1 Overview

1.1 Purpose and Scope

This section is informative

This specification, EPUB Media Overlays 3.1, defines a usage of [SMIL] (Synchronized Multimedia Integration Language), the Package Document, CSS Style Sheets, and EPUB® Content Documents for representation of audio synchronized with the EPUB Content Document.

The text and audio synchronization enabled by Media Overlays provides enhanced accessibility for any user who has difficulty following the text of a traditional book. Media Overlays also provide a continuous listening experience for readers who are unable to read the text for any reason, something that traditional audio embedding techniques cannot offer. They are even useful for purposes not traditionally considered accessibility concerns (e.g., for language learning or reading of commercial audio books).

This specification is one of a family of specifications that compose [EPUB 3.1], an interchange and delivery format for digital publications based on XML and Web Standards. It is meant to be read and understood in concert with the other specifications that make up EPUB 3.1.

Refer to [EPUB3 Changes] for more information on the differences between this specification and its predecessor.

1.2 Relationship to Other Specifications

This section is informative

1.2.1 Relationship to SMIL

This specification relies on a subset of [SMIL], from which the EPUB Media Overlays elements and attributes defined in Media Overlay Document Definition are derived.

1.3 Terminology

Terms with meanings specific to EPUB 3.1 are capitalized in this document (e.g., "Author", "Reading System"). A complete list of these terms and definitions is provided in [EPUB 3.1].

Only the first instance of a term in a section is linked to its definition.

1.4 Typographic Conventions

The following typographic conventions are used in this specification:

markup: All markup (elements, attributes, properties), code (JavaScript, pseudo-code), machine-readable values (string, characters, media types) and file names are in red monospace font.
markup link: Links to markup and code definitions are in underlined red monospace font.
http://www.idpf.org/: URIs are in navy blue monospace font.
hyperlink: Hyperlinks are underlined and blue.
[reference]: Normative and informative references are enclosed in square brackets.
Term: Terms defined in the Terminology are in capital case.
Term Link: Links to term definitions have a dotted blue underline.

: Normative element, attribute and property definitions are in blue boxes.

Informative markup examples are in light gray boxes.

note

Informative notes are in green boxes with a "Note" header.

caution

Informative cautionary notes are in red boxes with a "Caution" header.

1.5 Conformance Statements

The keywords 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].

All sections and appendixes of this specification are normative except where identified by the informative status label "This section is informative". The application of informative status to sections and appendixes applies to all child content and subsections they contain.

All examples in this specification are informative.

1.6 Namespace prefix mappings

For convenience, the following namespace prefixes [XMLNS] are used in this specification without explicitly being declared. To use any of these prefixes, a declaration is required.

Prefix mappings

prefix	URI
`epub`	`http://www.idpf.org/2007/ops`

2 Media Overlay Document Definition

2.1 Introduction

This section is informative

Synchronized audio narration is found in mainstream ebooks, educational tools and ebooks formatted for persons with print disabilities. In EPUB 3.1, these types of books are created using Media Overlay Documents to describe the timing for the pre-recorded audio narration and how it relates to the EPUB Content Document markup. The file format for Media Overlays is defined as a subset of [SMIL], a W3C recommendation for representing synchronized multimedia information in XML.

The Media Overlays feature is designed to be transparent to EPUB Reading Systems that do not support the feature. The inclusion of Media Overlays in an EPUB Publication has no impact on the ability of Media Overlay-unaware Reading Systems to render the EPUB Publication as though the Media Overlays are not present.

Although future versions of this specification might incorporate support for video media (e.g., synchronized text/sign-language books), this version supports only synchronizing audio media with the EPUB Content Document.

2.2 Content Conformance

A Media Overlay Document must meet all of the following criteria:

Document Properties

It must meet the conformance constraints for XML documents defined in XML Conformance [EPUB 3.1].

It must be valid to the Media Overlays schema as defined in Appendix A, Media Overlays Schema and conform to all content conformance constraints expressed in Media Overlay Document Definition.

It must be authored to reflect the structure of the EPUB Content Document with which it is associated, as stated in Structure.

It may refer to more than one EPUB Content Document, but an EPUB Content Document must not be referenced by more than one Media Overlay Document.

Authors should avoid using scripts to control audio and video embedded in the EPUB Content Document, as stated in Embedded Media.

It should use semantic markup where appropriate, as described in Semantic Inflection.

It must be packaged with the EPUB Publication as shown in Packaging.

File Properties

The Media Overlay Document filename should use the file extension .smil.

2.3 Reading System Conformance

EPUB Reading System support for Media Overlays is optional. A Reading System that supports Media Overlays must meet the following criteria:

  It must process the Media Overlay Document in conformance with all Reading System conformance constraints expressed in Media Overlay Document Definition.

  It must support XHTML Content Documents, and it may support SVG Content Documents.

  It must render Media Overlay elements as described in Basic Playback.

  It must allow user navigation while a Media Overlay is being played, as discussed in Navigation.

