Newsgroups: bit.listserv.deaf-l
Date:         Thu, 12 Nov 1992 10:07:30 EST
From:         "Dave Windley, Data Comm Specialist X799" <AADW@CATCC.BITNET>
Subject:      PC as TDD?

>
Greetings Bill:

> I have a TDD device which I used to communicate over the phone line with some

Just FYI: TDD is an acronym used by Hearing persons to represent
Telecommunications Device for the Deaf.  Saying "TDD device" is
redundant.  This acronym is not typically used by Deaf persons.  They
prefer to keep the original Western Union term "TTY".  TDD, like
"hearing-impaired", "speech-impaired", "visually-impaired", and
"crippled" are terms that are approved terminology by our government,
but were never discussed with the community they are supposed to
represent.  For this reason, Deaf people typically do not use them.

> As I understand things, the TDD service for the deaf runs at some non-standard
> (per today's modern computer equipment), settings.  I think it is something
> like 75 baudot, which defines its binary character set and transmission speed.

There is no "TDD service" that I know of.  The machine that you have,
called a TTY (Tele TYpewriter) by Deaf people, transmits and receives at
45.5 BAUD, which is a measure of character transmission.  The term
baudot is not associated with any speed per se.  BAUDOT is not a speed,
but a language for character transmission same as ASCII.  In fact,
BAUDOT is the predecessor of ASCII.


> (Suppossedly this is still used because the TDD service was originally set up
> by the phone company using old mechanical teletype devices, and newer TDD unit
> must adhere to the standard for compatibility.)
>

This very slow speed of 45.5 BAUD with the ancient language BAUDOT is
still maintained to guarantee downward compatability.  The phone
company has nothing to do with it.  A deaf man named Wechbreight(sp?)
is credited with developing the first acoustic coupler(modem) for deaf
use.  This coupler easily connected to the Army's old Western Union TTY
machines.  These early machines were basically motorized typewriters
with a set of five contacts to translate characters into groups of five
on and off pulses.  This five bit coding is known as BAUDOT.  It has no
error checking (parity) and no start or stop bits typical of the modern
coding schemes.

> Now for my question:
>
> Is there a product available (or plans for a kit), that would allow me to
> use my PC as a TDD device?
>

The only plans that I have heard of is for a Commodore 64 that allows
you to build a cable to connect to a port on the side of the computer.
The Commodore's port directly accesses the computer's memory differently
than other more current designs.  IBM and Apple products require the
BAUDOT 5 bit code to be translated to ASCII (8 bit) before communication
can occur.  There is no software available that can accomplish this
alone.  Hardware and software are required for this.

All major TTY manufacturers offer what are termed "dual-mode" modems
that can do that translation job for you.  They are very expensive
($300+) because of the limited market they apply to.  Their ASCII only
modes are a maximum of 1200 BAUD, which is a snail's pace compared to
the typical 2400-9600 BAUD computer modem user of today.

Most people find it better to buy a cheap $150 briefcase sized TTY
and keep their computer seperate for ASCII communication.

There is some hope on the horizon though.  Many manufacturers offer
ASCII modes on current TTY models.  This allows the devices to
communicate with computers and Telephone Relay Services at a more
reasonable speed of 300 BAUD.

> Besides the luxury of a nice keyboard and monitor, it would be convenient
> for storing messages or sending pre-written messages.  (With appropriate
> software.)  And a direct phone connection via my modem would be nicer.

The more expensive TTY models have direct connection via standard
modular (RJ-11/4) socket on the back of their cases.  This makes the
device much less prone to the interference typical of acoustic couplers.
>
> Any help would be greatly appreciated,
>

Hope this information clears up any misunderstanding you might have.

======================================================================
   Dave Windley,                       Catonsville Community College
      Data Communications Specialist          800 South Rolling Road
      Sign Interpreter                     CATONSVILLE MD 21228-9987
      ALL Inc. Co-Founder                      Voice: (410) 455-4799
      ITR Club Vice President                             AADW@CATCC
======================================================================
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