EPUB 3 Best Practices

Chapter 6. Media Overlays

Having looked at embedding audio content within documents, the natural progression is to look now at how to provide synchronized audio narration, another of the key new additions introduced in EPUB 3. Media overlays, as you’re about to see, are the secret behind how this magic works.

When you watch words get highlighted in your reading system as a narrator speaks, the term media overlay probably doesn’t immediately jump to mind to describe the experience, but what you are in fact witnessing is a media type (audio) being layered on top of your text content. Apple was the first to adopt this technology, adding it to their support of EPUB 2 under the name Read Aloud while the EPUB 3 specification was still being finalized, but other reading systems, like Readium, have since appeared offering beta support. Even Amazon has jumped into the synchronization game, if not with the same media overlays technology.

But the ability to synchronize text and audio in ebooks isn’t a new development in the grander scheme of ebook formats. It goes back fifteen years in DAISY digital talking books, and even further in antecedents to that format. The value of text and audio synchronization for learning has an equally long history, which is why the technology is so important across the entire spectrum of readers. Media overlays are more than just an accessibility feature of EPUB 3, in other words.

Overlays enable any reader to quickly and easily switch from one reading modality ...