  It must adhere to rules regarding referenced audio and video embedded in the EPUB Content Document, as stated in Embedded Audio and Video.

  Text-to-Speech-capable Reading Systems should conform to Reading System Text-to-Speech Conformance Requirements [EPUB 3.1].

  It should offer the skippability and escapability features described in Skippability and Escapability.

A Reading System that does not support Media Overlays must meet the following criteria:

It must ignore both the media-overlay attribute on Manifest item elements and the manifest item elements where the media-type attribute value equals application/smil+xml.

2.4 Media Overlay Document Definition

All elements [XML] defined in this section are in the https://www.w3.org/ns/SMIL namespace [XMLNS] unless otherwise specified.

2.4.1 The `smil` Element

The smil element is the root element of all Media Overlay Documents.

Element Name

smil

Usage

The smil element is the root element of the Media Overlay Document.

Attributes

version [required]

Specifies the version number of the [SMIL] specification to which the Media Overlay adheres.

This attribute must have the value "3.0".

id [optional]

The ID [XML] of the element, which must be unique within the document scope.

epub:prefix [optional]

Declares additional metadata vocabulary prefixes.

Refer to Semantic Inflection for more information.

Content Model

In this order:

head [0 or 1]
body [exactly 1]

2.4.2 The `head` Element

The head element is the container for metadata in the Media Overlay Document.

Element Name: head
Usage: The head element is the optional first child of the smil element.
Attributes: None
Content Model: metadata [0 or 1]

As this specification does not define any metadata properties that have to occur in the Media Overlay Document, the head element is optional.

2.4.3 The `metadata` Element

The metadata element represents metadata for the Media Overlay Document. The metadata element is an extension point that allows the inclusion of metadata from any metainformation structuring language.

Element Name: metadata
Usage: As a child of the head element.
Attributes: None
Content Model: [0 or more] elements from any namespace

This specification defines no metadata properties that must occur in the Media Overlay Document; the metadata element is provided for custom metadata requirements.

2.4.4 The `body` Element

The body element is the starting point for the presentation contained in the Media Overlay Document. It contains the main sequence of par and seq elements.

Element Name

body

Usage

The body element is the required second child of the smil element.

Attributes

epub:type [optional]

An expression of the structural semantics of the corresponding element in the EPUB Content Document.

The value is a white space separated list of property [Packages 3.1] types. Refer to Semantic Inflection for more information.

id [optional]

The ID [XML] of the element, which must be unique within the document scope.

epub:textref [optional]

The relative IRI reference [RFC3987] of the corresponding EPUB Content Document, including a fragment identifier that references the specific element as per the [XPTRSH].

Content Model

In any order:

seq [0 or more]
par [0 or more]

At least one par or seq is required.

2.4.5 The `seq` Element

The seq element contains media objects which are to be rendered sequentially.

Element Name

seq

Usage

One or more seq elements may occur as children of the body element and of the seq element.

Attributes

epub:type [optional]

An expression of the structural semantics of the corresponding element in the EPUB Content Document.

The value is a white space separated list of property [Packages 3.1] types. Refer to Semantic Inflection for more information.

id [optional]

The ID [XML] of the element, which must be unique within the document scope.

epub:textref [required]

The relative IRI reference [RFC3987] of the corresponding EPUB Content Document, including a fragment identifier that references the specific element as per the [XPTRSH].

Refer to Structure for more information.

Content Model

In any order:

seq [0 or more]
par [0 or more]

At least one par or seq is required.

2.4.6 The `par` Element

The par element contains media objects which are to be rendered in parallel.

Element Name

par

Usage

One or more par elements may occur as children of the body and seq elements.

Attributes

epub:type [optional]

An expression of the structural semantics of the corresponding element in the EPUB Content Document.

The value is a white space separated list of property [Packages 3.1] types. Refer to Semantic Inflection for more information.

id [optional]

The ID [XML] of the element, which must be unique within the document scope.

Content Model

In any order:

text [exactly 1]
audio [0 or 1]

The audio element is optional only if its sibling text element refers to audio or video media (see Embedded Media), or to textual content intended for rendering via Text-to-Speech (TTS).

2.4.7 The `text` Element

The text element references an element in the EPUB Content Document. A text element typically refers to a textual element, but can also refer to other EPUB Content Document media elements (see Embedded Media).

Element Name

text

Usage

As a required child of the par element.

Attributes

src [required]: The relative IRI reference [RFC3987] of the corresponding EPUB Content Document, including a fragment identifier that references the specific element as per the [XPTRSH].
id [optional]: The ID [XML] of the element, which must be unique within the document scope.

Content Model

Empty

2.4.8 The `audio` Element

The audio element represents a clip of audio media.

Element Name

audio

Usage

A required child of the par element unless its sibling text element refers to audio or video media, in which case it is optional (see Embedded Media).

