Techniques for WCAG 2.0

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FLASH26: Applying audio descriptions to Flash video


This technique relates to:

User Agent and Assistive Technology Support Notes

See User Agent Support Notes for FLASH26.


The objective of this technique is to provide a way for people who are blind or otherwise have trouble seeing the video in audio-visual material to be able to access the material. With this technique a description of the video is provided via audio description that will fit into the gaps in the dialogue in the audio-visual material.


Example 1: Playing descriptions when cue points are reached

In this example, the FLVPlayback component is used to create a video player. A custom class called "AudioDescriptions" is added to manage the playback of extended audio descriptions. This class provides event listeners to listen for cue points in the media that have been identified by the audio description provider. When these cuepoints are reached, an mp3 file containing the corresponding description will start playing. The recorded descriptions have been timed to fit with in the gaps in the movie's dialog.

By default, audio descriptions will be enabled. A button (which must itself be accessible to meet other success criteria) is provided below the video player that allows the user to turn audio descriptions on or off.

Example Code:

package {
  import *;
  import *;
  import flash.display.Sprite;
  public class AudioDescriptions extends Sprite {
    private var channel: SoundChannel = new SoundChannel;
    private var myPlayer: FLVPlayback;
    private var _enabled: Boolean = true;
    private var _toggleBtn: Button;
    private var snd: Sound = new Sound();
    public function AudioDescriptions() {
      // point myPlayer to the FLVPlayback component instance on the stage, 
      // which should be loaded with a valid video source.
      myPlayer = my_FLVPlybk;
      // add cue points. When any of these are reached, the 
      // MetadataEvent.CUE_POINT event will fire
      myPlayer.addASCuePoint(8.35, "ASpt1");
      myPlayer.addASCuePoint(23.23, "ASpt2");
      enable_AD_btn.addEventListener(MouseEvent.CLICK, handleBtnClick);
    private function handleBtnClick(e) {
      _enabled = ! _enabled;
      if (! _enabled) {
        enable_AD_btn.label = "Enable Audio Descriptions";
      } else {
        enable_AD_btn.label = "Disable Audio Descriptions";
    public function enable() {
      // set up an event handler which will be called each time a cue point is reached
      myPlayer.addEventListener(MetadataEvent.CUE_POINT, cp_listener);
    public function disable() {
      // remove the event handler called each time a cue point is reached, so 
      // that audio description is disabled.
      myPlayer.removeEventListener(MetadataEvent.CUE_POINT, cp_listener);
    private function cp_listener(eventObject: MetadataEvent): void {
      snd = new Sound();
      //recreate sound object as it can only load one mp3 file
      //check to see which cue point was reached
      switch ( {
        case "ASpt1":
        snd.load(new URLRequest("sphere.mp3"));
        //create a new Sound object, and load the appropriate mp3
        channel =;
        // play the audio description, and assign it to the SoundChannel object
        case "ASpt2":
        snd.load(new URLRequest("transfrm.mp3"));
        channel =;

The result can be viewed in the working version of Playing descriptions when cue points are reached. The source of Playing descriptions when cue points are reached is available.

Example 2: Providing an additional audio track for descriptions

Audio description can also be provided via an additional audio track that is the same length and plays simultaneously as the primary media, but that only includes sound for the segments when audio description needs to be played and silence at other times. A Flash author can provide a toggle to turn this additional audio track on or off, based on the listener's preference. When the additional track is enabled, there are two parallel audio tracks, one being the primary audio, and the second being the one containing only audio description. It is still necessary to ensure that the audio description and primary audio do not overlap in ways that make comprehension difficult. This method will achieve the same result as the method used in Example 1, but may be chosen because of the type of audio description files that are provided to the Flash author.



When Flash content contains video with an audio soundtrack, confirm that:

  1. Audio descriptions have been made available using separate sound files.

  2. A button is provided that allows users to enable or disable the audio descriptions

Expected Results

If this is a sufficient technique for a success criterion, failing this test procedure does not necessarily mean that the success criterion has not been satisfied in some other way, only that this technique has not been successfully implemented and can not be used to claim conformance.

Techniques are Informative

Techniques are informative—that means they are not required. The basis for determining conformance to WCAG 2.0 is the success criteria from the WCAG 2.0 standard—not the techniques. For important information about techniques, please see the Understanding Techniques for WCAG Success Criteria section of Understanding WCAG 2.0.