Overview

The need for clear and accurate [=Text-to-Speech=] (TTS) rendering of publications is imperative for their readability and comprehension. Unfortunately, the complexities of voicing natural languages and the limitations of built-in vocabularies in TTS engines often leads to incorrect and illegible voicing. Users either have to infer the correct meaning, when possible, or stop reading and have the garbled words spelled out. Anyone who has tried to read educational or instructional material using basic TTS playback will understand the frustration of this experience.

W3C has defined a variety of technologies to aid in improving the voice rendering of markup content: the Synthetic Speech Markup Language [[ssml]], pronunciation lexicons [[pronunciation-lexicon]], and the CSS Speech module.

SSML and pronunciation lexicons provide enhanced speech rendering. Lexicons are like dictionaries of common terms a TTS engine can use, while SSML provides the ability to add individual voicing for specific phrases. [=EPUB creators=] can use these technologies together or separately depending on the complexity of the text. Despite these advantages, the technologies have not been adapted for easy use within the XHTML and SVG formats that EPUB relies on. This document proposes an approach to enable their authoring and rendering in EPUB content documents.

This document also covers the use of CSS Speech for improved aural rendering in EPUB. CSS Speech covers a different domain than SSML and pronunciation lexicons. Instead of controlling the specific voicing of words and phrases, these properties allow EPUB creators to aspects of the aural playback itself — what text to render, at what volume, with what preferred voice, etc.

This document covers the use of these technologies for rendering by [=EPUB reading systems=]. Although it is anticipated that general assistive technologies such as screen readers could take advantage of the technologies, use by them is out of scope.

Terminology

This specification uses terminology defined in EPUB 3.3 [[epub-33]].

It also defines the following term:

text-to-speech

The rendering of the textual content of an [=EPUB publication=] by a [=reading system=] as artificial human speech using a synthesized voice.

The ssml:ph attribute

The ssml:ph attribute specifies a phonemic/phonetic pronunciation of the text represented by its carrying element.

Attribute Name

ph

Namespace

https://www.w3.org/2001/10/synthesis

Usage

EPUB creators MAY specify on any element in EPUB content documents with which they can logically associate a phonetic equivalent (i.e., that has descendant text content that a Text-to-Speech engine would otherwise render).

EPUB creators MUST NOT specify the attribute on a descendant of an element that already carries this attribute.

Value

A phonemic/phonetic expression, syntactically valid with respect to the phonemic/phonetic alphabet used.

The ssml:ph attribute inherits the authoring requirements of the [[ssml]] phoneme element's ph attribute.

When the ssml:ph attribute appears on an element that has text node descendants, the corresponding document text to which the pronunciation applies is the string that results from concatenating the descendant text nodes, in document order. The specified phonetic pronunciation must therefore logically match the element's textual data in its entirety (i.e., not just an isolated part of its content).

EPUB creators SHOULD NOT use the ssml:ph attribute on elements without text content that a Text-to-Speech engine would normally render (e.g., on empty div or span elements). The attribute is not intended to add additional voicing only for TTS playback, and reading systems are expected to ignore the attribute if it does not replace text they would normally render.

Similarly, EPUB creators SHOULD NOT add empty ssml:ph attributes to try and suppress the rendering of text. Reading systems are expected to ignore empty attributes. (See the aria-hidden attribute [[?wai-aria]] for specifying that content is only for visual rendering.)

The ssml:alphabet attribute

The ssml:alphabet attribute specifies which phonemic/phonetic pronunciation alphabet is used in the value of the ssml:ph attribute.

Attribute Name

alphabet

Namespace

https://www.w3.org/2001/10/synthesis

Usage

EPUB creators MAY specify on any element in an EPUB content document that can contain descendant text content.

Value

The name of the pronunciation alphabet used to express the value of the ssml:ph attribute.

The ssml:alphabet attribute inherits the authoring requirements of the [[ssml]] phoneme element's alphabet attribute.