Attributes

id [optional]

The ID [XML] of the element, which must be unique within the document scope.

src [required]

The relative or absolute IRI reference [RFC3987] of an audio file. The audio file must be one of the audio formats listed in the Core Media Types [EPUB 3.1] table.

clipBegin [optional]

A clock value that specifies the offset into the physical media corresponding to the start point of an audio clip.

must be a [SMIL] clock value.

See Appendix B, Examples of Clock Values.

clipEnd [optional]

A clock value that specifies the offset into the physical media corresponding to the end point of an audio clip.

must be a [SMIL] clock value.

See Appendix B, Examples of Clock Values.

The chronological offset of the terminating position must be after the starting offset specified in the clipBegin attribute.

Content Model

Empty

3 Creating Media Overlays

3.1 Introduction

This section is informative

A pre-recorded narration of a publication can be represented as a series of audio clips, each corresponding to part of the EPUB Content Document. A single audio clip, for example, typically represents a single phrase or paragraph, but infers no order relative to the other clips or to the text of a document. Media Overlays solve this problem of synchronization by tying the structured audio narration to its corresponding text (or other media) in the EPUB Content Document using SMIL markup. Media Overlays are, in fact, a simplified subset of SMIL 3.0 that allow the playback sequence of these clips to be defined.

The SMIL elements primarily used for structuring Media Overlays are body (used for the main sequence), seq (sequence) and par (parallel). (Refer to Media Overlay Document Definition for more information on these and other SMIL elements.)

The par element is the basic building block of an Overlay and corresponds to a phrase in the EPUB Content Document. The element provides two key pieces of information for synchronizing content: 1) the audio clip containing the narration for the phrase; and 2) a pointer to the associated EPUB Content Document fragment. The par element uses two media element children to represent this information: an audio element and a text element. Since par elements render their children in parallel, the audio clip and EPUB Content Document fragment are played at the same time, resulting in a synchronized presentation.

The text element src attribute references the associated phrase, sentence, or other segment of the EPUB Content Document by its IRI reference. The audio element src attribute similarly references the location of the corresponding audio clip, and adds the optional clipBegin and clipEnd attributes to indicate a specific offset within the clip.

The following example shows the Media Overlays markup for a single phrase or sentence.

<par>                        
    <text src="chapter1.xhtml#sentence1"/>
    <audio src="chapter1_audio.mp3" clipBegin="23s" clipEnd="30s"/>
</par>

par elements are placed together sequentially to form a series of phrases or sentences. Not every element of the EPUB Content Document will have a corresponding par element in the Media Overlay, only those relevant to the audio narration.

The following example shows a basic Media Overlay Document containing a sequence of phrases. The body element acts as the main sequence for the whole document.

<smil xmlns="http://www.w3.org/ns/SMIL" 
      version="3.0">
    <body>
        <par id="par1">
            <text src="chapter1.xhtml#sentence1"/>
            <audio src="chapter1_audio.mp3" clipBegin="0s" clipEnd="10s"/>
        </par>
        <par id="par2">
            <text src="chapter1.xhtml#sentence2"/>
            <audio src="chapter1_audio.mp3" clipBegin="10s" clipEnd="20s"/>
        </par>
        <par id="par3">
            <text src="chapter1.xhtml#sentence3"/>
            <audio src="chapter1_audio.mp3" clipBegin="20s" clipEnd="30s"/>
        </par>
    </body>
</smil>

par elements can also be added to seq elements to define more complex structures such as parts and chapters (see Structure).

3.2 Relationship to the EPUB Content Document

note

In this section, the EPUB Content Document is assumed to be an XHTML Content Document. While Media Overlays can be used with SVG Content Documents, playback behavior might not be consistent and therefore interoperability is not guaranteed.

3.2.1 Structure

The body of a Media Overlay Document consists of two elements: the par element and the seq element. The ordering of these elements must match the default reading order of the EPUB Content Document.

The par element represents phrases. Each element identifies a text and audio component to sychronize during playback.

The seq element represents sequences. It is used to represent nested containers such as sections, asides, headers, and footnotes. It allows the structure inherent in these containers to be retained in the Media Overlay Document.

The seq element must contain an epub:textref attribute. The IRI [RFC3987] value of this attribute must reference the corresponding structural element in an EPUB Content Document. As seq elements do not provide synchronization instructions, this attribute allows a Reading System to still match the element to a location in the text.

The following example shows a Media Overlay Document with nested seq elements, representing a chapter with both a section header and a figure.

