Synchronized Audio Description

Synchronized audio description is a voice narration inserted during pauses in a video to describe visual elements that the soundtrack does not convey. It enables people who are blind or visually impaired to follow the action, scene changes, and on-screen text. WCAG requires it at level AA for all prerecorded video content.

A video shows a character opening an envelope, reading the letter silently, and smiling. Without audio description, a blind person only hears the paper rustling. They do not know what causes the smile, or even that there is a smile at all.

#What WCAG Requires

WCAG criterion 1.2.5 (level AA) requires audio description for all prerecorded video in synchronized media. The description is inserted during pauses between dialogue: actions, expressions, on-screen text, scene changes.

If the audio track already conveys all visual information, no additional audio description is necessary. This is the case with an interview where the presenter announces the name and role of each speaker instead of displaying them only in a banner.

For videos without sufficient pauses, criterion 1.2.7 (level AAA) provides for extended audio description: the player automatically pauses the video while the description is spoken, then resumes playback.

The RGAA adopts these requirements in its Multimedia section (4).

#The Technical Pitfall No One Anticipates

HTML provides a <track kind="descriptions"> element to associate a descriptions file with a video. In theory, browsers could synthesize speech from the text.

<video controls>
  <source src="demo.mp4" type="video/mp4" />
  <track kind="descriptions" src="ad.vtt" srclang="en" label="Audio description" />
</video>

In practice, no major browser reliably supports this functionality. The most common solution remains to produce an alternative version of the video with the audio description mixed directly into the soundtrack, or to use JavaScript to control a separate audio file synchronized with playback.

#Plan Ahead at the Scenario Stage

Adding audio description after editing is expensive. Dialogue overlaps, pauses are too short, descriptions do not fit. The least costly approach: plan for sufficient silences between lines when writing the scenario, and integrate visual information directly into the script when possible. The W3C calls this technique "integrated description."

In France, nearly 2 million people with visual impairments are affected by audio description according to the CFPSAA.

#In Summary

Synchronized audio description describes the visual content of a video during pauses in dialogue. WCAG requires it at level AA. Plan for it from the scenario stage rather than in post-production: you will save time and your videos will be accessible without an alternative version.