TTY Basic

Norman Williams and Judy Harkins
RERC on Universal Telecommunications Access
Gallaudet University
6/30/98 

Brief History of TTY

• Method of coupling TeleTypewriters to the PSTN invented in 1964 by deaf physicist Robert Weitbrecht
• AT&T donated many old TTYs to make the network grow
• In the mid-1970s portable units entered the market
• New models developed throughout 1980s and 1990s
• TTY modems introduced in 1980s
• TTYs for pay phones developed in late 1980s

Which Term to Use?

• TTY refers to the TeleTypewriter, used for telegram and military communications.
• When a smaller unit came out, it was decided to label this class of equipment as Telecommunications Device for the Deaf, or TDD.
• When FCC issued a rule on telecommunications relay service, it adopted the European term text telephone but this term fell into disuse because, when signed, it looks like the sign for "bathroom."
• Deaf and hard of hearing people today use "TTY."

Telecommunications Relay Service

• Telecommunications Relay Service bridges the communication gap between voice telephone users and people who have disabilities of hearing or speech.
• Mandated by the Americans with Disabilities Act, Title IV, for 24/7 service nationwide.
• Funded by LECs and IECs, provided by vendors. Most TRS contracts are with IECs.
• Use of service is free.
• Supports Voice Carry Over and Hearing Carry Over.
• Supports Baudot, ASCII, and in some states Turbocode.
• About 95% of calls through relay are in Baudot.

Baudot is the default code for U.S. TTY

• Baudot is carrierless
• Both ends share one channel which requires agreement to take turns like CB radio
• Uses only two frequencies: 1400 and 1800 hertz
• Many regular modems use two channels which require more than two frequencies
• Does not require handshaking
• Uses five-bit code (not ASCII).
• Has two set of characters: LETTERS & FIGURES
• Supports about 60 characters including uni-cased A-Z, 0-9, and selected other characters.
• No error correction
• 45 Baud vs. 50 Baud
• USA uses 45 baud, England and South Africa use 50
• Same frequencies and coding

Functional characteristics

• TTY should be viewed more as a phone than as a data modem.
• Does not automatically disconnect (except when an automatic answering machine finishes taking the call).
• Calls can be transferred without being dropped. Regular modems cannot.
• Can alternate between voice and TTY.
• Extremely easy to use; no set-up or software required for basic TTY.
• Instantaneous connection.
• Usually can be shared with an extension phone for 3^rd TTY to join the conversation.
• Users cannot type at same time or interrupt each other; user signals end of turn by typing GA for "go ahead."
• Because of two set of characters, shifting errors can result in several bad characters. Usually it takes about 8 characters before user corrects incoming characters by pressing the space bar.
• TTY calls tend to be silent calls when answered. Some users will tap the space bar on outgoing calls when they detect a voice answer.

Call Progress Information on TTY

• Simple sound light is more important than call progress result codes.
• Can be achieved by simply putting an LED light on a speaker, or implemented as a graphic, even while on-line.
• Deaf users can interpret what is happening on the call by looking at light pattern.
• Example of patterns deaf people can determine using a simple light: busy, ringing, voice answer, answering machine, data modem.
• If a digital level is used, be sure to have a good threshold level.

Connection and DTMF

• Uses two modes of connection: Acoustic coupling and direct connection, audio in/out not supported in current models.
• Full DTMF set cannot be generated by most older models.
• Many TTYs and TTY modems cannot generate DTMF after call is connected, a problem for using TTY-adapted interactive voice response products.

Other Protocols

• Turbo Code Brief

• Patented carrierless,7 bit, 110 baud coding on single channel by Ultratec
• Provides interrupt command/protocol
• Transmits as fast as a person can type
• Beginning to be implemented in TRS centers
• All new Ultratec TTY models have it, other brands do not

• Bell 103 Brief

• Implemented in many domestic TTYs
• 300 bps
• Uses two channels and four frequencies
• Similar to international standard of V.21 but has different frequencies (Old Bell system standard)
• Is not popular with users because of setup difficulty especially on acoustic coupling where the system does not know which channel it is using: ANSWER or ORIGINATE
• Once the connection is made, it is preferred by deaf users over Baudot
• If Bell 103 is to be supported in your design, answering tone should be between 3 to 5 seconds before going to TTY mode

• V.18 Brief

• Standard was developed with support from deaf organizations, but deaf users have not had an opportunity to evaluate outcome in user environment

TTY modem brief

• Currently, regular modems do not support TTY calls
• There are several TTY modems on the market:
• Nxi NexCom 300 / 370 TTY -- 19.2k Hayes-compatible internal modem
• Phone-TTY -- CM-4 Dumb TTY/110/300 baud external modem
• Microflip Mic300i-- internal TTY/110/300 baud modem
• Ultratec Intele-modem -- TTY/110/300 baud Hayes compatible external modem

When implementing TTY in products:

• Use 1800 hertz and 1400 hertz frequencies
• Use Baudot character table (included)
• Use 5 bits, 1.0 start, 1.0 - 2.0 stop bits
• Sending tolerance should be set at 1% while receiving at 5%
• When user presses space bar on incoming characters, shift it to LETTERS immediately
• Be sure to allow user to transmit at anytime even before detecting TTY, for alerting hearing people this is a TTY call, and to fix shifting when space baring on incoming characters
• Keyboard must be comfortable to type for long conversations
• Incorporate simple line power indicator for sound light
• Support easy touch tone dialing while in conversation
• Interpret return key as a space character on one line display

Options found in higher end TTYs

• Answering machine
• Remote retrieval
• Message notification via paging
• Bell 103
• Printer

Other advice

• Get deaf consumer involved for beta testing (try NAD, TDI, and local deaf communities)
• Be sure to test with relay services
• For V.18 and Baudot specifications including Baudot table, go to   http://tap.gallaudet.edu/Text/v18.html

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