Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!usc!zaphod.mps.ohio-state.edu!casbah.acns.nwu.edu!accuvax.nwu.edu!nucsrl!telecom-request
From: hayes!tnixon@uunet.uu.net (Toby Nixon)
Newsgroups: comp.dcom.telecom
Subject: Re: How Does One Access a Hearing Impaired TTY
Message-ID: <15419@accuvax.nwu.edu>
Date: 14 Dec 90 10:29:56 GMT
Sender: news@accuvax.nwu.edu
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 54
Approved: Telecom@eecs.nwu.edu
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 10, Issue 882, Message 5 of 9


In article <15335@accuvax.nwu.edu>, molloy@emerald.uucp (Phil Molloy)
writes:

> A friend of mine would like to be able to contact her brother who has
> a TTY connected to his phone line.  She has access to various modems
> and computers to dial her bother's phone, but doesn't know if it is
> possible to communicate with this piece of equipment.  I don't know
> what kind of modulation/demodulation scheme is used by the TTY.  Does
> anyone out there know if what she wants to do is possible or should
> she simply by another TTY and do it that way?  

Telecommunications Devices for the Deaf (TDD) are sometimes called
"TTY" because the earliest ones were indeed based on Teletype
hardware; the newer ones are sleek, compact, battery-powered, with LED
or LCD displays and quite portable.

TDDs use a special, unique half-duplex modulation scheme at 45.45
baud, which is unlike any that are used in commonly-available modems.
The carrier is turned on and off for each character as it is typed,
rather than being continuously on all the time, and both TDDs on the
line use the same frequencies. Therefore, you can't use a standard
modem to communicate with a TDD.

Also, TDDs use five-bit Baudot code rather than 7- or 8-bit ASCII
code, so most standard comm software won't work, either.

There _are_ TDDs in existence which use Bell 103 modulation and ASCII
code, but they aren't very common.  And, you can also find special
modems (from companies such as Krown and Ultratec) that support both
Bell 103 and TDD modulation schemes and also automatically translate
between ASCII and Baudot, but these cost as much as many 2400bps
modems.

Telecommunications Industry Association committee TR-30.1 has a
project open to define a standard method for automatic interworking
between data modems and TDDs.  Once this standard is developed, we may
see many more modem companies begin to include TDD modulation in
standard PC-class modem products.

Your friend will need to either buy a TDD (they're not all that
expensive; $300-$500), or buy her brother a PC and a modem (which
would be a nice Christmas gift, and open up to him the whole world of
BBSing).


Toby Nixon, Principal Engineer    | Voice   +1-404-449-8791  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
P.O. Box 105203                   | UUCP uunet!hayes!tnixon  AT&T    !tnixon
Atlanta, Georgia  30348  USA      | Internet       hayes!tnixon@uunet.uu.net


[Moderator's Note: For further information, readers are referred to
the file on this subject in the Telecom Archives mentioned earlier.  PAT]