<smil xmlns="http://www.w3.org/ns/SMIL" xmlns:epub="http://www.idpf.org/2007/ops" version="3.0">
    <body>

        <!-- a chapter -->
        <seq id="id1" epub:textref="chapter1.xhtml#s01" epub:type="chapter">

            <!-- the section title -->
            <par id="id2">
                <text src="chapter1.xhtml#section1_title"/>
                <audio src="chapter1_audio.mp3"
                       clipBegin="0:23:23.84"
                       clipEnd="0:23:34.221"/>
            </par>

            <!-- some sentences in the chapter -->
            <par id="id3">
                <text src="chapter1.xhtml#text1"/>
                <audio src="chapter1_audio.mp3"
                       clipBegin="0:23:34.221"
                       clipEnd="0:23:59.003"/>
            </par>
            <par id="id4">
                <text src="chapter1.xhtml#text2"/>
                <audio src="chapter1_audio.mp3"
                       clipBegin="0:23:59.003"
                       clipEnd="0:24:15.000"/>
            </par>

            <!-- a figure -->
            <seq id="id7" epub:textref="chapter1.xhtml#figure">
                <par id="id8">
                    <text src="chapter1.xhtml#photo"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:24:18.123"
                           clipEnd="0:24:28.764"/>
                </par>
                <par id="id9">
                    <text src="chapter1.xhtml#caption"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:24:28.764"
                           clipEnd="0:24:50.010"/>
                </par>
            </seq>

            <!-- more sentences in the chapter (outside the figure) -->
            <par id="id12">
                <text src="chapter1.xhtml#text3"/>
                <audio src="chapter1_audio.mp3"
                       clipBegin="0:25:45.515"
                       clipEnd="0:26:30.203"/>
            </par>
            <par id="id13">
                <text src="chapter1.xhtml#text4"/>
                <audio src="chapter1_audio.mp3"
                       clipBegin="0:26:30.203"
                       clipEnd="0:27:15.000"/>
            </par>

        </seq>
    </body>
</smil>

note

The reason for grouping structures like sections, figures, tables, and footnotes in a seq element is so that their start and end positions can be identified during playback. Reading Systems can then offer playback options tailored to the layout of the given Rendition, such as jumping past a long figure, turning off rendering of page break announcements (see Skippability and Escapability), or customizing the reading mode to suit structures such as tables.

The following example shows the EPUB Content Document that corresponds to the previous Media Overlay example.

<html xmlns="http://www.w3.org/1999/xhtml" 
      xmlns:epub="http://www.idpf.org/2007/ops" 
      xml:lang="en" 
      lang="en">
    <head>
        <title>Media Overlays Example of EPUB Content Document</title>
    </head>
    <body id="sec1">
        <section id="sectionstart" epub:type="chapter">
            <h1 id="section1_title">The Section Title</h1>
            <p id="text1">The first phrase of the main text body.</p>
            <p id="text2">The second phrase of the main text body.</p>
            <figure id="figure">
                <img id="photo" 
                     src="photo.png" 
                     alt="a photograph for which there is a caption" />
                <figcaption id="caption">The photo caption</figcaption>
            </figure>
            <p id="text3">The third phrase of the main text body.</p>
            <p id="text4">The fourth phrase of the main text body.</p>
        </section>
    </body>
</html>

3.2.2 Granularity

This section is informative

Media Overlay text elements' src attributes refer to EPUB Content Document elements by their IDs [XML]. The granularity level of the Media Overlay therefore depends on how the EPUB Content Document is marked up. If the finest level of markup is at the paragraph level, then that is the finest possible level at which Media Overlay synchronization can be authored. Likewise, if sub-paragraph markup is available, such as [HTML] span elements representing phrases or sentences, then finer granularity is possible in the Media Overlay. Finer granularity gives users more precise results for synchronized playback when navigating by word or phrase and when searching the text, but increases the file size of the Media Overlay Documents.

3.2.3 Embedded Media

Any EPUB Content Document associated with a Media Overlay may contain embedded media such as video, audio, and images. The Media Overlay text element may be used in such instances to reference the embedded media by its ID [XML] value.

When a text element references embedded media that contains audio, the audio sibling element is optional.

Authors should avoid using scripts to control playback of referenced embedded EPUB Content Document media, as this might conflict with Media Overlays playback behavior.

3.2.4 Text-to-Speech

This specification allows the use of Text-to-Speech (TTS) in addition to pre-recorded audio clips. When a Media Overlay text element with no audio sibling element references an element within the target EPUB Content Document, the contents of that referenced element must be appropriate for rendering via TTS. For example, it could be a textual EPUB Content Document element or contain a text fallback.

3.3 Semantic Inflection

In order to express semantic inflections, the epub:type attribute [Content Docs 3.1] may be attached to Media Overlay par, seq, and body elements.

Values for the Media Overlay epub:type attribute are constrained identically to the epub:type attribute in EPUB Content Documents. Refer to Semantic Inflection [Content Docs 3.1] for details.

The epub:type attribute facilitates Reading System behavior appropriate for the semantic type(s) indicated. Examples of these behaviors are Skippability and Escapability and Note.

The following example shows the semantic markup for a Media Overlay containing a figure.

