Pronunciation Lexicon Specification (PLS) Version 1.0

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is the Recommendation of "Pronunciation Lexicon Specification (PLS) Version 1.0". It has been produced by the Voice Browser Working Group, which is part of the Voice Browser Activity.

Comments are welcome on www-voice@w3.org (archive). See W3C mailing list and archive usage guidelines.

The design of PLS 1.0 has been widely reviewed (see the disposition of comments) and satisfies the Working Group's technical requirements. A list of implementations is included in the PLS 1.0 Implementation Report, along with the associated test suite. The Working Group made a few editorial changes to the 18 August 2008 Proposed Recommendation in response to comments. Changes from the Proposed Recommendation can be found in Appendix D.

Section 2. Pronunciation Alphabets describes the legal values of the alphabet attribute for specifying a pronunciation alphabet. The Working Group is requesting the creation of a Pronunciation Alphabet registry with IANA so that pronunciation alphabets other than "ipa" can be also used. The location of the registry will be provided at http://www.w3.org/2001/10/synthesis when the registry becomes available. A future version of the PLS specification may permit values from this registry to be used in the alphabet attribute.

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures 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 Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

The sections in the main body of this document are normative unless otherwise specified. The appendices in this document are informative unless otherwise indicated explicitly.

Table of Contents

• 1. Introduction to Pronunciation Lexicon Specification
  • 1.1 How TTS Uses PLS
  • 1.2 How ASR Uses PLS
  • 1.3 How VoiceXML Applications Use PLS
  • 1.4 What PLS does not Support
  • 1.5 Glossary of Terms
• 2. Pronunciation Alphabets
• 3. PLS Documents
  • 3.1 Document Form
  • 3.2 Conformance
    • 3.2.1 Conforming Pronunciation Lexicon Specification Documents
    • 3.2.2 Using PLS with other Namespaces
    • 3.2.3 Conforming Pronunciation Lexicon Specification Processors
• 4. Pronunciation Lexicon Markup Language Definition
  • 4.1 <lexicon> Element
  • 4.2 <meta> Element
  • 4.3 <metadata> Element
  • 4.4 <lexeme> Element
  • 4.5 <grapheme> Element
  • 4.6 <phoneme> Element
  • 4.7 <alias> Element
  • 4.8 <example> Element
  • 4.9 Multiple Pronunciations for ASR and TTS
    • 4.9.1 Multiple Pronunciations for ASR
    • 4.9.2 Multiple Pronunciations for TTS
    • 4.9.3 Examples of Multiple Pronunciations
• 5. Examples
  • 5.1 Simple Use Case
  • 5.2 Multiple Pronunciations
  • 5.3 Multiple Orthographies
  • 5.4 Homophones
  • 5.5 Homographs
  • 5.6 Pronunciation by Orthography (Acronyms, Abbreviations, etc.)
• 6. References
  • 6.1 Normative References
  • 6.2 Informative References
• 7. Contributors and Acknowledgements
• Appendix A. Schema for Pronunciation Lexicon Specification (normative)
• Appendix B. MIME Type and File Suffix (normative)
• Appendix C. Issues in Retrieving Lexical Content (informative)
• Appendix D. Changes (informative)

1. Introduction to Pronunciation Lexicon Specification

This section is informative.

The accurate specification of pronunciation is critical to the success of speech applications. Most Automatic Speech Recognition (ASR) and Text-To-Speech (TTS) engines internally provide extensive high quality lexicons with pronunciation information for many words or phrases. To ensure a maximum coverage of the words or phrases used by an application, application-specific pronunciations may be required. For example, these may be needed for proper nouns such as surnames or business names.

The Pronunciation Lexicon Specification (PLS) is designed to enable interoperable specification of pronunciation information for both ASR and TTS engines. The language is intended to be easy to use by developers while supporting the accurate specification of pronunciation information for international use.

The language allows one or more pronunciations for a word or phrase to be specified using a standard pronunciation alphabet or if necessary using vendor specific alphabets. Pronunciations are grouped together into a PLS document which may be referenced from other markup languages, such as the Speech Recognition Grammar Specification [SRGS] and the Speech Synthesis Markup Language [SSML].

In its most general sense, a lexicon is merely a list of words or phrases, possibly containing information associated with and related to the items in the list. This document uses the term "lexicon" in only one specific way, as "pronunciation lexicon". In this particular document, "lexicon" means a mapping between words (or short phrases), their written representations, and their pronunciations suitable for use by an ASR engine or a TTS engine. Pronunciation lexicons are not only useful for voice browsers; they have also proven effective mechanisms to support accessibility for persons with disabilities as well as greater usability for all users. They are used to good effect in screen readers and user agents supporting multimodal interfaces.

1.1. How TTS Uses PLS

A TTS engine aims to transform input content (either text or markup, such as SSML) into speech. This activity involves several processing steps:

• Text normalization
• Word pronunciation (lexical stress, phonetic transcription)
• Sentence structure (intonation, rhythm)
• Sentence level modification in phonetic transcription (co-articulation)
• Computation of prosodic parameters
• Generation of the acoustic signal

SSML enables a user to control and enhance TTS activity by acting through SSML elements on these levels of processing (see [SSML] for details).

PLS is intended to be the standard format of the documents referenced by the <lexicon> element of SSML (see Section 3.1.4 of [SSML]).

The following is a simple example of an SSML document. It includes an Italian movie title and the name of the director to be read in US English.

<?xml version="1.0" encoding="UTF-8"?>
<speak version="1.0" 
    xmlns="http://www.w3.org/2001/10/synthesis" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
      http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
    xml:lang="en-US">
    
    The title of the movie is: "La vita è bella" (Life is beautiful),
    which is directed by Roberto Benigni. 
</speak>

To be pronounced correctly the Italian title and the director's name might include the pronunciation inline in the SSML document.

<?xml version="1.0" encoding="UTF-8"?>
<speak version="1.0" 
    xmlns="http://www.w3.org/2001/10/synthesis" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
      http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
    xml:lang="en-US">
    
    The title of the movie is: 
    <phoneme alphabet="ipa" ph="ˈlɑ ˈviːɾə ˈʔeɪ ˈbɛlə">"La vita è bella"</phoneme>
    <!-- The IPA pronunciation is:
    "&#x02C8;l&#x0251; &#x02C8;vi&#x02D0;&#x027E;&#x0259;
     &#x02C8;&#x0294;e&#x026A; &#x02C8;b&#x025B;l&#x0259;" --> 
    (Life is beautiful),
    which is directed by 
    <phoneme alphabet="ipa" ph="ɹəˈbɛːɹɾoʊ bɛˈniːnji">Roberto Benigni.</phoneme>
    <!-- The IPA pronunciation is:
    "&#x0279;&#x0259;&#x02C8;b&#x025B;&#x02D0;&#x0279;&#x027E;o&#x028A;
     b&#x025B;&#x02C8;ni&#x02D0;nji" --> 
</speak>

Using PLS, all the pronunciations can be factored out into an external PLS document which is referenced by the <lexicon> element of SSML (see Section 3.1.4 of [SSML]).

<?xml version="1.0" encoding="UTF-8"?>
<speak version="1.0" 
    xmlns="http://www.w3.org/2001/10/synthesis" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
      http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
    xml:lang="en-US">

    <lexicon uri="http://www.example.com/movie_lexicon.pls"/>

    The title of the movie is: "La vita è bella" (Life is beautiful),
    which is directed by Roberto Benigni. 
</speak>

The referenced lexicon might look something like this:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>La vita &#x00E8;<!-- same as: è --> bella</grapheme>
    <phoneme>ˈlɑ ˈviːɾə ˈʔeɪ ˈbɛlə</phoneme>
    <!-- IPA string is:
     "&#x02C8;l&#x0251; &#x02C8;vi&#x02D0;&#x027E;&#x0259;
      &#x02C8;&#x0294;e&#x026A; &#x02C8;b&#x025B;l&#x0259;" --> 
  </lexeme>
  <lexeme>
    <grapheme>Roberto</grapheme>
    <phoneme>ɹəˈbɛːɹɾoʊ</phoneme>
    <!-- IPA string is:
     "&#x0279;&#x0259;&#x02C8;b&#x025B;&#x02D0;&#x0279;&#x027E;o&#x028A;" --> 
  </lexeme>
  <lexeme>
    <grapheme>Benigni</grapheme>
    <phoneme>bɛˈniːnji<!-- IPA string is:
     "b&#x025B;&#x02C8;ni&#x02D0;nji" --></phoneme>
  </lexeme>
</lexicon>

The PLS engine will load the external PLS document and transparently apply the pronunciations during the processing of the SSML document. An application may contain several distinct PLS documents to be used at different points within the application. Section 3.1.4 of [SSML] describes how to use more than one lexicon document referenced in a SSML document.

Informative Note:

Given that many platform/browser/text editor combinations do not correctly cut and paste Unicode text, IPA symbols may be entered as numeric character references (see Section 4.1 on Character and Entity References of either XML 1.0 [XML10] or XML 1.1 [XML11]) in the pronunciation. However, the UTF-8 representation of an IPA symbol should always be used in preference to its numeric character reference. In order to overcome potential problems with viewing the UTF-8 representation of IPA symbols in this document, pronunciation examples are also shown in a comment using numeric character references.

