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 as the best marketing buzzword to describe the experience. But what you are in fact witnessing is a media type (audio) being overlaid on your text content, and tech trumps marketing in standards!
The audio-visual magic that overlays enable in EPUBs is just the tip of the iceberg, however. Overlays represent a bridge between the audio and video worlds, and between mainstream and accessibility needs, which is what really makes this new technology so exciting. They offer accessible audio navigation for blind and low-vision readers. They can improve the reading experience for persons with trouble focusing across a page and improve reading comprehension for many types of readers. They can also provide a unique read-along experience for young readers.
From a mainstream publisher’s perspective, overlays provide a bridge between audiobook and ebook production streams. Create a single source using overlays and you could transform and distribute across the spectrum from audio-only to text-only. With full-text and full-audio synchronization ebooks, you can transform down to a variety of formats. If you’re going to create an audiobook version of your ebook, it doesn’t make sense not to combine production, which is exactly what EPUB 3 allows you to now do. Your source content is more valuable by virtue of its completeness, and you can also choose to target and distribute your ebook with different modalities enabled for different audiences.
From a reader’s perspective, they can purchase a format that provides adaptive modalities to meet their reading needs: they can choose which playback format they prefer, or purchase a book with multiple modalities and switch between them as the situation warrants—listening while driving and visually reading when back at home, for example.
Media overlays are the answer to a lot of problems on both sides of the accessibility fence, in other words.
If you’re coming to this guide from accessibility circles, however, you’re probably wondering why this is considered new and exciting when it sounds an awful lot like the SMIL technology that has been at the core of the DAISY talking book specifications for more than a decade. And you’re right…sort of. Overlays are not new technology, but represent a new evolution of the DAISY standard, which EPUB 3 is a successor to. What is really exciting from an accessibility perspective is the chance to move this production back to the source to get high-quality audio and text synchronized ebooks directly from publishers. Removing synchronization information from having to be encoded in content files is another benefit overlays provide over older talking book formats, greatly simplifying their creation.