Speech Synthesis Markup Language: An Introduction
Speech Synthesis Markup Language Specification (SSML 1.0), introduced in September 2004, is one of the standards enabling access to the Web using spoken interaction. It's designed to provide a rich, XML-based markup language for assisting the generation of synthetic speech in web and other applications. The essential role of SSML is to provide authors of synthesizable content a standard way to control aspects of speech such as pronunciation, volume, pitch, rate, etc., across different synthesis-capable platforms.
The SSML specification is based upon JSML and JSGF specifications, which are owned by Sun. Originally JSML (JSpeech Markup Language) was developed as a very simple XML format used by applications to annotate text input to speech synthesizers. JSML had characteristics very similar to SSML: it defined elements that described the structure of a document, provided pronunciations of words and phrases, indicated phrasing, emphasis, pitch and speaking rate, and controlled other important speech characteristics. The letter "J" in the markup language name has come from the Java(TM) Speech API, introduced by Sun in collaboration with leading speech technology companies, for incorporating speech technology into user interfaces of applets and applications based on Java technology. The design of JSML elements and its semantics are quite simple. Here is the typical self-explaining example:
<jsml> <voice gender="female" age="20"> <div type="paragraph"> You have an incoming message from <emphasis>Peter Mikhalenko</emphasis> in your mailbox. Mail arrived at <sayas class="time">7am</sayas> today. </div> </voice> <voice gender="male" age="30"> <div type="paragraph"> Hi, Steve! <break/> Hope you're OK. </div> <div> Sincerely yours, Peter. </div> </voice> </jsml>
The JSpeech Grammar Format (JSGF) is a representation of grammars for use in speech recognition. It defines a platform- and vendor-independent way to describe one type of grammar, a rule grammar (also known as a command and control grammar or regular grammar). Grammars are used by speech recognizers to determine what the recognizer should listen for and so describe the utterances a user may say. JSGF is not an XML format and is out of scope of this article.
Voice browsers are a very important part of Multimodal Interaction and Device Independence, making web applications accessible with multiple modes of interaction. A voice browser is a device that interprets a markup language and is capable of generating voice output or interpreting voice input, and possibly other input/output modalities. There is a whole set of markup specifications for voice browsers developed at W3C, and SSML is a part of it. Speech synthesis is a process of automatic generation of speech output from data input which may include plain text, marked up text or binary objects. It must be practical to generate speech synthesis output from a wide range of existing document representations. The common requirement to speech synthesis markup is that speech output from HTML, HTML with CSS, XHTML, XML with XSL, and DOM must be possible. The intended use of SSML is to improve the quality of synthesized content.
The key concepts of SSML are
The system of automatic generation of speech output from text or annotated text input that supports SSML must render a document as spoken output using the information contained in the markup to render the document as intended by the author. There are several steps in a speech synthesis process.
<say-as/>element can be used in the input document to explicitly indicate the presence and type of these constructs and to resolve ambiguities.
<phoneme/>element of SSML allows a phonemic sequence to be provided for any word or word sequence.
<prosody/>for prosody purposes, which I will describe below.
<voice/>element in SSML allows the document creator to request a particular voice or specific voice qualities (e.g. a young male voice).
SSML provides a standard way to specify gross properties of synthetic speech production such as pronunciation, volume, pitch, rate, etc. Exact specification of synthetic speech output behavior across disparate processors, however, is beyond the scope of the SSML specification. It should be noticed that markup values are merely indications rather than absolutes. For example, it is possible for an author to explicitly indicate the duration of a text segment and also indicate an explicit duration for a subset of that text segment. If the two durations result in a text segment that the synthesis processor cannot reasonably render, the processor is permitted to modify the durations as needed to render the text segment.
The root element of an SSML document
<lexicon/> elements must occur before all other
elements and text contained within the
<speak/> element. There are no other ordering
constraints on the elements in the specification. The root element
must have a mandatory
xml:lang attribute specifying the
language of the root document.
xml:lang attribute can be
<s/> elements. Also root element must
version attribute and must have the value "1.0". The
root element can only contain text to be rendered and the following
xml:lang can be used:
<?xml version="1.0"?> <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/TR/speech-synthesis/synthesis.xsd" xml:lang="en-US"> <p>I don't speak French.</p> <p xml:lang="fr">Bonjour monsieur!</p> </speak>
An SSML document may reference one or more external pronunciation lexicon documents. A lexicon document is identified by a URI with an optional media type. No standard lexicon media type has yet been defined as the default for SSML specification. A lexicon document contains pronunciation information for tokens that can appear in a text to be spoken. The pronunciation information contained within a lexicon is used for tokens appearing within the referencing document. Lexicons can be included thusly:
<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/TR/speech-synthesis/synthesis.xsd" xml:lang="en-US"> <lexicon uri="http://www.xml.com/lexicon.file"/> <lexicon uri="http://www.xml.com/slang-words.file" type="media-type"/> ... </speak>
You can include metadata for the document using a metadata schema. The recommended metadata format is the XML serialization of RDF.