<smil xmlns="http://www.w3.org/ns/SMIL" 
      xmlns:epub="http://www.idpf.org/2007/ops"
      version="3.0">
    <body>
        <seq id="id1" epub:textref="chapter1.xhtml#figure" epub:type="figure">
            <par id="id2">
                <text src="chapter1.xhtml#figuretitle"/>
                <audio src="chapter1_audio.mp3" clipBegin="0:24:15.000" clipEnd="0:24:18.123"/>
            </par>
            <par id="id3">
                <text src="chapter1.xhtml#figurecaption"/>
                <audio src="chapter1_audio.mp3" clipBegin="0:24:18.123" clipEnd="0:24:38.530"/>
            </par>
            <par id="id4">
                <text src="chapter1.xhtml#figuretext1"/>
                <audio src="chapter1_audio.mp3" clipBegin="0:24:38.530" clipEnd="0:25:00.515"/>
            </par>
        </seq>
    </body>
</smil>

This specification adopts the vocabulary association mechanisms defined in Vocabulary Association [Content Docs 3.1] unmodified. Terms from the default vocabulary must be used unprefixed in Overlay Documents.

Media Overlays may use additional vocabularies by defining them in the epub:prefix attribute on the root smil element.

3.4 Associating Style Information

Visual rendering information for the currently-playing EPUB Content Document element may be expressed in the CSS Style Sheet using author-defined classes. These author-defined class names should be declared in the Package Document metadata using the metadata properties active-class and playback-active-class [Overlays Vocab]. The class names are then discoverable by Reading Systems.

This example demonstrates how Authors can associate style information with the currently-playing EPUB Content Document.

note

Although this example uses the class names -epub-media-overlay-active and -epub-media-overlay-playing, any class names are permitted. The class names chosen can be used along with any supported CSS features.

The author-defined CSS class names, declared using the metadata properties active-class and playback-active-class [Overlays Vocab] in the Package Document:

<meta property="media:active-class">-epub-media-overlay-active</meta>
<meta property="media:playback-active-class">-epub-media-overlay-playing</meta>

The CSS Style Sheet containing the author-defined class names:

/* emphasize the active element */
.-epub-media-overlay-active {
    background-color: yellow;
    color: black !important;
}

/* fade out the inactive text */
html.-epub-media-overlay-playing * {
    color: gray;
}

The relevant EPUB Content Document excerpt:

<html>
    …
    <span id="txt1">This is the first phrase.</span>
    <span id="txt2">This is the second phrase.</span>
    <span id="txt3">This is the third phrase.</span>
    …
</html>

In this example, the Reading System would apply the author-defined -epub-media-overlay-active class to each text element in the EPUB Content Document as it became active during playback. Conversely, the class name is removed when the element is no longer active. The user would see each EPUB Content Document element styled with a yellow background for the duration of that element's playback.

The Reading System would also apply the author-defined -epub-media-overlay-playing class to the document element of the EPUB Content Document when Media Overlays playback begins. The class name is removed when playback stops. In the case of an XHTML Content Document, the class name would be applied to the html element. In the case of an SVG Content Document, it would be applied to the svg element. The user would see all the inactive text elements turn gray during Media Overlays playback. When playback stopped, the elements’ colors would return to their defaults.

3.5 Packaging

3.5.1 Including Media Overlays

Manifest item elements [Packages 3.1] that reference EPUB Content Documents may specify a Media Overlay via the media-overlay attribute. The attribute must reference the ID [XML] of the manifest item for the corresponding Media Overlay Document.

Manifest items for Media Overlay Documents must have the media type application/smil+xml.

The duration of an EPUB Content Document must be specified in a duration attribute [Packages 3.1] on its manifest item.

The following example shows how to include Media Overlays in the manifest of a Package Document.

<manifest>
   <item id="ch1" 
         href="chapter1.xhtml" 
         media-type="application/xhtml+xml" 
         media-overlay="ch1_audio"
         duration="0:32:29"/>
   
   <item id="ch1_audio" 
         href="chapter1_audio.smil" 
         media-type="application/smil+xml"/>
   …
</manifest>

If an EPUB Content Document is wholly or partially referenced by a Media Overlay, then its manifest item entry must include a media-overlay attribute.

The media-overlay attribute must not be attached to manifest item elements that do not reference EPUB Content Documents.

3.5.2 Package Metadata

The Package Document must include the duration of the entire Rendition in a meta element with the duration [Overlays Vocab] property.

Authors also may include narrator [Overlays Vocab] information in the Package Document, as well as author-defined CSS class names to be applied to the currently-playing EPUB Content Document element.

The following example shows a Package Document with metadata about Media Overlays.

<metadata>
   …
   <meta property="media:duration">1:36:20</meta>
   <meta property="media:narrator">Joe Speaker</meta>
   <meta property="media:active-class">-epub-media-overlay-active</meta>
   <meta property="media:playback-active-class">-epub-media-overlay-playing</meta>
   …
</metadata>

note

The prefix media: is reserved by [Packages 3.1] for the inclusion of these properties in package metadata.

