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TTY Basic

Norman Williams and Judy Harkins
RERC on Universal Telecommunications Access
Gallaudet University

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 3rd 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
Was approved by ITU (CCITT). Goal is universal design by incorporation of TTY into digital products.  Supports TTY (45 and 50 Baud).  Supports DTMF, EDT, V.21, and V.23, used in Europe.  Unavailable in commercial form at the time of this presentation. 
  • 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

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