The WCAG guidelines make extensive use of the term time-based media. In practice, the requirements refer to video and audio content. There are requirements for audio only, video only and a combination of both.
Pre-recorded audio-only media must have a media alternative that provides equivalent information to the audio content. There is an exception: when the media is a media alternative for text and is clearly labelled as such.
For live broadcasts, the media alternative requirement is at the AAA level.
Pre-recorded video-only media must have either a media alternative or captions. There is an exception: when the media is a media alternative for text and is clearly labelled as such. At the AAA level, a media alternative is required.
There are no requirements for video-only media in live broadcasting.
There are several requirements for pre-recorded media containing audio and video material. Captions are always required unless the media is a media alternative for text and is clearly labelled as such. At level A, there is also a requirement for media alternative or audio description. At level AA, an audio description is required. If you noticed, the audio description meets both the A and AA level requirements. So a media alternative is not mandatory for AA-level compliance. However, I strongly recommend adding a media alternative as this will help many users.
Captions are a requirement for live media in the WCAG criteria, but there is an exception in Finnish law.
At the highest AAA level, the requirements also include a media alternative, extended audio description and sign language interpretation.
Captions are a transcription of dialogue that is primarily used to help viewers who can't hear audio. The captions contain also background noises, speaker differentiation, and other relevant content, not only the dialogue.
Captions come in two forms, open or closed captions. Open captions are "burned" into the video and cannot be turned off. The viewer can turn off closed captions and the user can visually change the closed captions to increase accessibility.
Closed captions are better for accessibility. While the WCAG criteria allow using open captions, I recommend always aiming for closed captions.
Subtitles are used to provide a translation for viewers who don't understand the language that is being spoken.
Subtitles assume viewers can hear the audio, so they don't contain background noises, speaker differentiation, or other relevant visual information.
An audio description is a verbal description of the things a sighted person will see. Audio description should provide information on visual content that is essential to the comprehension of the video. It should include things like character descriptions, actions, facial expressions, costumes and any other important visual elements.
Audio description is a separate audio track that runs alongside the main audio of the media content. The user should be able to switch between the audio tracks.
A commonly used example of an audio description is The Hunger Games with audio description: Katniss hunting on YouTube.
At the AAA level, the requirement is for extended descriptive coverage, the success criterion states:
Where pauses in foreground audio are insufficient to allow audio descriptions to convey the sense of the video, extended audio description is provided for all prerecorded video content in synchronized media.
The WCAG Guidelines describe extended captioning as follows:
Audio description that is added to an audiovisual presentation by pausing the video so that there is time to add additional description.
The Web Content Accessibility Guidelines define the alternative for time-based media as:
Document including correctly sequenced text descriptions of time-based visual and auditory information and providing a means for achieving the outcomes of any time-based interaction.
The media alternative must correspond exactly to the final synchronised media after editing. For example, an original script is not sufficient if it does not exactly match the final output.
A text can also have a media alternative. In this case, we are talking about media content that does not present any more information than the text already contains. The text can be presented directly or via text alternatives. The media alternative for text may be audio-only, video-only, or contain both.