For logical and physical division purposes
<s/> elements exist. The former represents a
paragraph, the latter a sentence. This is example of their usage:
<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/TR/speech-synthesis/synthesis.xsd" xml:lang="en-US"> <p> <s>This is the first sentence of the paragraph.</s> <s>Here's another sentence.</s> </p> </speak>
<say-as/> element allows the author to
indicate information on the type of text construct contained within
the element and to help specify the level of detail for rendering the
contained text. Defining a comprehensive set of text format types is
difficult because of the variety of languages that have to be
considered and because of the innate flexibility of written
languages. SSML only specifies the say-as element, its attributes, and
their purpose. It does not enumerate the possible values for the
<say-as/> element has three
interpret-as attribute is always required; the
other two attributes are optional. The legal values for the format
attribute depend on the value of the
interpret-as attribute indicates the
content type of the contained text construct. Specifying the content
type helps the synthesis processor to distinguish and interpret text
constructs that may be rendered in different ways depending on what
type of information is intended. In addition, the optional format
attribute can give further hints on the precise formatting of the
contained text for content types that may have ambiguous
detail attribute is an optional attribute
that indicates the level of detail to be read aloud or rendered. Every
value of the detail attribute must render all of the informational
content in the contained text; however, specific values for the detail
attribute can be used to render content that is not usually
informational in running text but may be important to render for
<phoneme/> element provides a phonemic/phonetic
pronunciation for the contained text. The phoneme element may be
empty. However, it is recommended that the element contain
human-readable text that can be used for non-spoken rendering of the
ph attribute is a required attribute that
specifies the phoneme/phone string. This element is designed strictly
for phonemic and phonetic notations and is intended to be used to
provide pronunciations for words or very short phrases. The
phonemic/phonetic string does not undergo text normalization and is
not treated as a token for lookup in the lexicon. Briefly, phonemic
strings consist of phonemes, language-dependent speech units that
characterize linguistically significant differences in the language;
loosely, phonemes represent all the sounds needed to distinguish one
word from another in a given language. The
attribute is an optional attribute that specifies the
phonemic/phonetic alphabet. An alphabet in this context refers to a
collection of symbols to represent the sounds of one or more human
languages. The only valid values for this attribute
"ipa" and vendor-defined strings of the
"x-organization-alphabet". Here is an example of
<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/TR/speech-synthesis/synthesis.xsd" xml:lang="en-US"> <phoneme alphabet="ipa" ph="təmei̥ɾou̥"> tomato </phoneme> </speak>
<sub/> element is employed to indicate that
the text in the alias attribute value replaces the contained text for
pronunciation. This allows a document to contain both a spoken and
written form. The required alias attribute specifies the string to be
spoken instead of the enclosed string. The processor should apply text
normalization to the alias value. An example of such substitution
might look something like the following:
<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/TR/speech-synthesis/synthesis.xsd" xml:lang="en-US"> <sub alias="World Wide Web Consortium">W3C</sub> <!-- World Wide Web Consortium --> </speak>
<voice/> element is a production element
that requests a change in speaking
<emphasis/> element requests that the
contained text be spoken with emphasis (also referred to as prominence
or stress). The
<break/> element is an empty
element that controls the pausing or other prosodic boundaries between
words. The use of the break element between any pair of words is
optional. If the element is not present between words, the synthesis
processor is expected to automatically determine a break based on the
linguistic context. The
<prosody/> element permits
control of the pitch, speaking rate and volume of the speech
output. It has quite complicated attributes, so it's better to read
the original specification for further investigating. Here's a
typical SSML document:
<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/TR/speech-synthesis/synthesis.xsd" xml:lang="en-US"> <voice gender="female" age="20"> <p> You have an incoming message from <emphasis>Peter Mikhalenko</emphasis> in your mailbox. Mail arrived at <sayas class="time">7am</sayas> today. </p> </voice> <voice gender="male" age="30"> <p> Hi, Steve! <break/> Hope you're OK. </p> <p> Sincerely yours, Peter. </p> </voice> </speak>
There are several implementations of SSML available, some of them are open source, but others are proprietary, industry implementations. For an open source example, see FreeTTS. Speech technologies and telecommunications industry leaders include France Telecom, Loquendo S.p.A., ScanSoft, Voxpilot. All of them have provided implementation reports to W3C; for more information see W3C's SSML 1.0 Implementation Report.
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