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Lexikon

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", although there is some disagreement on this. The deaf themselves do not usually use the term "TDD", but prefer simply "ef="module.php?name=Lexikon&file=search&eid=1&query=TTY">TTY" -- possibly the original term. The ambiguity between this and the other meanings of "ef="module.php?name=Lexikon&file=search&eid=1&query=TTY">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 ef="module.php?name=Lexikon&file=search&eid=1&query=Baudot code">Baudot code implemented ef="module.php?name=Lexikon&file=search&eid=1&query=asynchronous">asynchronously at 45.5 or 50 ef="module.php?name=Lexikon&file=search&eid=1&query=baud">baud, 1 ef="module.php?name=Lexikon&file=search&eid=1&query=start bit">start bit, 5 data bits, and 1.5 ef="module.php?name=Lexikon&file=search&eid=1&query=stop bits">stop bits. This is generally incompatible with standard ef="module.php?name=Lexikon&file=search&eid=1&query=modems">modems. [Standards docs? ef="module.php?name=Lexikon&file=search&eid=1&query=i18n">i18n issues?] A typical TDD is a device about the size of a small {laptop computer} (resembling, in fact, a circa 1983 Radio Shack ef="module.php?name=Lexikon&file=search&eid=1&query=Model 100">Model 100 computer) with a ef="module.php?name=Lexikon&file=search&eid=1&query=QWeRTY keyboard">QWeRTY keyboard, and small screen (often one line high, often made of an array of ef="module.php?name=Lexikon&file=search&eid=1&query=LeD">LeDs). There is often a small printer for making transcripts of terminal sessions. Because of the ef="module.php?name=Lexikon&file=search&eid=1&query=Stone Age">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 ef="module.php?name=Lexikon&file=search&eid=1&query=lock-in">lock-in with a high cruft factor). Another scenario sees the use of TDDs being replaced by the use of ef="module.php?name=Lexikon&file=search&eid=1&query=personal computers">personal computers and ef="module.php?name=Lexikon&file=search&eid=1&query=talk">talk protocols, presumably over the ef="module.php?name=Lexikon&file=search&eid=1&query=Internet">Internet. (1998-06-15)

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