1.2. How ASR Uses PLS

An ASR engine transforms an audio signal into a recognized sequence of words or a semantic representation of the meaning of the utterance (see Semantic Interpretation for Speech Recognition [SISR] for a standard definition of Semantic Interpretation).

An ASR grammar is used to improve ASR performance by describing the possible words and phrases the ASR might recognize. SRGS is the standard definition of ASR grammars (see [SRGS] for details).

PLS may be used by an ASR processor to allow multiple pronunciations of words and phrases, and also to do limited text normalization, such as acronym expansion and abbreviations.

PLS entries are applied to the graphemes inside SRGS grammar rules to convert them into the phonemes to be recognized. See the example below and the example in Section 1.3 for a PLS document used for both ASR and TTS.

There might be other uses of PLS, for instance in a dictation system or for unconstrained ASR, which might be beyond the scope of this specification.

This is a very simple SRGS grammar that allows the recognition of sentences like "Boston Massachusetts" or "Miami Florida".

<?xml version="1.0" encoding="UTF-8"?>
<grammar version="1.0"
  xmlns="http://www.w3.org/2001/06/grammar"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
    http://www.w3.org/TR/speech-grammar/grammar.xsd"
  xml:lang="en-US" root="city_state" mode="voice">

  <rule id="city" scope="public">
    <one-of> <item>Boston</item> 
             <item>Miami</item> 
             <item>Fargo</item> </one-of> 
  </rule>
  <rule id="state" scope="public">
    <one-of> <item>Florida</item>
             <item>North Dakota</item>
             <item>Massachusetts</item> </one-of>
  </rule> 
  
  <rule id="city_state" scope="public"> 
     <ruleref uri="#city"/> <ruleref uri="#state"/>
  </rule>
</grammar>

If a pronunciation lexicon is referenced by a SRGS grammar it can allow multiple pronunciations of the word in the grammar to accommodate different speaking styles. Here is the same grammar with a reference to an external PLS document.

<?xml version="1.0" encoding="UTF-8"?>
<grammar version="1.0"
  xmlns="http://www.w3.org/2001/06/grammar"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
    http://www.w3.org/TR/speech-grammar/grammar.xsd"
  xml:lang="en-US" root="city_state" mode="voice">
  
  <lexicon uri="http://www.example.com/city_lexicon.pls"/>

  <rule id="city" scope="public">
    <one-of> <item>Boston</item> 
             <item>Miami</item> 
             <item>Fargo</item> </one-of> 
  </rule>
  <rule id="state" scope="public">
    <one-of> <item>Florida</item>
             <item>North Dakota</item>
             <item>Massachusetts</item> </one-of>
  </rule> 
  
  <rule id="city_state" scope="public"> 
     <ruleref uri="#city"/> <ruleref uri="#state"/>
  </rule>
</grammar>

Note also that an SRGS grammar might reference multiple PLS documents.

1.3. How VoiceXML Applications Use PLS

A VoiceXML 2.0 application ([VXML]) contains SRGS grammars for ASR and SSML prompts for TTS. The introduction of PLS into both SRGS and SSML will directly impact VoiceXML applications.

The benefits described in Section 1.1 and Section 1.2 are also available in VoiceXML applications. The application may use several contextual PLS documents at different points in the interaction, but may also use the same PLS document both in SRGS, to improve ASR, and in SSML, to improve TTS. Here is an example PLS document:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>judgment</grapheme>
    <grapheme>judgement</grapheme>
    <phoneme>ˈdʒʌdʒ.mənt</phoneme>
    <!-- IPA string is:
    "&#x02C8;d&#x0292;&#x028C;d&#x0292;&#x002E;m&#x0259;nt" --> 
  </lexeme>
  <lexeme>
    <grapheme>fiancé</grapheme>
    <grapheme>fiance</grapheme>
    <phoneme>fiˈɒns.eɪ</phoneme>
    <!-- IPA string is:
    "fi&#x02C8;&#x0252;ns&#x002E;e&#x026A;" --> 
    <phoneme>ˌfiː.ɑːnˈseɪ</phoneme>
    <!-- IPA string is:
    "&#x02CC;fi&#x02D0;&#x002E;&#x0251;&#x02D0;n&#x02C8;se&#x026A;" --> 
  </lexeme>
</lexicon>

which could be used to improve TTS as shown in the following SSML document:

<?xml version="1.0" encoding="UTF-8"?>
<speak version="1.0" 
    xmlns="http://www.w3.org/2001/10/synthesis" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
      http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
    xml:lang="en-US">

    <lexicon uri="http://www.example.com/lexicon_defined_above.xml"/>

    <p> In the judgement of my fiancé, Las Vegas is the best place for a honeymoon.
              I replied that I preferred Venice and didn't think the Venetian casino was an
              acceptable compromise.<\p>
</speak>

but also to improve ASR in the following SRGS grammar:

<?xml version="1.0" encoding="UTF-8"?>
<grammar version="1.0"
    xmlns="http://www.w3.org/2001/06/grammar"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
      http://www.w3.org/TR/speech-grammar/grammar.xsd"
    xml:lang="en-US" root="movies" mode="voice">
  
  <lexicon uri="http://www.example.com/lexicon_defined_above.xml"/>

  <rule id="movies" scope="public">
     <one-of>
             <item>Terminator 2: Judgment Day</item> 
             <item>My Big Fat Obnoxious Fiance</item> 
             <item>Pluto's Judgement Day</item>
     </one-of> 
  </rule>
</grammar>

1.4. What PLS does not Support

The current specification is focused on the major features described in the requirements document [REQS]. The most complex features have been postponed to a future revision of this specification. Some of the complex features not included, for instance, are the introduction of morphological, syntactic and semantic information associated with pronunciations (such as word stems, inter-word semantic links, pronunciation statistics, etc.). Many of these features can be specified using RDF [RDF-XMLSYNTAX] that reference lexemes within one or more pronunciation lexicons.

1.5. Glossary of Terms

Acronym

A word formed from the initial letters of a name, such as PC for Personal Computer, or by combining initial letters or parts of a series of words, such as radar for radio detection and ranging, or variations, such as W3C for World Wide Web consortium.

Acronym expansion

The action of replacing an acronym by the sequence of words it represents. Acronym expansion is typically performed to help TTS engines read acronyms and ASR to recognize them.

ASR, Automatic Speech Recognition, Speech Recognition

The process of using an automatic computation algorithm to analyze spoken utterances to determine what words and phrases or semantic information were present.

Grapheme

One of the set of the smallest units of a written language, such as letters, ideograms, or symbols, that distinguish one word from another; a representation of a single orthographic element.

Homophone

One of a set of words that are pronounced the same way but differ in meaning, origin, and sometimes spelling. E.g. night and knight in English. Note that color and colour are considered multiple orthographies for the same word and not homophones, because they are variations of spelling with the same pronunciation and meaning.

Homograph

'a word of the same written form as another but of different meaning and usually origin, whether pronounced the same way or not, as bear "to carry; support" and bear "animal" or lead "to conduct" and lead "metal."' [DICT] An example from French is fils (son) and fils (threads).

International Phonetic Alphabet (IPA)

The International Phonetic Alphabet [IPA] is a phonetic alphabet used by linguists to accurately and uniquely represent each of the wide variety of sounds (phones or phonemes) the human vocal apparatus can produce. It is intended as a notational standard for the phonetic representation of all languages.

Lexeme

An atomic unit in a language, like a word or a stem. In this specification, "lexeme" designates a collection of graphemic and pronunciation representations (e.g. IPA, SAMPA, Pinyin, etc.) of words or phrases.

Lexicon

In its most general sense, a lexicon is merely a list of words or phrases, possibly containing information associated with and related to the items in the list. This document uses the term "lexicon" in only one specific way, as "pronunciation lexicon", which means a mapping between words (or short phrases), their written representations, and their pronunciations suitable for use by an ASR engine or a TTS engine. However, the word "lexicon" can mean other things in other contexts.

Orthography

A notation for writing and displaying words. Orthography includes character sets, white space, case sensitivity, diacritics within languages such as Arabic or Persian, and accents within languages such as French.

Phoneme

One of the set of the smallest units of speech that can distinguish words: for example, the English language has over 40 phonemes (19 vowels and 24 consonants). In American English, /t/ and /p/ are phonemes that can distinguish the word tin from pin.

Phonetic alphabet

A set of symbols that represent the sounds in spoken languages, such as English, Chinese, or German, see also International Phonetic Alphabet.

Pronunciation lexicon

The term pronunciation lexicon means a mapping between words (or short phrases), their written representations, and their pronunciations suitable for use by an ASR engine or a TTS engine.

SAMPA

The Speech Assessment Methods Phonetic Alphabet [SAMPA]: a phonetic alphabet using only ASCII characters, rather than the extended character set used by the International Phonetic Alphabet.

Semantic Interpretation for Speech Recognition [SISR]

A W3C specification defining a process to produce a semantic result representing the meaning of a natural language utterance.

Speech Recognition Grammar Specification [SRGS]

A W3C specification defining a language to describe grammars (words or phrases) that an ASR engine can recognize.