4 Playback Behaviors

4.1 Loading the Media Overlay

When an EPUB Reading System loads a Package Document, it must refer to the Manifest item elements' media-overlay attributes to discover the corresponding Media Overlays for EPUB Content Documents. Playback must start at the Media Overlay element which corresponds to the desired EPUB Content Document starting point. Note that the start of an EPUB Content Document may correspond to an element at the start or in the middle of a Media Overlay. When the Media Overlay Document has finished playing, the Reading System should load the next EPUB Content Document (as specified in the Package Document spine) and also load its corresponding Media Overlay Document, provided that one is given.

4.2 Basic Playback

4.2.1 Timing and Synchronization

Reading Systems must render immediate children of the body element in a sequence. A seq element's children must be rendered in sequence, and playback completes when the last child has finished playing. A par element's children must be rendered in parallel (with each starting at the same time), and playback completes when all the children have finished playing. When the body element's last child has finished playing, playback of the Media Overlay Document is done.

4.2.2 Rendering Audio

When presented with a Media Overlay audio element, Reading Systems must play the audio resource referenced by the src attribute, starting at the clip offset time given by the clipBegin attribute and ending at the clip offset time given by the clipEnd attribute. The following rules must be observed:

If clipBegin is not specified, its value is assumed to be "0".
If clipEnd is not specified, its value is assumed to be the full duration of the physical media.
If clipEnd exceeds the full duration of the physical media, then its value is assumed to be the full duration of the physical media.

User-controllable audio playback options should include timescale modification, in which the playback rate is altered without distorting the pitch. The suggested range is half-speed to double-speed.

4.2.3 Rendering EPUB Content Document Elements

When presented with a Media Overlay text element, Reading Systems should ensure the EPUB Content Document element referenced by the src attribute is visible in the Viewport. During Media Overlays playback, Reading Systems with a Viewport should add the class names given by the metadata properties active-class and playback-active-class [Overlays Vocab] to the appropriate elements in the EPUB Content Document. Conversely, the class names should be removed when the playback state changes, as described in Associating Style Information.

The active-class and playback-active-class metadata properties are optional, and if omitted, Reading System behavior is implementation-specific.

4.3 Interacting with the EPUB Content Document

4.3.1 Navigation

Because the Media Overlay is closely linked to the EPUB Content Document, it is very easy for Reading Systems to locate a position in the EPUB Content Document based on the current position in the Media Overlay playback. If the user pauses synchronized playback and navigates to a different part of the EPUB Publication, synchronized playback must resume at that point. For example, if a specific page number in the EPUB Content Document is the desired location, then this same point is located in the Media Overlay and playback started there.

This same approach allows for synchronizing the Media Overlay playback with user selection of a navigation point in the EPUB Navigation Document. The Reading System loads the Media Overlay for that file and finds the correct point for starting playback based on the ID [XML] of the navigation point target.

note

A Media Overlay Document can be associated directly with a Navigation Document in order to provide synchronized playback of its contents, regardless of whether the XHTML Content Document in which it resides is included in the spine. See EPUB Navigation Document for more information.

note

Media Overlay Document elements can be associated with EPUB Content Document structures such as tables. Reading Systems need to ensure that Media Overlay playback remains synchronized with user navigation of table rows and cells. The Reading System might also play the corresponding table header preceding the contents of the cell.

4.3.2 Embedded Audio and Video

An EPUB Content Document with which a Media Overlay is associated may itself contain embedded video and audio media, which may be pointed to by Media Overlay elements. Unlike text and images, video and audio media has an intrinsic duration. Consequently, when a Reading System renders the synchronization described by a Media Overlay, the default playback behavior of audio and video media embedded within the associated EPUB Content Document must be overridden.

Note that the rules below apply only to referenced [HTML] video or audio elements within the associated EPUB Content Document. That is to say, the rules apply to only those elements pointed to by text elements within the Media Overlay (i.e., via the src attribute). Embedded media that is not referenced by Media Overlay elements is not subject to these rules.

