An audio description is a separate audio track that narrates or describes the visual content of a clip, show, or movie. You can listen to an audio description during a video to help you enjoy a program if you're visually impaired, or when you're unable to look at your screen.
Turn on audio descriptions by default
Not all videos contain audio descriptions. You can use these steps to automatically hear descriptions when they're available on your Mac or iOS device:
Create descriptive audio tracks
If you're creating a video in a professional video editing app like Final Cut Pro, you can add your own audio descriptions as a separate audio track. When creating audio tracks that contain descriptive video, apply a media characteristic tag to those tracks to indicate their purpose. If your media is tagged properly, Quicktime Player, iTunes, Safari, and other apps that play video can automatically select the appropriate audio track for the viewer.
In Final Cut Pro, you need to use the roles function to output the standard audio channel configuration for a descriptive audio track. Create and apply a role to any audio clips that you plan to use as part of the audio descriptive track. You can then export the audio as a separate channel in the QuickTime file when you share the finished project. You also tag that separate audio channel as an audio descriptive track like any audio channel in a QuickTime movie (see "For movie files" below). Learn more about applying roles and exporting with roles in Final Cut Pro.
For movie files
- Apply the media characteristic tag "public.accessibility.describes-video" to the descriptive video track.
- Apply a language tag to the descriptive video track.
For HLS Audio Streams
- Add the media characteristics tag "public.accessibility.describes-video” to the audio rendition containing the descriptive video stream.
- Set the AUTOSELECT attribute for the descriptive video stream to YES.
- Apply a language tag to the descriptive video stream.
Surround Sound audio content
If your content includes a 5.1 Surround audio track in a given language, that audio configuration should be preserved when a video description is added. The description track should be mixed into the Center channel of the 5.1 mix. The 5.1 mix should not be mixed down to stereo for these tracks.