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Telecommunications devices for the deaf

A Telecommunications Device for the Deaf (TDD) is an electronic device used for telephone communications by deaf persons and those with other hearing difficulties.

The typical TDD is a device about the size of a small laptop computer with a QWERTY keyboard and small screen that uses LEDss to display typed text electronically. The TDD allows for the transmission of input text via the telephone. Direct communication can only be between telephones both fitted with the system. In certain countries there are telecommunications relay services, so that a deaf person can communicate with a person lacking a textphone using a human translator, voice carry over.

Original TDD devices were called TTYs, dervied from teletype, and was invented by deaf-physicist Robert Weitbrecht in 1964. Communications was through frequency key shifting and single tone, allowing only one-way (simplex) communication at once. During the mid-1970s, portable TTYs were developed and was also the time period when the term "TDD" began being used, largely by those outside the deaf community. The deaf community, interestingly, does not usually use the term "TDD", but instead prefer "TTY".

The original standard used by TDDs is the Baudot code implemented asynchronously at either 45.5 or 50 baud, 1 start bit, 5 data bits, and 1.5 stop bits. The TDD/TTY standard is generally incompatible with standard Hayes-compatible modems. In 1994 the ITU approved the V.18 standard, an ASCII full- or half-duplex modulation method which has been widely adopted. Computers can, with appropriate software and modem, emulate a V.18 TDD.

In addition to TDD there are a number of pieces of additional equipment that can be coupled to telephones to improve their utility. For those with hearing difficulties the telephone ring and conversation sound level can be amplified or pitch adjusted, ambient noise can also be filtered. The amplifier can be a simple addition or through an inductive coupler to interact with suitable hearing aids. The ring can also be supplimented with extension bells or a visual call indicator.

The use of voice recognition systems is in limited use due to technical difficulties. Newer text based communication methods, such SMS and IRC, have also been adopted.