All referenced audio and video media embedded within an EPUB Content Document must have their public playback interface deactivated (typically: play/pause control, time slider, volume level, etc.). This behavior is necessary to avoid interference between the scheduled playback sequence defined by the Media Overlay, and the arbitrary playback behavior due to user interaction or script execution. As a result, when the Reading System is in playback mode, it should:
- Hide the individual video/audio UI controls from the page, which overrides the default behavior defined by the [HTML] controls attribute.
- Prevent scripts embedded within the EPUB Content Document from invoking the JavaScript audio/video playback API (i.e., authored as part of the default behavior). It is recommended that content producers avoid publishing embedded scripts dedicated to controlling the playback of embedded audio/video media. The published Media Overlay can then retain full control of the synchronized presentation without any risk of interference from script-enabled custom behaviors.
All referenced audio and video media embedded within an EPUB Content Document must be initialized to their "stopped" state, and be ready to be played from the zero-position within their content stream (possibly displaying the image specified using the [HTML] poster attribute). This requirement overrides the default behavior defined by the [HTML] autoplay attribute.
When an EPUB Content Document element becomes active, the CSS Style Sheet visual highlighting rules apply regardless of the content type referred to by that element's src attribute (e.g., the CSS class name defined by the active-class [Overlays Vocab] metadata property should be applied to visible video and audio player controls within the host EPUB Content Document).
In addition to the default behavior of Media Overlay activation for textual fragments and images, audio and video playback must be started and stopped according to the duration implied by the authored Media Overlay synchronization (as per the standard [SMIL] timing model). There are two possible scenarios:
- When a Media Overlay text element has no audio sibling within its par parent container, the referenced EPUB Content Document audio or video media must play until it ends, at which point the text element's lifespan terminates. In this case, the implicit duration of the text element (and by inference, of the parent par container) is that of the referenced audio or video clip.
- When a Media Overlay text element has an audio sibling within its par parent container, the playback duration of the referenced EPUB Content Document audio or video media must be constrained by the duration of the audio sibling. In this case, the actual duration of the parent par container is that of the child audio clip, regardless of the duration of the video or audio media pointed to by the text element. This behavior can result in embedded video or audio media ending playback prematurely (before reaching its full duration), or ending before the playback of the parallel Media Overlay audio is finished (in which case the last-played video frame should remain visible until the parent par container finally ends). This behavior is equivalent of the Media Overlay audio element implicitly carrying the behavior of the [SMIL] endsync attribute.
  
  Furthermore, Reading Systems should expose user controls for the volume levels of each independent audio track (i.e., from the audio element of the Media Overlay, and from the embedded audio or video media within the EPUB Content Document), so that audio output can be adjusted to match listeners' requirements. Note that having overlapping audio tracks is typically an authoring-time concern: content producers usually add a layer of audio information over a video track for description purposes. It is recommended that overlapping audio situations are carefully examined and dealt with at production stage, as Reading Systems are not required to handle simultaneous volume levels in any particular way.
When a text element becomes inactive in the Media Overlay, and when it points to embedded video or audio media, that referenced media must be reset to its initial "stopped" state, ready to be played from the zero-position within their content stream (possibly displaying the poster image specified using the [HTML] markup).

4.3.3 Text-to-Speech

When a Media Overlay text element with no audio sibling element references text within the target EPUB Content Document, Reading Systems capable of Text-to-Speech (TTS) should render the referenced text using TTS.

As per Reading System conformance requirements, the speech-related information provided in the target EPUB Content Document should be used to play the audio stream as part of the Media Overlay rendering. See Reading System Text-to-Speech Conformance Requirements [EPUB 3.1].

The Media Overlay text element's lifespan corresponds to the rendering time of the associated speech synthesis. The implicit duration of the text element (and by inference, of the parent par element) is therefore determined by the execution of the Text-to-Speech engine, and cannot be known at authoring time (factors like speech rate, pauses and other prosody parameters influence the audio output).

4.4 Skippability and Escapability

4.4.1 Skippability

While reading, users might want to turn on or off certain features of the content, such as footnotes, page numbers, or other types of secondary content. This feature is called skippability. Reading Systems should use the semantic information provided by Media Overlay elements' epub:type attribute to determine when to offer users the option of skippable features.

The following example shows a Media Overlay Document with a pagebreak. A Reading System could offer the user the option of turning on and off the page break/page number announcements, which are often cumbersome to listen to.

<smil xmlns="http://www.w3.org/ns/SMIL" 
      xmlns:epub="http://www.idpf.org/2007/ops"
      version="3.0">
    <body>
        <!-- a paragraph -->
        <par id="id1">
            <text src="chapter1.xhtml#para1"/>
            <audio src="chapter1_audio.mp3"
                   clipBegin="0:23:22.000"
                   clipEnd="0:24:15.000"/>
        </par>

        <!-- a page number -->
        <par id="id2" epub:type="pagebreak">
            <text src="chapter1.xhtml#pgbreak1"/>
            <audio src="chapter1_audio.mp3"
                   clipBegin="0:24:15.000"
                   clipEnd="0:24:18.123"/>
        </par>

        <!-- another paragraph -->
        <par id="id3">
            <text src="chapter1.xhtml#para2"/>
            <audio src="chapter1_audio.mp3"
                   clipBegin="0:24:18.123"
                   clipEnd="0:25:28.530"/>
        </par>
    </body>
</smil>

The following example shows an EPUB Content Document with a pagebreak.

<html … >
    …
    <body>
        <p id="para1">This is the paragraph before the pagebreak … </p>
        
        <span id="pgbreak1" epub:type="pagebreak" title="234"/>
        
        <p id="para2">This is the paragraph after the pagebreak …</p>
    </body>
</html>

The following non-exhaustive list represents terms from the [Structure Vocab] for which Reading Systems might offer the option of skippability:

footnote
endnote
pagebreak

Reading System support for skippability based on epub:type values should not be assumed.

4.4.2 Escapability

Escapable items are nested structures, such as tables and lists, that users might wish to skip over, continuing to read from the point immediately after the nested structure. The escapability feature differs from the skippability feature in that it does not enable or disable entire types of items, but provides an exit from them (e.g., a user can listen to some of the content before choosing to escape).

