For users who have hearing impairments the audio output must be conveyed visually on the screen. Most desktops provide visual audio alerts and warnings. In console mode the system can also be configured to provide visual bells. There is a "Visual Bells mini-HOWTO" written by Alessandro Rubini that provides the configuration details available at: http://www.ibiblio.org/pub/Linux/docs/HOWTO/mini/.
Zapata is a computer-based, high-density telephony project. The current version is available for download as source code on at: http://www.zapatatelephony.org/project.html.
Closed captioning provides text translation of spoken words to video display. Closed captioning can be used for distance learning, video-teleconferencing, audio from a CD-ROM, and other types of interactive technology.
Ccdecoder is a closed captioned, extended data services decoder for the bttv and video4linux based tv video cards: http://sourceforge.net/projects/ccdecoder/.