Speech Synthesis Markup Language [SSML]

A W3C XML language for specifying the rendering of text by a TTS engine.

TTS, Text-To-Speech, Speech Synthesis

Converting text into sounds using speech synthesis techniques.

URI: Uniform Resource Identifier

A global identifier in the context of the World Wide Web [WEB-ARCH]. A URI is defined as any legal anyURI primitive as defined in Section 3.2.17 of XML Schema Part 2: Datatypes [XML-SCHEMA-2]. For informational purposes only, [RFC3986] and [RFC2732] may be useful in understanding the structure, format, and use of URIs. Note that IRIs (see [RFC3987]) are permitted within the above definition of URI.

2. Pronunciation Alphabets

A phonemic/phonetic alphabet is used to specify a pronunciation. An alphabet in this context refers to a collection of symbols to represent the sounds of one or more human languages. In the PLS specification the pronunciation alphabet is specified by the alphabet attribute (see Section 4.1 and Section 4.6 for details on the use of this attribute). The only valid values for the alphabet attribute are "ipa" (see the next paragraph) and vendor-defined strings of the form "x-organization" or "x-organization-alphabet". For example, the Japan Electronics and Information Technology Industries Association [JEITA] might wish to encourage the use of an alphabet such as "x-jeita" or "x-jeita-2000" for their phoneme alphabet [JEIDAALPHABET]. Another example might be "x-sampa" [X-SAMPA], an extension of the SAMPA phonetic alphabet [SAMPA] to cover the entire range of characters in the International Phonetic Alphabet [IPA].

A compliant PLS processor MUST support "ipa" as the value of the alphabet attribute. This means that the PLS processor MUST support the Unicode representations of the phonetic characters developed by the International Phonetic Association [IPA]. In addition to an exhaustive set of vowel and consonant symbols, this character set supports a syllable delimiter, numerous diacritics, stress symbols, lexical tone symbols, intonational markers and more. For this alphabet, legal phonetic/phonemic values are strings of the values specified in Appendix 2 of [IPAHNDBK]; note that an IPA transcription may contain white space characters to assist readability, which have no implications for the pronunciation. Informative tables of the IPA-to-Unicode mappings can be found at [IPAUNICODE1] and [IPAUNICODE2]. Note that not all of the IPA characters are available in Unicode. For processors supporting this alphabet,

• The processor MUST syntactically accept all legal values.
• The processor SHOULD handle all Unicode IPA codes that can reasonably be considered to belong to the current language.

Informative Note:

Note that there are peculiarities in the IPA alphabet which might have implications for implementers, for instance equivalent, withdrawn and superseded IPA symbols; see Appendix 2 of [IPAHNDBK] for further details.

Informative Note:

When IPA symbols are used to represent the phonemes of a language, there can be an ambiguity concerning which allophonic symbol to select to represent a phoneme. Note that this may result in inconsistencies between lexicons which were composed for the identical language.

Informative Note:

Currently there is no ready way for a blind or partially sighted person to read or interact with a lexicon containing IPA symbols. It is hoped that implementers will provide tools which will enable such an interaction.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">

3.2 Conformance

This section enumerates the conformance rules of this specification.

All sections in this specification are normative, unless otherwise indicated. The informative parts of this specification are identified by "Informative" labels within sections.

Individual conformance requirements or testable statements are identifiable in the PLS specification through imperative voice statements. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. However, for readability, these words do not appear in all uppercase letters in this specification.

4. Pronunciation Lexicon Markup Language Definition

The Pronunciation Lexicon markup language consists of the following elements and attributes:

4.1 <lexicon> Element

The root element of the Pronunciation Lexicon markup language is the <lexicon> element. This element is the container for all other elements of the PLS language. A <lexicon> element MUST contain zero or more <meta> elements, followed by an OPTIONAL <metadata> element, followed by zero or more <lexeme> elements. Note that a PLS document without any <lexeme> elements may be useful as a placeholder for future lexical entries.

The <lexicon> element MUST specify an alphabet attribute which indicates the default pronunciation alphabet to be used within the PLS document. The values of the alphabet attribute are described in Section 2. The default pronunciation alphabet MAY be overridden for a given lexeme using the <phoneme> element.

The REQUIRED version attribute indicates the version of the specification to be used for the document and MUST have the value "1.0".

The REQUIRED xml:lang attribute allows identification of the language for which the pronunciation lexicon is relevant. IETF Best Current Practice 47 [BCP47] is the normative reference on the values of the xml:lang attribute.

Note that xml:lang specifies a single unique language for the entire PLS document. This does not limit the ability to create multilingual SRGS [SRGS] and SSML [SSML] documents. These documents may reference multiple pronunciation lexicons, possibly written for different languages.

The namespace URI for PLS is "http://www.w3.org/2005/01/pronunciation-lexicon". All PLS markup MUST be associated with the PLS namespace, using a Namespace Declaration as described in either Namespaces in XML 1.0 [XML-NS10] or Namespaces in XML 1.1 [XML-NS11]. This can for instance be achieved by declaring an xmlns attribute on the <lexicon> element, as the examples in this specification show.

PLS documents MAY include the xml:base attribute as defined in [XML-BASE]. Note that as in the HTML 4.01 specification [HTML], this is a URI which all the relative references within the document take as their base.

Informative Note:

Note that in this version of the specification, only the contents of metadata can potentially use relative URIs.

Example:

A simple PLS document for the word "tomato" and its pronunciation.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>tomato</grapheme>
    <phoneme>təmei̥ɾou̥</phoneme>
    <!-- IPA string is: "t&#x0259;mei&#x325;&#x027E;ou&#x325;" -->
  </lexeme>
</lexicon>

4.2 <meta> Element

The <metadata> and <meta> elements are containers in which information about the document can be placed. The <metadata> element provides more general and powerful treatment of metadata information than <meta> by using a metadata schema.

A <meta> element associates a string to a declared meta property or declares http-equiv content. Either a name or http-equiv attribute is REQUIRED. It is an error to provide both name and http-equiv attributes. A content attribute is also REQUIRED. The only <meta> property defined by this specification is "seeAlso". It is used to specify a resource that might provide additional metadata information about the content. This property is modeled on the "seeAlso" property from Section 5.4.1 of "RDF Vocabulary Description Language 1.0: RDF Schema" [RDF-SCHEMA]. The http-equiv attribute has a special significance when documents are retrieved via HTTP. Although the preferred method of providing HTTP header information is to use HTTP header fields, the http-equiv content MAY be used in situations where the PLS document author is unable to configure HTTP header fields associated with their document on the origin server, for example, cache control information. Note that HTTP servers and caches are not required to inspect the contents of <meta> in PLS documents and thereby override the header values they would send otherwise.

The <meta> element is an empty element.

Informative Note:

This section is modeled after the <meta> description in the HTML 4.01 Specification [HTML]. Despite the fact that the name/content model is now being replaced by better ways to include metadata, see for instance Section 20.6 of XHTML 2.0 [XHTML2], and the fact that the http-equiv directive is no longer recommended in Section 3.3 of XHTML Media Types [XHTML-MTYPES], the Working Group has decided to retain this for compatibility with the other specifications of the first version of the Speech Interface Framework (VoiceXML, SSML, SRGS, CCXML). Future versions of the framework will align with more modern metadata schemes.

Example:

This is an example of how <meta> elements can be included in a PLS document to specify a resource that provides additional metadata information.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
    <meta http-equiv="Cache-Control" content="no-cache"/>
    <meta name="seeAlso" content="http://example.com/my-pls-metadata.xml"/>
    <!--  If lexemes are to be added to this lexicon, they start below -->
</lexicon>

4.3 <metadata> Element

The <metadata> element is a container in which information about the document can be placed using metadata markup. The behavior of software processing the content of a <metadata> element is not described in this specification. Therefore, software implementing this specification is free to ignore that content.

Although any metadata markup can be used within <metadata>, it is RECOMMENDED that the RDF/XML Syntax [RDF-XMLSYNTAX] be used, in conjunction with the general metadata properties defined by the Dublin Core Metadata Initiative [DC] (e.g., Title, Creator, Subject, Description, Rights, etc.)

Example:

This is an example of how metadata can be included in a PLS document using the "Dublin Core Metadata Element Set, Version 1.1" [DC-ES] describing general document information such as title, description, date, and so on:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <metadata>
    <rdf:RDF
       xmlns:rdf = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
       xmlns:dc  = "http://purl.org/dc/elements/1.1/">

     <!-- Metadata about the PLS document -->
     <rdf:Description rdf:about=""
       dc:title="Pronunciation lexicon for W3C terms"
       dc:description="Common pronunciations for many W3C acronyms and abbreviations, i.e. I18N or WAI"
       dc:publisher="W3C"
       dc:date="2005-11-29"
       dc:rights="Copyright 2002 W3C"
       dc:format="application/pls+xml">
       <dc:creator>The W3C Voice Browser Working Group</dc:creator>
     </rdf:Description>
    </rdf:RDF>
  </metadata>
  <!--  If lexemes are to be added to this lexicon, they start below -->
</lexicon>

4.4 <lexeme> Element

The <lexeme> element is a container for a lexical entry which MAY include multiple orthographies and multiple pronunciation information.

