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", 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).

Another scenario sees the use of TDDs being replaced by the

use of personal computers and talk protocols, presumably

over the Internet.

(1998-06-15)