As you might suspect at this point, the reading system can’t synchronize or play content back any way but what has been defined; as a reader you cannot, for example, dynamically change from word-to-word to paragraph-by-paragraph read-back as you desire. The magic is only as magical as you make it, at least at this time.

With only a single level of playback granularity available, the decision on how fine a playback experience to provide has typically been influenced by the disability you’re targeting, going back to the origins of the functionality in talking books. Books for blind and low-vision readers are often only synchronized to the heading level, for example, and omit the text entirely. Readers with dyslexia or cognitive issues, however, may benefit more from word-level synchronization using full-text full-audio playback.

Coarser synchronization—for example at the phrase or paragraph level—can be useful in cases where the defining characteristics of a particular human narration (flow, intonation, emphasis) add an extra dimension to the prose, such as with spoken poetry or religious verses. The production costs associated with synchronizing human-narrated ebooks to the word level, however, has typically meant that only short-prose works (such as children’s books) get this treatment.

Let’s turn to the practical construction of an overlay to discover why the complexity increases by level, though. Understanding the issues will give better insight into which model you ultimately decide to use.