The <lexeme> element contains one or more <grapheme> elements, one or more pronunciations (either by <phoneme> or <alias> elements or a combination of both), and zero or more <example> elements. The children of the <lexeme> element MAY appear in any order, but note that the order will have an impact on the treatment of multiple pronunciations (see Section 4.9).

The <lexeme> element has an OPTIONAL xml:id [XML-ID] attribute, allowing the element to be referenced from other documents (through fragment identifiers or XPointer [XPOINTER], for instance). For example, developers may use external RDF statements [RDF-CONC] to associate metadata (such as part of speech or word relationships) with a lexeme.

The <lexeme> element has an OPTIONAL role attribute which takes as its value one or more white space separated QNames as defined in Section 4 of Namespaces in XML (1.0 [XML-NS10] or 1.1 [XML-NS11], depending on the version of XML being used).

The role attribute describes additional information to help the selection of the most relevant pronunciation for a given orthography. The main use is to differentiate words that have the same spelling but are pronounced in different ways (cf. homographs and see also Section 5.5). A QName in the attribute content of the role attribute is expanded into an expanded-name using the namespace declarations in scope for the containing <lexeme> element. Thus, each QName provides a reference to a specific item in the designated namespace. In the second example below, the QName "claws:VVI" within the role attribute expands to the "VVI" item in the "http://www.example.com/claws7tags" namespace. This mechanism allows for referencing defined taxonomies of word classes, with the expectation that they are documented at the specified namespace URI.

Example:

A pronunciation lexicon for the Italian language with two lexemes. One of them is for the loan word "file" which is often used in technical discussions to have the same meaning and pronunciation as in English. This is distinct from the homograph noun "file" which is the plural form of "fila" meaning "queue". Note that this user-specified pronunciation for "file" takes precedence over any system-defined pronunciation.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="it">
  <lexeme>
    <grapheme>file</grapheme>
    <phoneme>faɪl</phoneme>
    <!-- This is the pronunciation
      of the loan word "file" in Italian.
      IPA string is: "fa&#x026A;l" -->
  </lexeme>
  <lexeme>
    <grapheme>EU</grapheme>
    <alias>Unione Europea
      <!-- This is a substitution of the European
      Union acronym in Italian language.  --></alias>
  </lexeme>
</lexicon>

The following is an example of a pronunciation lexicon for the word "read":

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      xmlns:claws="http://www.example.com/claws7tags" alphabet="ipa"
      xml:lang="en">
  <lexeme role="claws:VVI claws:VV0 claws:NN1">
    <!-- verb infinitive, verb present tense, singular noun -->
    <grapheme>read</grapheme>
    <phoneme>ri&#x02D0;d<!-- same as riːd --></phoneme>
  </lexeme>
  <lexeme role="claws:VVN claws:VVD">
    <!-- verb past participle, verb past tense -->
    <grapheme>read</grapheme>
    <phoneme>red</phoneme>
  </lexeme>
</lexicon>

Note that the role attribute is based on qualified values (in this example from the UCREL CLAWS7 tagset of part-of-speech) to distinguish the verb infinitive, present tense and singular noun from the verb past tense and past participle pronunciation of the word "read".

The following is an example document which references the above lexicon and includes an extension element to show how the role attribute may be used to select the relevant pronunciation of the word "read" in the dialog.

<?xml version="1.0" encoding="UTF-8"?>
<speak version="1.0" 
      xmlns="http://www.w3.org/2001/10/synthesis" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
        http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
      xmlns:myssml="http://www.example.com/ssml_extensions"
      xmlns:claws="http://www.example.com/claws7tags"
      xml:lang="en">
  <lexicon uri="http://www.example.com/lexicon.pls"
      type="application/pls+xml"/>
  <voice gender="female" age="3">
      Can you <myssml:token role="claws:VVI">read</myssml:token> this book
      to me?
  </voice>
  <voice gender="male" age="43">
      I've already <myssml:token role="claws:VVN">read</myssml:token> it
      three times!
  </voice>
</speak>

Here is another example in Chinese that uses SSML 1.1 [SSML-11].

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0"
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      xmlns:claws="http://www.example.com/claws7tags"
      alphabet="x-myorganization-pinyin"
      xml:lang="zh-CN">
  <lexeme role="claws:VV0">
    <!-- base form of lexical verb -->
    <grapheme>处</grapheme>
    <phoneme>chu3</phoneme>
    <!-- pinyin string is: "chǔ" in 处罚 处置 -->
  </lexeme>
  <lexeme role="claws:NN">
    <!-- common noun, neutral for number -->
    <grapheme>处</grapheme>
    <phoneme>chu4</phoneme>
    <!-- pinyin string is: "chù" in 处所 妙处 -->
  </lexeme>
</lexicon>

This is a sample document which references the above lexicon and shows how the role attribute may be used to select the relevant pronunciation of the Chinese word "处" in the dialog.

<?xml version="1.0" encoding="UTF-8"?>
<speak version="1.1"
      xmlns="http://www.w3.org/2001/10/synthesis"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
        http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
      xmlns:claws="http://www.example.com/claws7tags"
      xml:lang="zh-CN">
  <lexicon uri="http://www.example.com/lexicon.pls"
      type="application/pls+xml"
      xml:id="mylex"/>
  <lookup ref="mylex">
    他这个人很不好相<w role="claws:VV0">处</w>。
    此<w role="claws:NN">处</w>不准照相。
  </lookup>
</speak>  

Informative Note:

The SRGS 1.0 [SRGS] and SSML 1.0 [SSML] specifications do not currently support a selection mechanism based on the role attribute. Future versions of these specifications are expected to allow the selection of relevant pronunciations on the basis of the role attribute.

4.5 <grapheme> Element

A <lexeme> contains at least one <grapheme> element. The <grapheme> element contains text describing the orthography of the <lexeme>.

The <grapheme> element MUST contain 'character' child information items. The <grapheme> element MUST NOT contain 'element' child information items from any namespace, i.e. PLS or foreign namespace.

In more complex situations there may be alternative textual representations for the same word or phrase; this can arise due to a number of reasons, for example:

• Regional spelling variations e.g. "colour" and "color";
• Free spelling variations e.g. "judgment" and "judgement"
• Alternate writing systems, e.g. Japanese uses a mixture of Han ideographs (Kanji), and phonemic spelling systems (Katakana or Hiragana) for representing the orthography of a word or phrase, and such mixture sometimes has several variations as in kana suffixes following kanji stems (Okurigana) for example "okonau" (行なう vs. 行う);
• Reformed spellings e.g. in German some of the words which used to have "ß" before are now to be written with "ss".

In order to remove the need for duplication of pronunciation information to cope with the above variations, the <lexeme> element MAY contain more than one <grapheme> element to define the base orthography and any variants. Note that all the pronunciations given within the <lexeme> apply to each and every <grapheme> within the <lexeme>.

Examples:

An example of a single grapheme and a single pronunciation.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>Sepulveda</grapheme>
    <phoneme>səˈpʌlvɪdə</phoneme>
    <!-- IPA string is: "s&#x0259;&#x02C8;p&#x028C;lv&#x026A;d&#x0259;" -->
  </lexeme>
</lexicon>

Another example with more than one written form for a lexical entry, where the first orthography uses Latin characters for "Romaji" orthography, the second one uses "Kanji" orthography and the third one uses the "Hiragana" orthography:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="jp">
  <lexeme>
    <grapheme>nihongo<!-- "Romaji" --></grapheme>
    <grapheme>日本語<!-- "Kanji" --></grapheme>
    <grapheme>にほんご<!-- "Hiragana" --></grapheme>
    <phoneme>ɲihoŋo
      <!-- IPA string is: "&#x0272;iho&#x014B;o" --></phoneme>
  </lexeme>
</lexicon>

4.6 <phoneme> Element

A <lexeme> MAY contain one or more <phoneme> elements. The <phoneme> element contains text describing how the <lexeme> is pronounced.

The <phoneme> element MUST contain 'character' child information items. The <phoneme> element MUST NOT contain 'element' child information items from any namespace, i.e. PLS or foreign namespace.

A <phoneme> element MAY have an alphabet attribute, which indicates the pronunciation alphabet that is used for this <phoneme> element only. See Section 4.1 for the default pronunciation alphabet. The legal values for the alphabet attribute are described in Section 2.

The prefer is an OPTIONAL attribute, which indicates the pronunciation that MUST be used by a speech synthesis engine when it is set to "true". See Section 4.9 for required behavior when multiple pronunciations have prefer set to "true". The possible values are: "true" or "false". The default value is "false".

The prefer mechanism spans both the <phoneme> and <alias> elements. Section 4.9 describes how multiple pronunciations are specified in PLS for ASR and TTS, and gives many examples in Section 4.9.3.

