For prerecorded audio-only and prerecorded video-only media, the following are true, except when the audio or video is a media alternative for text and is clearly labeled as such:
(From Understanding WCAG 2.0: Understanding Success Criterion 1.2.1)
There are three defined types of real-time media: audio-only, video-only, or synchronised media. A media item will be considered synchronised if:
In both cases, the sychronisation relates to time synchronisation between actions. For instance:
Most Success Criteria apply to synchronised media, except where it is made clear that it applies to audio-only or video-only content.
For the purposes of WCAG 2.0, "live" content is defined as
information captured from a real-world event and transmitted to the receiver with no more than a broadcast delay
- Note 1: A broadcast delay is a short (usually automated) delay, for example used in order to give the broadcaster time to queue or censor the audio (or video) feed, but not sufficient to allow significant editing.
- Note 2: If information is completely computer generated, it is not live.
The typical "seven-second delay" used to edit out profanity the like in live radio is an example of a broadcast delay. Transmission delays, such as latency of a link-up to a live event, also should not affect "live" status.
Pre-recorded content is simply anything that is "not live".
Understanding that providing alternatives for live content is more challenging than for pre-recorded media, most of the Success Criteria apply to the latter, although a couple do apply to live content, such as provision of captions at AA level. The titles of each Success Criterion in Guideline 1.2 clearly state whether they apply to live or pre-recorded media.
The definition of "synchronised content", as well as the basic Success Criterion 1.2.1 for audio-only or video-only content, have the following exclusion in their definition:
[...that] the media is a media alternative for text and is clearly labeled as such
where a "media alternative for text" is defined in the WCAG 2.0 glossary as
media that presents no more information than is already presented in text (directly or via text alternatives)
- Note: A media alternative for text is provided for those who benefit from alternate representations of text. Media alternatives for text may be audio-only, video-only (including sign-language video), or audio-video.
An example given in Success Criterion 1.2.1 is of a text-based tutorial that is backed up by a video that essentially provides a visual alternative to the textual content on the page. In this case, as long as the text alternative for this video points to the main text in its description, it does not require its own alternative.
Note that "[Standard]" will be replaced with "WCAG2A", "WCAG2AA" or "WCAG2AAA", depending on the level being tested.
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If this embedded object contains pre-recorded audio only, and is not provided as an alternative for text content, check that an alternative text version is available.
Elements which may contain audio.
If this embedded object contains pre-recorded video only, and is not provided as an alternative for text content, check that an alternative text version is available, or an audio track is provided that presents equivalent information.
Elements which may contain video.