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From: KLH@nic.ddn.mil (Ken Harrenstien)
Newsgroups: comp.dcom.telecom
Subject: Re: TDD Specs
Message-ID: <8281@accuvax.nwu.edu>
Date: 26 May 90 01:06:31 GMT
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[Posting a response is easier than calling those phone numbers.]

I recall that the TDD manufacturers, under EIA auspices, were working
on a standard to pin down all of the various TDD specifications.  I
don't know, however, if the EIA actually has a final result or not.
My guess would be not, but it's certainly worth a try.  I don't have a
number handy but their office is in the other Washington (DC); try
asking directory assistance for the Electronic Industries Association.
Let us know if there's any news!

Basically the old-style TDDs use 5-bit Baudot (American Communications
set), 1.5 stop bit, 45.45 baud, with the Weitbrecht modem (U.S. Pat.
No. 3,507,997 if you must know).  The latter is half-duplex, 1400Hz
mark, 1800Hz space, plus a somewhat undefined 100ms "holding tone"
duration after last bit (otherwise the quiescent marking state would
maintain a continuous 1400Hz tone).

I don't know of any easily available published references.  They do
exist; we even wrote some of them as SRI project reports (you don't
think I memorize patent numbers, do you?), but they're hard to get.
That's why I suggest finding out if the EIA has come out with anything
yet.  Maybe Curtis Reid, another TDD-cognizant Telecom reader, knows
of other alternatives.  You can always send direct e-mail if you have
specific technical questions.


Ken