Examples:

More than one pronunciation per lexical entry:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>huge</grapheme>
    <phoneme prefer="true">hjuːdʒ</phoneme>
    <!-- IPA string is: "hju&#x02D0;d&#x0292;" -->
    <phoneme>juːdʒ</phoneme>
    <!-- IPA string is: "ju&#x02D0;d&#x0292;" -->
  </lexeme>
</lexicon>

More than one written form and more than one pronunciation:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>theater</grapheme>
    <grapheme>theatre</grapheme>
    <phoneme prefer="true">ˈθɪətər</phoneme>
    <!-- IPA string is: "&#x02C8;&#x03B8;&#x026A;&#x0259;t&#x0259;r" -->
    <phoneme>ˈθiːjətər</phoneme>
    <!-- IPA string is: "&#x02C8;&#x03B8;i&#x02D0;j&#x0259;t&#x0259;r" -->
  </lexeme>
</lexicon>

An example of a <phoneme> that changes the pronunciation alphabet to a proprietary one.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>color</grapheme>
    <phoneme>ˈkʌlər</phoneme>
    <!-- IPA string is: "&#x02C8;k&#x028C;l&#x0259;r" -->
  </lexeme>
  <lexeme>
    <grapheme>XYZ</grapheme>
    <phoneme alphabet="x-example-alphabet">XYZ</phoneme>
    <!-- The above pronunciation is given in a proprietary alphabet 
      called: "x-example-alphabet" -->
  </lexeme>
</lexicon>

4.7 <alias> Element

A <lexeme> element MAY contain one or more <alias> elements which are used to indicate the pronunciation of an acronym or an abbreviated term, in terms of other orthographies, or other substitutions as necessary; see examples below and in Section 4.9.3.

The <alias> element MUST contain 'character' child information items. The <alias> element MUST NOT contain 'element' child information items from any namespace, i.e. PLS or foreign namespace.

In a <lexeme> element, both <alias> elements and <phoneme> elements MAY be present. If authors want explicit control over the pronunciation, they can use the <phoneme> element instead of the <alias> element.

The <alias> element has an OPTIONAL prefer attribute analogous to the prefer attribute for the <phoneme> element; see Section 4.6 for a normative description of the prefer attribute.

Pronunciations of <alias> element contents MUST be generated by the processor using pronunciations described by the <phoneme> element of any constituent graphemes in the PLS document and without invoking recursive access to the PLS document on the <alias> elements of any constituent graphemes. The processor SHOULD determine the pronunciations of the remaining <alias> element contents by the same process that it determines the pronunciation of out-of-lexicon graphemes.

Examples:

Acronym expansion using the <alias> element:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>W3C</grapheme>
    <alias>World Wide Web Consortium</alias>
  </lexeme>
</lexicon>

The following example illustrates a combination of <alias> and <phoneme> elements. The indicated acronym, "GNU", has only two pronunciations. Note that the pronunciation described by the <alias> element of "Unix" is not used as part of the pronunciation of the <alias> element contents of "GNU" as recursion of <alias> is not permissible. The pronunciations described by the <phoneme> elements of "GNU" and "Unix" are used by the processor to generate the pronunciation of "GNU is Not Unix".

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>GNU</grapheme>
    <alias><!-- be careful about recursion here -->GNU is Not Unix</alias>
    <phoneme>gəˈnuː</phoneme>
    <!-- IPA string is: "g&#x0259;&#x02C8;nu&#x02D0;" -->
  </lexeme>
  <lexeme>
    <grapheme>Unix</grapheme>
    <grapheme>UNIX</grapheme>
    <alias>a multiplexed information and computing service</alias>
    <phoneme>ˈjuːnɪks</phoneme>
    <!-- IPA string is: "&#x02C8;ju&#x02D0;n&#x026A;ks" -->
  </lexeme>
</lexicon>

4.8 <example> Element

The <example> element includes an example sentence that illustrates an occurrence of this lexeme. Because the examples are explicitly marked, automated tools can be used for regression testing and for generation of pronunciation lexicon documentation.

The <example> element MUST contain 'character' child information items. The <example> element MUST NOT contain 'element' child information items from any namespace, i.e. PLS or foreign namespace.

Zero, one or many <example> elements MAY be provided for a single <lexeme> element.

Example:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>lead</grapheme>
    <phoneme>led</phoneme>
    <example>My feet were as heavy as lead.<!-- possible comment --></example>
  </lexeme>
  <lexeme>
    <grapheme>lead</grapheme>
    <phoneme>liːd</phoneme>
    <!-- IPA string is: "li&#x02D0;d" -->
    <example>The guide once again took the lead.</example>
  </lexeme>
</lexicon>

4.9 Multiple Pronunciations for ASR and TTS

This section describes the treatment of multiple pronunciations specified in a PLS document for ASR and TTS.

4.9.1 Multiple Pronunciations for ASR

If more than one pronunciation for a given <lexeme> is specified (either by <phoneme> elements or <alias> elements or a combination of both), an ASR processor MUST consider each of them as valid pronunciations for the <grapheme>. See Example 2 and following examples in Section 4.9.3.

If more than one <lexeme> contains the same <grapheme>, all relevant pronunciations (see discussion in Section 4.4 regarding the selection of relevant pronunciations using the role attribute) will be collected in document order and an ASR processor MUST consider all of them as valid pronunciations for the <grapheme>. See Example 7 and Example 8 in Section 4.9.3.

4.9.2 Multiple Pronunciations for TTS

If more than one pronunciation for a given <lexeme> is specified (either by <phoneme> elements or <alias> elements or a combination of both), a TTS processor MUST use the first one in document order that has the prefer attribute set to "true". If none of the pronunciations has prefer set to "true", the TTS processor MUST use the first one in document order unless the TTS processor is documented as having a method of selecting pronunciations, in which case the processor MUST use any one of the pronunciations. See Example 2 and following examples in Section 4.9.3.

If more than one <lexeme> contains the same <grapheme>, all relevant pronunciations (see discussion in Section 4.4 regarding the selection of relevant pronunciations using the role attribute) will be collected in document order and a TTS processor MUST use the first one in document order that has the prefer attribute set to "true". If none of the relevant pronunciations has prefer set to "true", the TTS processor MUST use the first one in document order unless the TTS processor is documented as having a method of selecting pronunciations, in which case the processor MUST use any one of the relevant pronunciations. See Example 7 and Example 8 in Section 4.9.3.

Note that a TTS processor may have language-dependent internal mechanisms enabling it to automatically choose between multiple pronunciations. See Example 9 in Section 4.9.3.

4.9.3 Examples of Multiple Pronunciations

This section is informative.

The following examples are designed to describe and illustrate the most common examples of multiple pronunciations. Both ASR and TTS behavior is described.

Example 1:

In the following example, there is only one pronunciation. It will be used by both ASR and TTS processors.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>bead</grapheme>
    <phoneme>biːd</phoneme>
    <!-- IPA string is: "bi&#x02D0;d" -->
  </lexeme>
</lexicon>

Example 2:

In the following example, there are two pronunciations. An ASR processor will recognize both pronunciations, whereas a TTS processor will only use one. Since none of the pronunciations has prefer set to "true", unless the processor is documented to have a different strategy, it will use the first of the pronunciations because it is first in document order.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>read</grapheme>
    <phoneme>red</phoneme>
    <phoneme>riːd</phoneme>
    <!-- IPA string is: "ri&#x02D0;d" -->
  </lexeme>
</lexicon>

Example 3:

In the following example, there are two pronunciations. An ASR processor will recognize both pronunciations, whereas a TTS processor will only use the second one (because it has prefer set to "true").

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>lead</grapheme>
    <phoneme>led</phoneme>
    <phoneme prefer="true">liːd</phoneme>
    <!-- IPA string is: "li&#x02D0;d" -->
  </lexeme>
</lexicon>

Example 4:

In the following example, "read" has two pronunciations. The first one is specified by means of an alias to "red", which is defined just below it. An ASR processor will recognize both pronunciations, whereas a TTS processor will only use one. Since none of the pronunciations has prefer set to "true", unless the processor is documented to have a different strategy, it will use the first of the pronunciations because it is first in document order. In this example, the alias refers to a lexeme later in the lexicon, but in general, this order is not relevant.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>read</grapheme>
    <alias>red</alias>
    <phoneme>riːd</phoneme>
    <!-- IPA string is: "ri&#x02D0;d" -->
  </lexeme>
  <lexeme>
    <grapheme>red</grapheme>
    <phoneme>red</phoneme>
  </lexeme>
</lexicon>

Example 5:

In the following example, there are two pronunciations for "lead". Both are given with prefer set to "true". An ASR processor will recognize both pronunciations, whereas a TTS processor will only use the first one (because it is first in document order that has prefer set to "true").