Reading Systems should allow escaping of nested structures. Reading Systems must determine the start of nested structures by the value of the epub:type attribute and should offer users the option to skip playback of that structure and resume with whatever content comes after it.

The following example shows the Media Overlay Document for an EPUB Content Document containing a paragraph, a table, and another paragraph. A Reading System that supported escapability would give the user the option to interrupt playback of the table to continue playing the next paragraph.

<smil xmlns="http://www.w3.org/ns/SMIL" 
      xmlns:epub="http://www.idpf.org/2007/ops"
      version="3.0">
    <body>
        <!-- a paragraph, part of the regular document text -->
        <par id="id1">
            <text src="c01.xhtml#para1"/>
            <audio src="chapter1_audio.mp3"
                   clipBegin="0:23:22.000"
                   clipEnd="0:24:15.000"/>
        </par>

        <!-- a table with two nested rows -->
        <seq id="id2" epub:textref="c01.xhtml#t1" epub:type="table">
            <seq id="id3" epub:textref="c01.xhtml#tr1" epub:type="table-row">
                <par id="id4" epub:type="table-cell">
                    <text src="c01.xhtml#td1"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:24:15.000"
                           clipEnd="0:24:18.123"/>
                </par>
                <par id="id5" epub:type="table-cell">
                    <text src="c01.xhtml#td2"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:24:18.123"
                           clipEnd="0:25:28.530"/>
                </par>
                <par id="id6" epub:type="table-cell">
                    <text src="c01.xhtml#td3"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:25:28.530"
                           clipEnd="0:25:45.515"/>
                </par>
            </seq>
            <seq id="id7" epub:textref="c01.xhtml#tr2" epub:type="table-row">
                <par id="id8" epub:type="table-cell">
                    <text src="c01.xhtml#td4"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:25:45.515"
                           clipEnd="0:26:45.700"/>
                </par>
                <par id="id9" epub:type="table-cell">
                    <text src="chapter1.xhtml#td5"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:26:45.700"
                           clipEnd="0:28:02.033"/>
                </par>
                <par id="id10" epub:type="table-cell">
                    <text src="chapter1.xhtml#td6"/>
                    <audio src="chapter1_audio.mp3"
                           clipBegin="0:28:02.033"
                           clipEnd="0:28:52.207"/>
                </par>
            </seq>
        </seq>

        <!-- another paragraph -->
        <par id="id11">
            <text src="c01.xhtml#para2"/>
            <audio src="chapter1_audio.mp3"
                   clipBegin="0:28:52.207"
                   clipEnd="0:30:01.000"/>
        </par>
    </body>
</smil>

The following non-exhaustive list represents terms from the [Structure Vocab] for which Reading Systems might offer the option of escapability:

table
table-row
table-cell
list
list-item
figure

Status of this Document

1 Overview

1.1 Purpose and Scope

1.2 Relationship to Other Specifications

1.2.1 Relationship to SMIL

1.3 Terminology

1.4 Typographic Conventions

note

caution

1.5 Conformance Statements

1.6 Namespace prefix mappings

2 Media Overlay Document Definition

2.1 Introduction

2.2 Content Conformance

2.3 Reading System Conformance

2.4 Media Overlay Document Definition

2.4.1 The smil Element

2.4.2 The head Element

2.4.3 The metadata Element

2.4.4 The body Element

2.4.5 The seq Element

2.4.6 The par Element

2.4.7 The text Element

2.4.8 The audio Element

3 Creating Media Overlays

3.1 Introduction

3.2 Relationship to the EPUB Content Document

note

3.2.1 Structure

note

3.2.2 Granularity

3.2.3 Embedded Media

3.2.4 Text-to-Speech

3.3 Semantic Inflection

3.4 Associating Style Information

note

3.5 Packaging

3.5.1 Including Media Overlays

3.5.2 Package Metadata

note

4 Playback Behaviors

4.1 Loading the Media Overlay

4.2 Basic Playback

4.2.1 Timing and Synchronization

4.2.2 Rendering Audio

4.2.3 Rendering EPUB Content Document Elements

4.3 Interacting with the EPUB Content Document

4.3.1 Navigation

note

note

4.3.2 Embedded Audio and Video

4.3.3 Text-to-Speech

4.4 Skippability and Escapability

4.4.1 Skippability

4.4.2 Escapability

5 EPUB Navigation Document

note

Appendix A. Media Overlays Schema

note

Appendix B. Examples of Clock Values

Acknowledgements and Contributors

IDPF Members

Invited Experts/Observers

References

Normative References

Informative References

2.4.1 The `smil` Element

2.4.2 The `head` Element

2.4.3 The `metadata` Element

2.4.4 The `body` Element

2.4.5 The `seq` Element

2.4.6 The `par` Element

2.4.7 The `text` Element

2.4.8 The `audio` Element