Reading Time: 2 minutes

Speech Synthesis Markup Language (SSML) is an XML-based markup language used to control speech output in web and application environments. It enables applications to interact with users through synthesized speech in a natural and flexible way.

SSML allows developers and content authors to define how text should be spoken, including pronunciation, volume, pitch, speaking rate, and voice characteristics. It is a W3C recommendation developed by the Voice Browser Working Group.

Purpose of SSML

The primary purpose of SSML is to assist text-to-speech (TTS) engines in producing high-quality spoken output. An SSML document is processed by a speech synthesizer and converted into audio.

Different SSML elements influence different stages of the speech synthesis process, allowing fine-grained control over how speech is rendered.

Stages of the Speech Synthesis Process

The conversion of an SSML document into speech typically involves the following stages:

  1. XML parsing
  2. Structure analysis
  3. Text normalization
  4. Text-to-phoneme conversion
  5. Prosody analysis
  6. Waveform production

XML Parsing

During XML parsing, an XML parser extracts content from the SSML document tree. This content forms the basis for all subsequent processing stages.

Structure Analysis

In this stage, the structure of the document is analyzed. The order and grouping of elements influence how speech is sequenced in the final audio output.

Text Normalization

Text normalization determines how written text should be spoken. Different languages and contexts may result in different spoken interpretations.

For example, the text 3/4 could be spoken as “three quarters”, “third of April”, or “fourth of March” depending on context.

Text-to-Phoneme Conversion

At this stage, words are broken down into phonemes, which are the basic units of pronunciation used by the speech synthesizer.

Prosody Analysis

Prosody analysis determines pitch, timing, pauses, and emphasis. These characteristics are known as prosodic features and are critical
for natural-sounding speech.

SSML elements such as emphasis, break, and prosody are used to control this stage.

Waveform Production

The final stage generates the audio waveform. Information from phoneme conversion and prosody analysis is combined to produce the spoken output.

Structure of an SSML Document

The structure of an SSML document can be understood through the following example:


<?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">

  <lexicon uri="http://www.somelexiconfile.com/lexicon.file"/>

  <voice gender="female">
    <p>
      <s>I speak <emphasis>French</emphasis>.</s>
      <s>I also speak <emphasis>German</emphasis>.</s>
    </p>

    <sub alias="International Phonetic Association">IPA</sub>
  </voice>

  <audio src="royal.wav">
    <emphasis>Welcome</emphasis> to the Royal Club.
  </audio>

</speak>

Key SSML Elements

speak

The speak element is the root element of an SSML document. It defines the SSML version, namespaces, schema location, and language used for speech output.

p and s

The p element represents a paragraph, while the s element represents a sentence. These elements help structure spoken content.

emphasis

The emphasis element is used to stress words or phrases. The effect of emphasis may vary depending on language, dialect, or voice.

voice

The voice element allows selection of voice characteristics such as gender, name, and age.

For example:


<voice gender="female" age="6">
  Hello!
</voice>

This would render the text using a child’s female voice.

lexicon

The lexicon element references an external pronunciation dictionary used to control how specific words are spoken.

sub

The sub element specifies an alias for abbreviations or acronyms. For example, “IPA” can be spoken as “International Phonetic Association”.

audio

The audio element allows insertion of prerecorded audio files into the synthesized speech output.

Conclusion

Speech Synthesis Markup Language enables rich, natural, and customizable speech output for web and application environments. By controlling pronunciation, prosody, voice selection, and structure, SSML plays a key role in modern voice-enabled systems.

For a complete list of SSML elements and specifications, refer to the official W3C documentation.