Pure-digital services (as on satellite and digital cable TV) cannot transmit the Secondary Audio Program. Some satellite and digital-cable platforms offer multiple audio tracks using proprietary systems, and an original SAP channel can be mapped onto one of them. Nonetheless, in practice it is difficult for viewers at home to watch descriptions recorded on SAP via satellite or digital cable unless the satellite or cable operator takes relatively extraordinary measures.
In all broadcast systems in North America, blind and visually-impaired consumers face the accessibility obstacle of having to turn the description track on and off using visual menu systems they can't see.
Audio description (AD) on Digital Terrestrial Television in the United Kingdom is delivered on a separate track, making it possible to adjust the AD volume separately from that of the audio from the television programme. However most people receive audio description via digital satellite television or cable television, as you don't need to buy a special box and can just select audio description (or Narrative) via the menus on your set-top box. On satellite television and cable television the AD soundtrack is pre-mixed - the traditional way of experiencing description through TV.
DVDs very rarely include audio description using one of the eight audio tracks available on DVD-Video discs. However, even when an audio description track is available, the visual interface used to turn the description track on or off can be inaccessible to a blind person.
Researchers at the Centre for Learning Technology at Ryerson University in Toronto have been investigating various methods to overcome the limitations of audio description, including audible menus, extended description, first-person narrative and live description.
Recently the Center for Learning Technology has launched a new free web based video description service at (http://www.livedescribe.com). Describers can download LiveDescribe, a free video description authoring tool and create video description for video files such as avi, mov and mp4. Once the descriptions are complete, the description can be uploaded with one click to livedescribe.com. Blind or low vision audience members can then download the descriptions and play them back with the LiveDescribe Player (http://www.livedescribe.com/wiki/download.php). Unfortunately due to copyright restrictions both the describer and viewer of video description must have their own individual copy of the video file.