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>lead</grapheme>
    <alias prefer="true">led</alias>
    <phoneme prefer="true">liːd</phoneme>
    <!-- IPA string is: "li&#x02D0;d" -->
  </lexeme>
  <lexeme>
    <grapheme>led</grapheme>
    <phoneme>led</phoneme>
  </lexeme>
</lexicon>

Example 6:

In the following example, there are two pronunciations for "lead". ASR processor will recognize both pronunciations, whereas a TTS processor will only use the second one (because it has prefer set to "true"). Note that the alias entry for "lead" as "led" does not inherit the preference of the pronunciation of the alias.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>lead</grapheme>
    <alias>led</alias>
    <phoneme prefer="true">liːd</phoneme>
    <!-- IPA string is: "li&#x02D0;d" -->
  </lexeme>
  <lexeme>
    <grapheme>led</grapheme>
    <phoneme prefer="true">led</phoneme>
  </lexeme>
</lexicon>

Example 7:

In the following example, "lead" has two different entries in the lexicon. An ASR processor will recognize both pronunciations given here, but a TTS processor will only recognize one. Since none of the pronunciations has prefer set to "true", unless the processor is documented to have a different strategy, it will use the "led" pronunciation because it is first in document order.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>lead</grapheme>
    <phoneme>led</phoneme>
  </lexeme>
  <lexeme>
    <grapheme>lead</grapheme>
    <phoneme>liːd</phoneme>
    <!-- IPA string is: "li&#x02D0;d" -->
  </lexeme>
</lexicon>

Example 8:

In the following example, there are two pronunciations in each of two different lexeme entries in the same lexicon document. An ASR processor will recognize both pronunciations given here, but a TTS processor will only recognize the "liːd" pronunciation, because it is the first one in document order that has prefer set to "true".

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>lead</grapheme>
    <alias>led</alias>
    <phoneme prefer="true">liːd</phoneme>
    <!-- IPA string is: "li&#x02D0;d" -->
  </lexeme>
  <lexeme>
    <grapheme>lead</grapheme>
    <phoneme prefer="true">led</phoneme>
    <phoneme>liːd</phoneme>
    <!-- IPA string is: "li&#x02D0;d" -->
  </lexeme>
</lexicon>

Example 9:

In the following example in French, "1" has three pronunciations. The latter two pronunciations are specified by means of an alias to "une", which is defined just below it. An ASR processor will recognize all three pronunciations given here, but a TTS processor will only recognize the "un" pronunciation, unless otherwise documented by the processor. A TTS processor documented capable of automatically choosing between multiple pronunciations will select either the "un" or "une" alias (given a grammatical context). If it selects the "une" alias then the "yn" pronunciation will be used because it has prefer set to "true".

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0"
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="fr">
  <lexeme>
    <grapheme>1</grapheme>
    <alias>un</alias>
    <alias>une</alias>
  </lexeme>
  <lexeme>
    <grapheme>une</grapheme>
    <phoneme prefer="true">yn</phoneme>
    <phoneme>ynə</phoneme>
    <!-- IPA string is: "yn&#x0259;" -->
  </lexeme>
</lexicon>

5. Examples

This section is informative.

5.1 Simple Case

In its simplest form the Pronunciation Lexicon language allows orthographies (the textual representation) to be associated with pronunciations (the phonetic/phonemic representation). A Pronunciation Lexicon document typically contains multiple entries. So, for example, to specify the pronunciation for proper names, such as "Newton" and "Scahill", the markup will look like the following.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-GB">
  <lexeme>
    <grapheme>Newton</grapheme>
    <phoneme>ˈnjuːtən</phoneme>
    <!-- IPA string is: "&#x02C8;nju&#x02D0;t&#x0259;n" -->
  </lexeme>
  <lexeme>
    <grapheme>Scahill</grapheme>
    <phoneme>ˈskɑhɪl</phoneme>
    <!-- IPA string is: "&#x02C8;sk&#x0251;h&#x026A;l" -->
  </lexeme>
</lexicon>

Here we see the root element <lexicon> which contains the two lexemes for the words "Newton" and "Scahill". Each <lexeme> is a composite element consisting of the orthographic and pronunciation representations for the entry. For each of the two <lexeme> elements there is a single <grapheme> element which includes the orthographic text and a single <phoneme> element which includes the pronunciation. In this case the alphabet attribute of the <lexicon> element is set to "ipa", so the International Phonetic Alphabet [IPA] is being used for all the pronunciations.

5.2 Multiple pronunciations for the same orthography

For ASR systems it is common to rely on multiple pronunciations of the same word or phrase in order to cope with variations of pronunciation within a language. In the Pronunciation Lexicon language, multiple pronunciations are represented by more than one <phoneme> (or <alias>) element within the same <lexeme> element.

In the following example the word "Newton" has two possible pronunciations.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-GB">
  <lexeme>
    <grapheme>Newton</grapheme>
    <phoneme>ˈnjuːtən</phoneme>
    <!-- IPA string is: "&#x02C8;nju&#x02D0;t&#x0259;n" -->
    <phoneme>ˈnuːtən</phoneme>
    <!-- IPA string is: "&#x02C8;nu&#x02D0;t&#x0259;n" -->
  </lexeme>
</lexicon>

In the situation where only a single pronunciation needs to be selected among multiple pronunciations that are available (for example where a pronunciation lexicon is being used by a speech synthesis system), then the prefer attribute on the <phoneme> element may be used to indicate the preferred pronunciation.

5.3 Multiple orthographies

In some situations there are alternative textual representations for the same word or phrase. This can arise due to a number of reasons. See Section 4.5 for details. Because these are representations that have the same meaning (as opposed to homophones), it is recommended that they be represented using a single <lexeme> element that contains multiple graphemes.

Here are two simple examples of multiple orthographies: alternative spelling of an English word and multiple writings of a Japanese word.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <!-- English entry showing how alternative spellings are handled -->
  <lexeme>
    <grapheme>colour</grapheme>
    <grapheme>color</grapheme>
    <phoneme>ˈkʌlər</phoneme>
    <!-- IPA string is: "&#x02C8;k&#x028C;l&#x0259;r" -->
  </lexeme>
</lexicon>

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="jp">
  <!-- Japanese entry showing how multiple writing systems are handled
          romaji, kanji and hiragana orthographies -->
  <lexeme>
    <grapheme>nihongo</grapheme>
    <grapheme>日本語</grapheme>
    <grapheme>にほんご</grapheme>
    <phoneme>ɲihoŋo</phoneme>
    <!-- IPA string is: "&#x0272;iho&#x014B;o" -->
  </lexeme>
</lexicon>

In some cases the pronunciations may overlap rather than being exactly the same. For example the English names "Smyth" and "Smith" share one pronunciation, but "Smyth" has a pronunciation that is only relevant to itself. Hence this needs to be represented using multiple <lexeme> elements.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>Smyth</grapheme>
    <grapheme>Smith</grapheme>
    <phoneme>smɪθ/phoneme>
    <!-- IPA string is: "sm&#x026A;&#x03B8;" -->
  </lexeme>
  <lexeme>
    <grapheme>Smyth</grapheme>
    <phoneme>smaɪð</phoneme>
    <!-- IPA string is: "sma&#x026A;&#x00F0;" -->
  </lexeme>
</lexicon>

5.4 Homophones

Most languages have homophones, words with the same pronunciation but different meanings (and possibly different spellings), for instance "seed" and "cede". It is recommended that these be represented as different lexemes.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>cede</grapheme>
    <phoneme>siːd</phoneme>
    <!-- IPA string is: "si&#x02D0;d" -->
  </lexeme>
  <lexeme>
    <grapheme>seed</grapheme>
    <phoneme>siːd</phoneme>
    <!-- IPA string is: "si&#x02D0;d" -->
  </lexeme>
</lexicon>

5.5 Homographs

Most languages have words with different meanings but the same spelling (and sometimes different pronunciations), called homographs. For example, in English the word bass (fish) and the word bass (in music) have identical spellings but different meanings and pronunciations. Although it is recommended that these words be represented using separate <lexeme> elements that are distinguished by different values of the role attribute (see Section 4.4), if a pronunciation lexicon author does not want to distinguish between the two words they could simply be represented as alternative pronunciations within the same <lexeme> element. In the latter case the TTS processor will not be able to distinguish when to apply the first or the second transcription.

In this example the pronunciations of the homograph "bass" are shown.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>bass</grapheme>
    <phoneme>bæs</phoneme>
    <!-- IPA string is: b&#x00E6;s -->
    <phoneme>beɪs</phoneme>
    <!-- IPA string is: be&#x026A;s -->
  </lexeme>
</lexicon>

Note that English contains numerous examples of noun-verb pairs that can be treated either as homographs or as alternative pronunciations, depending on author preference. Two examples are the noun/verb "refuse" and the noun/verb "address".

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      xmlns:mypos="http://www.example.com/my_pos_namespace"
      alphabet="ipa" xml:lang="en-US">
  <lexeme role="mypos:verb">
    <grapheme>refuse</grapheme>
    <phoneme>rɪˈfjuːz</phoneme>
    <!-- IPA string is: "r&#x026A;&#x02C8;fju&#x02D0;z" -->
  </lexeme>
  <lexeme role="mypos:noun">
    <grapheme>refuse</grapheme>
    <phoneme>ˈrefjuːs</phoneme>
    <!-- IPA string is: "&#x02C8;refju&#x02D0;s" -->
  </lexeme>
</lexicon>

5.6 Pronunciation by Orthography (Acronyms, Abbreviations, etc.)

For some words and phrases pronunciation can be expressed quickly and conveniently as a sequence of other orthographies. The developer is not required to have linguistic knowledge, but instead makes use of the pronunciations that are already expected to be available. To express pronunciations using other orthographies the <alias> element may be used.

This feature may be very useful to deal with acronym expansion.

