Provide accessible alternatives to significant audio and video.
On this page:
Audio content is not accessible to someone who is deaf or hard-of-hearing. Video content is not accessible to someone who is blind or visually impaired. Important audio and video content must be provided in an alternative format that is accessible. For multimedia content containing both audio and video elements, section 508 requires that equivalent alternatives be synchronized with the presentation.
Someone who is blind or visually impaired must have access to the visual portion of multimedia content. Multimedia presentations must provide an audio description or a full text alternative so the content is accessible to someone who is blind. The audio description compliments the existing audio track with information that someone who is blind or visually impaired would otherwise be unable to access. With an audio description, a narrator describes important visual details during pauses in the existing audio track. The requirement for accessible multimedia presentations can also be met by providing a full text alternative. The alternative describes everything that is happening in the multimedia presentation. The text alternative must include a transcript of all dialogue in addition to the description of the video.
Someone who is deaf or hard of hearing must have access to the audio portion of multimedia content. If the content is audio only, a transcript is an acceptable alternative. Transcripts for audio content do not have to be synchronized because it is not a multimedia presentation. If the content includes audio and video, a transcript is not sufficient. Synchronized captions must be provided for multimedia content. Captions identify speakers, dialogue, music, and sound effects. There are two types of captions. Open captions are "burned" into the image, similar to subtitles, and cannot be turned off. Closed captions are a separate data stream that is synchronized with the multimedia. The user can turn closed captions on or off.
Required development techniques
The following techniques are the minimum required to meet Checkpoint 3.2 from the IBM Software Accessibility Checklist:
- For audio-only content, providing a text transcript of the audio.
- For video-only content, providing a text or audio description of the video.
- For multimedia content, providing synchronized captions (closed captions or open captions).
- In addition to providing synchronized captions, one of the following techniques is required for multimedia content:
a) Providing a sound track that includes audio description as the primary sound track.
b) Providing a sound track that includes audio description AND associating it with the multimedia content.
c) Providing an audio description in its own sound track AND merging the description track with the original soundtrack of the multimedia content at runtime.
d) Providing a full text alternative including any interaction.
Examples of required techniques
1. For audio-only content, providing a text transcript of the audio.
A transcript is a word-for-word textual version of the audio. The user should be given the choice of reading the transcript or listening to the audio, or both. In addition to spoken dialog, the transcript should include a description of any background sounds or sound effects that are important to the understanding of the audio-only content. Because there is no video, captions are not required.
Audio podcasts must a transcript for every audio podcast.
2. For video-only content, providing a text or audio description of the video.
The transcript is required so that visual information in the video is also available to someone who is blind.
The WebAIM site has an example of an audio transcript that describes the Universal Studios logo so a blind user can understand what is happening on the screen.
3. For multimedia content, providing synchronized captions (closed captions or open captions).
Captions provide a text alternative for audio content so it is accessible to someone who is deaf or hard of hearing. Captions must be synchronized so the user can read the captions while watching the video content. Captions must be provided for multimedia content including:
Tools like Media Access Generator (MAGpie) from WGBH can be used to add captions. MAGpie creates captions and audio descriptions for rich media in multiple formats including Apple's QuickTime, Synchronized Multimedia Integration Language (SMIL) and Microsoft's Synchronized Accessible Media Interchange (SAMI) format. MAGpie can be downloaded for free from the MAGpie page on the WGBH National Center for Accessible Media (NCAM) website.
The 2006 Annual Meeting of Stockholders Webcast is another example of open captions. The Webcast is posted with captions turned on by default. A separate download is available of the Webcast with captions turned off. The Webcast does not require an audio description since the video only shows Sam Palmisano speaking. There is no interaction with other people and no additional information provided in the video portion of the Webcast.
An online training class has PowerPoint slides which are displayed while you hear the speaker discussing the information on the slides. The transcript for the audio of each slide is displayed on the left side of the screen. In this case, captions are recommended but not required because there is no additional video beyond the slide. Of course, the slides must follow the techniques for accessible PowerPoint and the controls must be keyboard accessible.
4. For multimedia content, providing a sound track that includes an audio description as the primary sound track.
The audio description enables someone who is blind to understand the visual content in the multimedia presentation. It should include things like actions, characters or any information on the screen visually that is not already described in the dialog. The audio descriptions are inserted in pauses in the dialog. The sound track with the descriptions can be provided as an alternate sound track or it can be integrated into the main sound track that all users hear.
5. For multimedia content, providing a sound track that includes audio description AND associating it with the multimedia content.
The National Center for Accessible Multimedia (NCAM), which is part of the Media Access Group at WGBH, maintains a small multimedia clip that shows the opening to the movie "The Lion King". The clip contains audio descriptions (as well as captions). Visit the NCAM site to play the Lion King video clip, and to hear the audio descriptions added during pauses in the dialog.
6. For multimedia content, providing an audio description in its own sound track AND merging the description track with the original soundtrack of the multimedia content at runtime.
SMIL can be used to merge the description track with the original sound track. The National Center for Accessible Multimedia provides a set of SMIL templates for incorporating accessibility into rich media. For more examples of this technique see WCAG 2.0 examples for providing audio descriptions in SMIL 1.0 or WCAG 2.0 examples for providing audio descriptions in SMIL 2.0.
7. For multimedia content, providing a full text alternative including any interaction.
A full text alternative will allow a person to experience all of the video information, audio information, and interaction that the multimedia presents, in an alternative format. An example of this technique (using the Lion King example above) would be to provide a text alternative similar to a screenplay, which is modified to exactly match what happens on the screen. The alternative would include everything that would be found in a transcript, along with the text from the audio description, sounds, scene changes, character expressions, and additional descriptive text if necessary.
Required test techniques
Test the software to ensure that it complies with accessibility requirements.
A manual test must be conducted to verify the text description of visual elements and non-verbal audio elements that are important to understanding the video or audio components of an application. Verify that a screen reader can announce the text equivalents of the visual elements as well as verify that the transcript for multimedia elements is synchronized with the audio or video track.
All text descriptions of video and audio elements in the application must meet the pass criteria described in the Test techniques table below for this checkpoint to be satisfied.
Required test software
Install the following software as needed to test this checkpoint:
The following test techniques are required to verify this checkpoint. Perform the following steps to verify the accessibility of the text equivalents for video and audio elements.
For audio-only elements:
(For text equivalent transcripts of the audio element, see step 5 below for the required screen reader test.)
For video-only elements:
(For text equivalent descriptions of video content, see step 5 below for the required screen reader test.)
For multimedia content (containing both audio and video):
For multimedia content (containing both audio and video):
Verify that an audio or text description of the visual elements of the video (the “action”) is provided using at least one of the following methods: (For text equivalent descriptions of multimedia content, see step 5 below for the required screen reader test.)
For all text equivalent transcripts provided for audio-only, and text equivalent descriptions of video-only and multimedia elements:
©2009 IBM Corporation
Last updated October 1, 2009.