Formerly also known as telecommunications device for the def (TDD) or teletypewriter (TTY)– a text telephone is a telecommunications device used by those who cannot understand on the phone. A typewriter-like unit shows the conversation on a screen so that it can be read. This devide is used by those people with hearing problems or speech dificulties. The text telephone looks like a laptopt. It has a QWERTY keyboard and a LCD screen on which the text is typed. It also has a printer. This is a modern version of a text telephone. The older version only had a printer, no screen. This device is connected to the house line, and when the person on the other side is speaking, it shows up as a text on the screen. The text is transmitted live.
The term TDD is sometimes discouraged because people who are deaf are increasingly using mainstream devices and technologies to carry out most of their communication. The devices described here were developed for use on the analog Public Switched Telephone Network [PSTN]. They do not work well on the new internet protocol (IP) networks. Thus as society increasingly moves toward IP based telecommunication, the telecommunication devices used by people who are deaf will not be TDDs. In the US for example, the use of the term TDD is thus discouraged to avoid confusion with these analog devices being referred to as TTYs