The value of the ssml:alphabet attribute is inherited in the document tree. The pronunciation alphabet used for each ssml:ph attribute value is determined by locating the first occurrence of the ssml:alphabet attribute starting with the element on which the ssml:ph attribute appears, followed by the nearest ancestor element.

EPUB creators SHOULD ensure that an alphabet is defined in scope for all phonemes expressed in ssml:ph attributes. Interoperability of playback cannot be guaranteed in the absence of a declaration — reading systems may apply a default alphabet, for example, or may not voice the phoneme.

Lexicon conformance

A pronunciation lexicon:

• MUST meet the conformance constraints for XML documents defined in XML Conformance [[epub-33]].

• MUST be valid to the grammar defined in [[pronunciation-lexicon]].

Associating with EPUB content documents

EPUB creators MAY associate zero or more pronunciation lexicons [[pronunciation-lexicon]] with an [=EPUB content document=].

To associate a pronunciation lexicon with an [=XHTML content document=], EPUB creators MUST use the [[html]] link element. Similarly, to associate a pronunciation lexicon with an [=SVG content document=], EPUB creators MUST use the [[svg]] link element.

For both types of EPUB content document, the link element MUST have its rel attribute set to "pronunciation" and its type attribute set to the media type "application/pls+xml".

EPUB creators SHOULD specify the link element hreflang attribute on each link, and its value MUST match the language for which the pronunciation lexicon is relevant [[pronunciation-lexicon]] when specified.

Introduction

[=Reading systems=] may implement [=Text-to-Speech=] playback in different ways depending on the type of engine they use — one might only feed the text content of the document to the engine, for example, while another could support full markup. This document tries to provide flexibility in its requirements to allow for these differences. The only requirement is that the correct rendering behavior result.

Although this document frames the enhancements in the context of a reading system with built-in Text-to-Speech rendering capabilities, it is anticipated that any application or assistive technology that can access the markup of an EPUB publication will be able to use these features to provide improved voice rendering. Ensuring the technologies works with these applications is outside the scope of this work, however.

Conformance

[=Reading systems=] with [=Text-to-Speech=] (TTS) capabilities SHOULD support SSML attributes, pronunciation lexicons and CSS Speech as follows:

SSML

Reading systems that support SSML:

• MUST process the ssml:ph attribute per the requirements for the phoneme element's ph attribute [[ssml]] with the additional requirements that it:

  • MUST ignore ssml:ph attributes whose value is an empty string or consists only of ASCII whitespace [[infra]].

  • MUST ignore ssml:ph attributes on elements whose descendant text content is an empty string or consists only of ASCII whitespace [[infra]].

  • MUST ignore ssml:ph attributes on elements whose descendant text content represents a fallback.

• MUST process the ssml:alphabet attribute per the requirements for the phoneme element's alphabet attribute [[ssml]].

Pronunciation Lexicons

Reading systems that support pronunciation lexicons:

• MUST process all linked pronunciation lexicons in an EPUB content document as defined in [[pronunciation-lexicon]].

• MUST apply the supplied lexemes to all text nodes in the EPUB content document whose language matches the language for which the pronunciation lexicon is relevant [[pronunciation-lexicon]]. [[bcp47]] defines the algorithm for matching language tags.

It is not required that the reading system use a Text-to-Speech engine that supports pronunciation lexicons so long as the lexemes are processed and applied correctly. A reading system might, for example, transform the lexicon into an alternative dictionary format its TTS engine supports.

SSML and Pronunciation Lexicons

Reading systems that support SSML and pronunciation lexicons:

• MUST let any pronunciation instructions provided via the ssml:ph attribute take precedence in cases where a grapheme element [[pronunciation-lexicon]] matches a text node of an element that carries the ssml:ph attribute.

CSS Speech

This document adds no additional requirements for reading system support to those defined in [[css-speech-1]].