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Audio Synchronization

This page details the resources and references to help in creation of EPUB 3 publication with media overlays.

 Media Overlays (MO) is the technical term for the part of the EPUB 3 specification specifying that the (visual) rendition of the text of the eBook must be accompanied by, and synchronized to, the (aural) rendition of some pre-recorded audio file, embedded in the eBook container.

Media overlays are used to synchronize audio narration with text. EPUB 3 allows 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.

Reading systems that support media overlays do so by enabling three functions:

  1. the audio rendition (hopefully with dedicated audio controls)
  2. the synchronous highlighting of the text fragment being narrated
  3. the "tap-to-play" function, which allows the user to tap (or click) on the text, and have the aural rendition restarting from the touched (or clicked) text fragment.

You need to perform the following steps, for each XHTML file you want to attach audio to:

  1. produce an audio file,
  2. add suitable id attributes to elements in the page, one for each desired SMIL fragment,
  3. create a SMIL file specifying the sync information between the text and the audio,
  4. adding the highlighting class to the CSS,
  5. add the suitable metadata to the OPF file of the eBook.


Additional resources 

  • Audio recording and editing software
    • Audacity: a free and open-source digital audio editor and recording application software


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