More info.

Best regards,

Luciano

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Telecommunications devices for the deaf:

http://www.tutorgig.com/ed/Telecommu...e_for_the_Deaf

Original TDD devices were called TTYs, derived from teletype, and was
invented by deaf-physicist Robert Weitbrecht in 1964. Communications
was through frequency key shifting and single tone, allowing only
one-way (simplex) communication at once. During the mid- 1970s, portable
TTYs were developed and was also the time period when the term "TDD" began
being used, largely by those outside the deaf community. The deaf
community, interestingly, does not usually use the
term "TDD", but instead prefer "TTY".

The original standard used by TDDs is the Baudot code implemented
asynchronously at either 45.5 or 50 baud, 1 start bit, 5 data bits,
and 1.5 stop bits. The TDD/TTY standard is generally incompatible
with standard Hayes-compatible modems. In 1994 the ITU approved
the V.18 standard, an ASCII full- or half-duplex modulation method
which has been widely adopted. Computers can, with appropriate software
and modem, emulate a V.18 TDD. Some voice modems, coupled with
appropriate software, can now be converted to TDD modems by using a
software-based decoder for TDD tones.

* * *

TTY FAQ:

http://www.deafweb.org/tty_faq.htm

* * *

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The V.18 standard

The V.18 standard gives specifications for the
implementation of text telephones, more commonly
called TDDs (Telecommunications Devices for the Deaf).


http://www.dinf.ne.jp/doc/english/Us...96/page169.htm

http://tap.gallaudet.edu/v18.htm


ITU Recommendation V.18 specifies interworking with
the following TTY protocols:

Baudot @ 45.45 baud (U.S. TTYs).

Baudot @ 50 baud (used in England, Australia, and some other
countries; also known as "international" Baudot).

V.21/text telephone version (used in Sweden, Norway, and Finland).

DTMF (used in Denmark, Holland, and some other countries).

EDT ("European Deaf Telephone," used in Germany, Austria,
Switzerland, and several other countries).

* * *

TWO DSP SOCKET MODEM with V.18 Standard

(Ask which TTY protocols are supported in their implementation of V.18).


MODULATIONS of the DSP SOCKET MODEM:

V.90 (server and client), V.34bis, V.34, V.33, V.32bis, V.32,
V.29, V.22bis, V.22, V22fast (HYPERCOM compatible), V.23,
V.21, V.18, Bell 212A, Bell 103


DSP SOCKET MODEM - SERIAL
http://www.telindusproducts.com/Tap/...ocketmodem.pdf

DSP SOCKET MODEM - PARALLEL
http://www.telindusproducts.com/Tap/...emparallel.pdf

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Radio Teletype (RTTY)

I am not an expert but RTTY and TTY/TDD are close.
Maybe you can use ideas from Amateur Radio RTTY circuits.
(If you use only the Baudot code at speed 45.45 for your project).

Both TDD and RTTY use five level Baudot/Murray code.
Baudot table:
http://burks.brighton.ac.uk/burks/foldoc/76/10.htm


The first difference I can see:

In TTY/TDD the “mark” tic is defined as 1400 Hz and
the “space” tic is defined as 1800 Hz.
See: "The Protocol Today"
http://www.ciscounitytools.com/HelpFiles/TTYAngel.htm


In RTTY the mark frequency is 2125Hz and the space is 2275Hz.
RTTY is commonly transmitted at speeds of 45.5 bits per second.
RTTY is either transmitted in Baudot or ASCII code.
http://www.roity.com/rc/rtty.html


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TDD devices:

Non-Printing TTYs:
http://www.phone-tty.com/PRODUCTS/OLD_PAGES/TTYs.htm

PocketComm the POCKET TTY:
http://www.krowntty.com/html/products/pocketcomm.html

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