Using the track element to provide audio descriptions

From WCAG WG


Status

  • New technique.

This technique would be an Advisory Technique in Situation B of Understanding 1.2.1 (for Video-Only), Understanding 1.2.3, Understanding 1.2.5, and Understanding 1.2.7. Once there is sufficient user agent support, it would be a Sufficient Technique.

  • Added to XML as H96

Applicability

  • HTML5

This technique relates to:

Note: While this is currently an advisory technique, if it becomes a sufficient technique it will be sufficient for 1.2.7 only if the user agent has the ability to pause the video to allow the extended audio description to play completely without overlapping other important audio in the primary program.

User Agent and Assistive Technology Support Notes

Description

The objective of this technique is to use the HTML5 track element to specify a descriptions timed text track for a video element. Audio description timed text tracks contain textual descriptions of the video component of the media resource, intended for audio synthesis when the visual component is obscured, unavailable, or not usable. The user agent makes the cues available to the user in a non-visual fashion, for instance, by synthesizing them into speech.

The src attribute of the track element is an URL providing the text track data.

The audio description cues must fit into the gaps available in the audio component of the media resource. If there is not enough time to synthesize the description text in the track cue's time interval, user agents may truncate the speech. This limits the amount of supplementary information that can be added.

User agents may also support extended audio descriptions by halting the video until the description has been completely synthesized, then restarting the video.

Examples

Example 1

A video element for a video in the English language. The audio descriptions are provided in the WebVTT format. 

  <video poster="myvideo.png" controls>
    <source src="myvideo.mp4" srclang="en" type="video/mp4">
    <track src="myvideo_en.vtt" kind="descriptions" srclang="en" label="English">
  </video>

Example 2

A video element for a video with both an English and French language source element, and with an English and a French audio description track using the WebVTT (vtt) file format. 

  <video poster="myvideo.png" controls>
    <source src="myvideo.mp4" srclang="en" type="video/mp4">
    <source src="myvideo.webm" srclang="fr" type="video/webm">
    <track src="myvideo_en.vtt" kind="descriptions" srclang="en" label="English">
    <track src="myvideo_fr.vtt" kind="descriptions" srclang="fr" label="French">
  </video>

Example 3

A video, "Google self-driving car". with an audio description track. 

  <video controls tabindex="1">
    <source src="cdgQpa1pUUE.webm" type="video/webm">
    <source src="cdgQpa1pUUE.mp4" type="video/mp4">
    <track id="audesc" src="cdgQpa1pUUE.vtt" kind="descriptions" label="English descriptions" srclang="en-us"></track>
  </video>

The live example can be tested on Chrome using the Chrome extensions listed above.

Resources

Related Techniques

Tests

Procedure

For the video element used to play a video:

  1. Check that the video contains a track element of kind descriptions in the language of the video.

Expected Results

  • Check that #1 is true
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