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Computing (FOLDOC) dictionary
Telecommunications Device for the Deaf
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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", although there is
some disagreement on this. 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 implemented asynchronously at 45.5 or 50
baud, 1 start bit, 5 data bits, and 1.5 stop bits. This
is generally incompatible with standard modems.
[Standards docs? i18n issues?]
A typical TDD is a device about the size of a small laptopcomputer (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.
Because of the Stone Age vintage of this technology (the
idiosyncrasy of which drives up the expense of individual
units), it is thought that TDD standards should transition to
use of standard modem line settings (e.g., ASCII 2400-8-N-1).
An obstacle to this is the millions of Baudot-only terminals
in use (an example of lock-in with a high cruft factor).