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <!-- 
        Acronym expansion
  -->
  <lexeme>
    <grapheme>W3C</grapheme>
    <alias>World Wide Web Consortium</alias>
  </lexeme>
  <!-- 
        number representation
  -->
  <lexeme>
    <grapheme>101</grapheme>
    <alias>one hundred and one</alias>
  </lexeme>
  <!-- 
        crude pronunciation mechanism
  -->
  <lexeme>
    <grapheme>Thailand</grapheme>
    <alias>tie land</alias>
  </lexeme>
  <!-- 
        crude pronunciation mechanism and acronym expansion
  -->
  <lexeme>
    <grapheme>BBC 1</grapheme>
    <alias>be be sea one</alias>
  </lexeme>
</lexicon>

6. References

6.1 Normative References

[BCP47]

Tags for the Identification of Languages, A. Phillips and M. Davis, Editors. IETF, September 2006. This RFC is available at http://www.rfc-editor.org/rfc/bcp/bcp47.txt.

[IPAHNDBK]

Handbook of the International Phonetic Association, International Phonetic Association, Editors. Cambridge University Press, July 1999. Information on the Handbook is available at http://www.arts.gla.ac.uk/IPA/handbook.html.

[RFC2119]

Key words for use in RFCs to Indicate Requirement Levels, S. Bradner, Editor. IETF, March 1997. This RFC is available at http://www.ietf.org/rfc/rfc2119.txt.

[RFC3986]

Uniform Resource Identifier (URI): Generic Syntax, T. Berners-Lee et al., Editors. IETF, January 2005. This RFC is available at http://www.ietf.org/rfc/rfc3986.txt.

[RFC3987]

Internationalized Resource Identifiers (IRIs), M. Duerst and M. Suignard, Editors. IETF, January 2005. This RFC is available at http://www.ietf.org/rfc/rfc3987.txt.

[RFC4267]

The W3C Speech Interface Framework Media Types: application/voicexml+xml, application/ssml+xml, application/srgs, application/srgs+xml, application/ccxml+xml, and application/pls+xml, Max Froumentin, Editor. IETF, November 2005. This RFC is available at http://www.ietf.org/rfc/rfc4267.txt.

[SRGS]

Speech Recognition Grammar Specification Version 1.0, Andrew Hunt and Scott McGlashan, Editors. World Wide Web Consortium, 16 March 2004. This version of the SRGS 1.0 Recommendation is http://www.w3.org/TR/2004/REC-speech-grammar-20040316/. The latest version is available at http://www.w3.org/TR/speech-grammar/.

[SSML]

Speech Synthesis Markup Language (SSML) Version 1.0, Daniel C. Burnett, et al., Editors. World Wide Web Consortium, 7 September 2004. This version of the SSML 1.0 Recommendation is http://www.w3.org/TR/2004/REC-speech-synthesis-20040907/. The latest version is available at http://www.w3.org/TR/speech-synthesis/.

[XML10]

Extensible Markup Language (XML) 1.0 (Fourth Edition), T. Bray et al. World Wide Web Consortium, 16 August 2006, edited in place 29 September 2006. This version of the XML 1.0 Recommendation is http://www.w3.org/TR/2006/REC-xml-20060816/. The latest version is available at http://www.w3.org/TR/REC-xml/.

[XML11]

Extensible Markup Language (XML) 1.1 (Second Edition), T. Bray et al. World Wide Web Consortium, 16 August 2006, edited in place 29 September 2006. This version of the XML 1.1 Recommendation is http://www.w3.org/TR/2006/REC-xml11-20060816/. The latest version is available at http://www.w3.org/TR/xml11/.

[XML-BASE]

XML Base, J. Marsh, editor. World Wide Web Consortium, 27 June 2001. This version of the XML Base Recommendation is http://www.w3.org/TR/2001/REC-xmlbase-20010627/. The latest version is available at http://www.w3.org/TR/xmlbase/.

[XML-ID]

xml:id Version 1.0, J. Marsh, D. Veillard, N. Walsh. World Wide Web Consortium, 9 September 2005. This version of the xml:id Recommendation is http://www.w3.org/TR/2005/REC-xml-id-20050909/. The latest version is available at http://www.w3.org/TR/xml-id/.

[XML-NS10]

Namespaces in XML 1.0 (Second Edition), T. Bray et al., Editors. World Wide Web Consortium, 16 August 2006. This version of the XML Namespaces Recommendation is http://www.w3.org/TR/2006/REC-xml-names-20060816/. The latest version is available at http://www.w3.org/TR/xml-names/.

[XML-NS11]

Namespaces in XML 1.1, T. Bray et al., Editors. World Wide Web Consortium, 4 February 2004. This version of the XML Namespaces Recommendation is http://www.w3.org/TR/2004/REC-xml-names11-20040204/. The latest version is available at http://www.w3.org/TR/xml-names11/.

[XML-SCHEMA-1]

XML Schema Part 1: Structures Second Edition, H. S. Thompson, et al., Editors. World Wide Web Consortium, 28 October 2004. This version of the XML Schema Part 1 Recommendation is http://www.w3.org/TR/2004/REC-xmlschema-1-20041028/. The latest version is available at http://www.w3.org/TR/xmlschema-1/.

[XML-SCHEMA-2]

XML Schema Part 2: Datatypes Second Edition, Paul V. Biron and Ashok Malhotra, Editors. World Wide Web Consortium, 28 October 2004. This version of the XML Schema Part 2 Recommendation is http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/. The latest version is available at http://www.w3.org/TR/xmlschema-2/.

6.2 Informative References

[DC]

Dublin Core Metadata Initiative.
See http://dublincore.org/.

[DC-ES]

Dublin Core Metadata Element Set, Version 1.1: Reference Description.
See http://dublincore.org/documents/dces/.

[DICT]

Dictionary.com Unabridged (v 1.0.1), Dictionary.com. Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006. Entries in the dictionary are available at http://dictionary.reference.com.

[HTML]

HTML 4.01 Specification, Dave Raggett, et al., Editors. World Wide Web Consortium, 24 December 1999. This version of the HTML 4.01 Recommendation is http://www.w3.org/TR/1999/REC-html401-19991224/. The latest version of HTML is available at http://www.w3.org/TR/html/.

[IPA]

International Phonetic Association.
See http://www.arts.gla.ac.uk/IPA/ipa.html for the organization's website.

[IPAUNICODE1]

The International Phonetic Alphabet, J. Esling. This table of IPA characters in Unicode is available at http://web.uvic.ca/ling/resources/ipa/charts/unicode_ipa-chart.htm.

[IPAUNICODE2]

The International Phonetic Alphabet in Unicode, J. Wells. This table of Unicode values for IPA characters is available at http://www.phon.ucl.ac.uk/home/wells/ipa-unicode.htm.

[JEIDAALPHABET]

JEIDA-62-2000 Phoneme Alphabet. JEITA. An abstract of this document (in Japanese) is available at http://it.jeita.or.jp/document/publica/standard/summary/JEIDA-62-2000.pdf.

[JEITA]

Japan Electronics and Information Technology Industries Association.
See http://www.jeita.or.jp/.

[RDF-CONC]

Resource Description Framework (RDF): Concepts and Abstract Syntax, G. Klyne and J.J. Carroll, Editors. World Wide Web Consortium, 10 February 2004. This version of the RDF Concepts and Abstract Syntax is http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/. The latest version is available at http://www.w3.org/TR/rdf-concepts/.

[RDF-SCHEMA]

RDF Vocabulary Description Language 1.0: RDF Schema, D. Brickley and R. Guha, Editors. World Wide Web Consortium, 10 February 2004. This version of the RDF Schema Recommendation is http://www.w3.org/TR/2004/REC-rdf-schema-20040210/. The latest version of RDF Schema is available at http://www.w3.org/TR/rdf-schema/.

[RDF-XMLSYNTAX]

RDF/XML Syntax Specification, D. Beckett, Editor. World Wide Web Consortium, 10 February 2004. This version of the RDF/XML Syntax Recommendation is http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/. The latest version of the RDF XML Syntax is available at http://www.w3.org/TR/rdf-syntax-grammar/.

[REQS]

Pronunciation Lexicon Specification (PLS) Version 1.0 Requirements, P. Baggia and F. Scahill, Editors. World Wide Web Consortium, 29 October 2004. This document is a work in progress. This version of the Pronunciation Lexicon Requirements is http://www.w3.org/TR/2004/WD-lexicon-reqs-20041029/. The latest version of the Pronunciation Lexicon Requirements is available at http://www.w3.org/TR/lexicon-reqs/.

[RFC2732]

Format for Literal IPv6 Addresses in URL's, R. Hinden, et al., Editors. IETF, December 1999. This RFC is available at http://www.ietf.org/rfc/rfc2732.txt.

[SAMPA]

SAMPA computer readable phonetic alphabet, J.C. Wells.
See http://www.phon.ucl.ac.uk/home/sampa/home.htm for information on it.

[SISR]

Semantic Interpretation for Speech Recognition (SISR) Version 1.0, Luc van Tichelen and Dave Burke, Editors. World Wide Web Consortium, 5 April 2007. This version of the SISR 1.0 Recommendation is http://www.w3.org/TR/2007/REC-semantic-interpretation-20070405/. The latest version is available at http://www.w3.org/TR/semantic-interpretation/.

