Audio descriptive videos are a national service that makes visual media accessible for people who are blind or visually impaired.
Audio description is a narrative track for the blind and visually impaired viewers of visual media. The descriptive narrator talks through the presentation, describing what is happening on the screen during the natural pauses in the audio and sometimes during dialogue if necessary.

On NTSC televisions, audio descriptions are usually transmitted on the secondary audio program (SAP)

• SAP must be turned on to hear the descriptions, which are pre-mixed with the original soundtrack
  
• The sound is monaural, by its nature