Telecommunications Device for the Deaf
<communications> (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
[Teletype for the deaf?].
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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