What is EPUB media overlays?

Why it matters when you convert a PDF to EPUB

A PDF carries no narration, so converting one never produces media overlays; they are added later, for read-along books. They explain the MED messages in EPUBCheck, which check that clips and text references match.

How EpubCC handles it: it never writes media overlays or SMIL files, because a PDF has no narration to synchronise. An EPUB from EpubCC can still be read aloud by a reading app's text-to-speech, which works better than with the PDF because the text is in reading order, the language is declared and running headers are no longer read out on every page.

Related terms

• What is EPUB 3?

EPUBCheck messages about this

• MED-008 The time specified in the clipBegin attribute must not be after clipEnd.
• MED-009 The time specified in the clipBegin attribute must not be the same as clipEnd.
• CSS-029 Found CSS class name "…" but no "…" property was declared in the package document.
• CSS-030 The package document declares media overlays styling class names but no CSS was found in the content document.
• MED-003 Picture "img" elements must reference core media type resources, but found resource "…" of type "…".
• MED-004 Image file header may be corrupted.
• MED-005 Media Overlay audio reference … to non-standard audio type … found.
• MED_007 Picture "source" elements must define a "type" attribute when they reference foreign resources; found resource "…" of type "…".
• MED_010 EPUB Content Documents referenced from a Media Overlay must specify the "media-overlay" attribute.
• MED_011 EPUB Content Document referenced from multiple Media Overlay Documents.