Update understanding/20/audio-description-prerecorded.html

Supported by TF, after discussion

understanding/20/audio-description-prerecorded.html

@@ -24,7 +24,8 @@ Intent of Audio Description (Prerecorded)
 
       
          This success criterion applies to all videos that contain "important visual details that cannot be understood from the main soundtrack alone". If the original audio
-            have sufficient pauses, and where no portion of the original audio can be omitted in
+            offers no pauses in the dialog where audio descriptions can be provided, then the content fails this success criterion -- no audio descriptions have been provided. However, the information can still be made available to users through other time-based media requirements at A and AAA levels.
+            Where all pauses in the dialogue contain audio descriptions, yet additional important visual information has not been described, the video can provide that additional content through either extended audio descriptions or a combination of A and AA. As noted in the [Time-based media guideline](https://www.w3.org/WAI/WCAG22/Understanding/time-based-media), "Sometimes, there is so much dialogue that audio description cannot fit into existing pauses in the dialogue. The option at Level A to provide an alternative for time-based media instead of audio description for synchronized media would allow access to all of the information in the synchronized media. This option also allows access to the visual information in non-visual form when audio description is not provided for some other reason."
             favour of an audio description without losing important information. In these cases,
             the content will fail this success criterion.