Add extended audio description in SMIL 1.0
SM1 is a sufficient technique when used with G8, the general technique for a movie with extended audio descriptions, and it is filed against three different description rules rather than the one shown above. Extended description exists for a specific problem. Ordinary audio description has to fit in the gaps between lines of dialogue, and some video has no gaps worth the name. A densely narrated demonstration leaves nowhere to say what is on screen. SM1's answer is to stop the video instead. The presentation is split into sequential segments, playback freezes at chosen points while the longer description plays, and then it carries on. W3C's own example pauses in four places. This technique assumes a SMIL 1.0 player. If you are not publishing SMIL, the same job today is a video that pauses under script for description, or a separately published described version alongside the original.
How we find it in an audit
Watching is the method. There is no substitute. A reviewer plays the video once as a sighted viewer and once with their eyes off the screen, and lists everything that only exists visually. On-screen text, a diagram being pointed at, a demonstration with no commentary, a result that appears without being named. Then we count how much of that could fit in the existing gaps. When the answer is not much, ordinary description is not the fix and extended description is. Automated tools can tell you a description track is absent. They have nothing to say about whether the one you have is enough.
How affected users experience it
A blind viewer hears a competent narrator explain the process. They never learn which button was pressed. The instructor says "and then you do this", and this happened on screen. Description written into the gaps cannot rescue that, because the gaps are where the narrator is drawing breath. Freezing the picture for a few seconds so somebody can say what changed costs a sighted viewer nothing they will remember and gives a blind viewer the step they were missing.
Passes vs. fails
Passes
<seq>
<par><video src="lesson-1.mp4"/></par>
<par><audio src="describe-1.mp3"/></par>
<par><video src="lesson-2.mp4"/></par>
</seq>
<!-- The picture holds still while each description plays, then carries on. -->Fails
<par>
<video src="lesson.mp4"/>
<audio src="narration.mp3"/>
</par>
<!-- Wall-to-wall narration. No gaps, no description, no pauses to make room for one. -->How this gets tested
The W3C publishes test rules that define what a checker looks for here.
- Video element visual content has accessible alternativeA tool finds candidates, you decide
- Video element visual content has strict accessible alternativeA tool finds candidates, you decide
Other ways to satisfy this rule
13 guides on this site are filed under 1.2.3 Audio Description or Media Alternative (Prerecorded). W3C lists this one as sufficient for that rule only alongside G8, so the pair is what passes and neither half does on its own.
- G8sufficientProvide a version with extended audio descriptions
- G58sufficientPut the media alternative link beside the media
- G69sufficientProvide a full transcript of the media
- G78sufficientOffer a selectable audio description track
- G173sufficientOffer a described version of every video
- G203sufficientDescribe talking-head video with static text
This guide is our interpretation of W3C technique SM1: Adding extended audio description in SMIL 1.0. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.