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| (communications) | Telecommunications Device for the Deaf - (TDD) A terminal device used widely by deaf
people for text communication over telephone lines.
The acronym TDD is sometimes expanded as "Telecommunication
Display Device" but is generally considered to be derived from
"Telecommunications Device for the Deaf". The deaf themselves
do not usually use the term "TDD", but prefer simply "TTY"
-- possibly the original term. The ambiguity between this and
the other meanings of "TTY" is generally not problematic.
The acronym "TTD" is also common
The standard most used by TDDs is reportedly a survivor of
Baudot code. It uses asynchronous transmission of 1400 Hz
and 1600 Hz tones at 45.5 or 50 baud, with one start bit,
5 data bits and 1.5 stop bits. This is generally
incompatible with standard modems.
A typical TDD is a device about the size of a small laptop computer (resembling, in fact, a circa 1983 Radio Shack
Model 100 computer) with a QWERTY keyboard, and small
screen (often one line high, often made of an array of
LEDs). There is often a small printer for making
transcripts of terminal sessions. An acoustic coupler
connects it to the telephone handset.
With the falling cost of personal computers and the
widespread use of Internet talk systems, there is now
little reason to use this Stone Age technology.
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