (must have)</h3>

<p>The speech synthesis markup may provide methods to mark points
in text output or segments of text output to generate callbacks,
event notification or other information that can be used to track
progress of text output, that determine timing and location of
barge-in for an appropriate resume, that can be used to trigger
other activities, or that can be used to synchronize speech
output with other output modalities. The mechanisms by which
event notifications are issues are outside the scope of the
speech synthesis markup specification. (See also, <a href=
"#multi-modal-req">Integration with SMIL</a>).</p>

<h3>5.3 Pause/resume behavior (should have)</h3>

<p>The speech synthesis markup specification may define behavior
of implementations with respect to pausing and resuming audio
output. Beyond the typical instant stop/start model (a tape
player paradigm) some consideration could be given to specifying
word boundaries or other locations where pausing is reasonable
for a listener. Similarly, the markup may enable a mechanism to
indicate appropriate locations to resume output that may be
different from the pause location. [These capabilities may be
more of an environment of API issue than a markup issue.]</p>

<h3>5.4 Comments (must have)</h3>

<p>The speech synthesis markup must support a mechanism for
inline comments. [Presumably the parent markup language, e.g.
XML, will provide such a mechanism.]</p>

<h3>5.5 Engine extensibility (should have)</h3>

<p>The speech synthesis markup may need to define a mechanism by
which specific speech synthesizer implementations can provide
enhancements or non-standard extensions without affecting the
core specification behavior. <a name="further-reading"></a></p>

<h2 class="diff">6. Further Reading Material</h2>

<p class="diff">The following resources are related to the Speech
Synthesis Markup Language requirements and specification.</p>

<dl class="diff">
<dt><a href="http://www.research.att.com/~rws/Sable.v1_0.htm">
SABLE</a></dt>

<dd><a href="http://www.research.att.com/~rws/Sable.v1_0.htm">
(http://www.research.att.com/~rws/Sable.v1_0.htm)</a><br>
SABLE is a markup language for controlling text to speech
engines. It has evolved out of work on combining three existing
text to speech languages: SSML, STML and JSML. Implementations
are available for the Bell Labs synthesizer and in the Festvial
speech synthesizer. The following are two of the papers written
about SABLE and its applications:<br>
<br>


<ul>
<li><a href="http://www.research.att.com/~rws/SABPAP/sabpap.htm">
SABLE: A Standard for TTS Markup</a>, Sproat et. al. <a href=
"http://www.research.att.com/~rws/SABPAP/sabpap.htm">
(http://www.research.att.com/~rws/SABPAP/sabpap.htm)</a></li>

<li><a href="http://www.bell-labs.com/project/tts/csssable.html">
SABLE: an XML-based Aural Display List For The WWW</a>, Sproat
and Raman. <a href=
"http://www.bell-labs.com/project/tts/csssable.html">
(http://www.bell-labs.com/project/tts/csssable.html)</a></li>
</ul>
</dd>

<dt><br>
<a href=
"http://java.sun.com/products/java-media/speech/forDevelopers/JSML/index.html">
Java Speech API Markup Language</a></dt>

<dd><a href=
"http://java.sun.com/products/java-media/speech/forDevelopers/JSML/index.html">
(http://java.sun.com/products/java-media/speech/forDevelopers/JSML/index.html)</a><br>

 JSML is an XML specification for controlling text-to-speech
engines. Implementations are available from IBM, Lernout &amp;
Hauspie and in the Festival speech synthesis platform and in
other implementations of the Java Speech API.</dd>

<dt><br>
<a href=
"http://www.cstr.ed.ac.uk/publications/1997/Sproat_1997_a.ps">Spoken
Text Markup Language</a></dt>

<dd><a href=
"http://www.cstr.ed.ac.uk/publications/1997/Sproat_1997_a.ps">
(http://www.cstr.ed.ac.uk/publications/1997/Sproat_1997_a.ps)</a><br>

 STML is an SGML language for controlling text to speech engines
developed jointly by Bell Laboratories and by the Centre for
Speech Technology Research, Edinburgh University.</dd>

<dt><br>
<a href="http://www.microsoft.com/iit/">Microsoft Speech API
Control Codes</a></dt>

<dd><a href="http://www.microsoft.com/iit/">
(http://www.microsoft.com/iit/)</a><br>
SAPI defines a set of inline control codes for manipulating
speech output by SAPI speech synthesizers.</dd>

<dt><br>
<a href="http://www.voicexml.com/">VoiceXML Prompts</a></dt>

<dd><a href="http://www.voicexml.com/">
(http://www.voicexml.com/)</a><br>
The Voice XML specification for dialog systems development
includes a set of prompt elements for generating speech synthesis
and other audio output that are very similar to elements of JSML
and SABLE.</dd>
</dl>
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