[SSML-11]

Speech Synthesis Markup Language (SSML) Version 1.1, Daniel C. Burnett and 双志伟 (Zhi Wei Shuang), Editors. World Wide Web Consortium, 20 June 2008. This version of the SSML 1.1 Working Draft is http://www.w3.org/TR/2008/WD-speech-synthesis11-20080620/. The latest version is available at http://www.w3.org/TR/speech-synthesis11/.

[VXML]

Voice Extensible Markup Language (VoiceXML) Version 2.0, Scott McGlashan et al., Editors. World Wide Web Consortium, 16 March 2004. This version of the VoiceXML 2.0 Recommendation is http://www.w3.org/TR/2004/REC-voicexml20-20040316/. The latest version is available at http://www.w3.org/TR/voicexml20/.

[XHTML2]

XHTML 2.0, J. Axelsson et al., Editors. World Wide Web Consortium, 22 July 2004. This version of the XML XHTML 2.0 Working Draft is http://www.w3.org/TR/2004/WD-xhtml2-20040722/. The latest version is available at http://www.w3.org/TR/xhtml2/.

[XHTML-MTYPES]

XHTML Media Types, Ishikawa Masayasu, Editor. World Wide Web Consortium, 1 August 2002. This version of the W3C Note is http://www.w3.org/TR/xhtml-media-types/xhtml-media-types.html. The latest version is available at http://www.w3.org/TR/xhtml-media-types/.

[XPOINTER]

XPointer Framework, P. Grosso, E. Maler, J. Marsh, N. Walsh. World Wide Web Consortium, 25 March 2003. This version of the XPointer Framework Recommendation is http://www.w3.org/TR/2003/REC-xptr-framework-20030325/. The latest version is available at http://www.w3.org/TR/xptr-framework/.

[X-SAMPA]

Computer-coding the IPA: a proposed extension of SAMPA, J.C. Wells, University College London, 28 April 1995. This version is available at http://www.phon.ucl.ac.uk/home/sampa/ipasam-x.pdf.

[WEB-ARCH]

Architecture of the World Wide Web, Volume One, I. Jacobs, N. Walsh, World Wide Web Consortium, 15 December 2004. This version of the Architecture of World Wide Web Recommendation is http://www.w3.org/TR/webarch/.

7. Contributors and Acknowledgements

The Contributors who provided ideas, comments, feedback and implementation experience to improve this specification. (listed in alphabetical order):

Jeff Adams, Nuance

Kazuyuki Ashimura, W3C

Patrizio Bergallo, Loquendo

Ellen Eide, IBM

Max Froumentin, W3C

Richard Ishida, W3C

严峻 (Yan Jun), iFLYTEK

Matt Oshry, Microsoft

Dave Pawson, RNIB

Luc Van Tichelen, Nuance

The editor wishes to thank the following W3C groups for their helpful comments: WAI and WAI/PF, I18N and MMI.

This specification was written with the help of the following people (listed in alphabetical order):

Debbie Dahl, Conversational Technologies

Ken Davies, HeyAnita

Kurt Fuqua, Vail Systems

Will Gardella, SAP

Makoto Hirota, Canon

Jim Larson, Intervoice

Dave Raggett, W3C/Volantis

Matt Womer, W3C

Appendix A - Schema for Pronunciation Lexicon Specification

This section is normative.

There are two schemas which can be used to validate PLS documents.
The latest version of the schemas are available at:

• The XML schema:
  "http://www.w3.org/TR/pronunciation-lexicon/pls.xsd"
• The RELAX NG schema:
  "http://www.w3.org/TR/pronunciation-lexicon/pls.rng"

For stability it is RECOMMENDED that you use the dated URI available at:

• The XML schema:
  "http://www.w3.org/TR/2008/PR-pronunciation-lexicon-20081014/pls.xsd"
• The RELAX NG schema:
  "http://www.w3.org/TR/2008/PR-pronunciation-lexicon-20081014/pls.rng"

Appendix B - MIME Type and File Suffix

This section is normative.

The media type associated to Pronunciation Lexicon Specification documents is "application/pls+xml" and the filename suffix is ".pls" as defined in [RFC4267].

Appendix C - Issues in Retrieving Lexical Content

This section is informative.

Speech applications that use a PLS document need a mechanism enabling them to retrieve appropriate lexical content. In the simplest of cases, an application will search the PLS document for <grapheme> elements with content that exactly matches the input and retrieve all corresponding lexemes. In general, however, the retrieval of lexical content is not so trivial; it is necessary to define what constitutes an exact match and which lexemes are to be retrieved when competing matches can apply.

Here is an example of an approach to retrieve appropriate lexical content.

• The text can be tokenized into the smallest orthographic units that might have some meaning to the written language addressed by the PLS document. By default, for written languages with an alphabet, all consecutive members of the alphabet can be grouped into a single token and all other characters (such as punctuation, hyphen and apostrophe) can be considered as individual tokens. Applications that address languages using a logographic writing system might consider each logogram as a separate token. Languages that are heavily agglutinative need a more elaborate default tokenization procedure.
• A speech application may provide a mechanism to segment the text into explicit tokens. Such a mechanism can be used to overcome a limitation of the default tokenization. For example, "don't" can be explicitly tokenized as "do" and "n't" in order to match a <grapheme> element with content "n't".
• Tokens should be processed in the direction of normal script reading. Thus, English is processed from left to right, and languages such as Arabic and Hebrew, from right to left.
• Precedence should be given to the retrieval of lexemes having a <grapheme> element whose content exactly matches the longest possible sequence of consecutive tokens. Thus, a lexeme for "they'll" should have precedence over a lexeme for "they" given the input "they'll'.
• Lexical retrieval should be performed by the bias of tokens rather than characters. Thus, a lexeme for "do" should not match the beginning of "done".
• A match can occur only when the Unicode characters are equal. Thus, matches should be sensitive to case and diacritics. "Lima" should not match "lima", "cure" should not match "curé", and "vitæ" should not match "vitae". An application may choose to expand ligatures (such as "æ") into corresponding letter sequences ("ae") and to use a collation table to further loosen the match.
• Multiple consecutive white space characters should be reduced to a unique single white space character. Thus, "New York" should match "New     York".

This outlined approach is designed principally with the needs of English in mind and should be modified to accommodate the particular requirements of other languages.

It is recommended for applications that use a PLS document to describe the approach they adopt in retrieving lexical content.

Example:

An application that uses the following PLS document:

<?xml version="1.0" encoding="UTF-8"?>
<lexicon version="1.0" 
      xmlns="http://www.w3.org/2005/01/pronunciation-lexicon"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://www.w3.org/2005/01/pronunciation-lexicon 
        http://www.w3.org/TR/2007/CR-pronunciation-lexicon-20071212/pls.xsd"
      alphabet="ipa" xml:lang="en-US">
  <lexeme>
    <grapheme>New York</grapheme>
    <alias>NY</alias>
  </lexeme>
  <lexeme>
    <grapheme>York   City</grapheme>
    <alias>YC</alias>
  </lexeme>
</lexicon>

should process "New   York City" as "NY City" rather than "New   YC" if it uses the above approach.

Appendix D - Changes

This section is informative.

Changes in this document:

• Modified the third example because there was a missing "uri" attribute of <lexicon> element (4.4).
• Updated IPA references (6.2).
• The schema files (pls.xsd and pls.rng) have been updated to allow for non-PLS attributes on all PLS elements (App. A).

Changes in the Proposed Recommendation (18 August 2008):

• Updated definition of URI (1.5).
• Clarified usage of white space in IPA transcriptions (2).
• Clarified definition of xml:base attribute (4.1).
• Updated example of alternative textual representations for German (4.5).
• Applied minor editorial changes.

Changes in the Candidate Recommendation (12 December 2007):

• Added support of IRI and XML 1.1.
• Expanded Conformance section (3).
• Clarified recursion language for alias element (4.7).
• Clarified prefer attribute definition (4.6, 4.7, 4.9).
• Updated schemas and added dated URI for stability (App. A).
• Added a note on QNames (4.4).
• Added informative notes on IPA (2).
• Updated Authors/Contributors/Acknowledgements (7).
• Added Appendix to list changes since previous drafts (App. D).
• Reviewed normative/informative attributions.
• Added Chinese example (4.4).
• Updated examples (i.e. added SchemaLocation, fixes to IPA codes, etc).
• Added clarifications and fixed spelling errors.

Changes in the second Last Call Working Draft (26 October 2006):

• Added a new role attribute to the lexeme element which helps to disambiguate Homographs (4.4).
• Added an informative appendix on issues in retrieving lexical content (App. C).
• Removed orthography attribute in the grapheme element (4.4).
• Removed recursion of alias elements (4.7).
• Styled normative language covered by RFC2119.
• Updated schemas (App. A).
• Modified examples.
• Added requested clarifications.

Changes in the first Last Call Working Draft (31 January 2006):

• Support for the IPA phonetic alphabet is mandatory (2).
• Added a new section on multiple pronunciations (4.9).
• Clarified the use of the "prefer" attribute (4.6, 4.7, 4.9).
• Added a glossary of terms (1.5).
• Fixed XML schema and added Relax NG schema (App. A).