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Telecommunications device for the deaf - Wikipedia, the free encyclopedia
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Telecommunications device for the deaf

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A telecommunications device for the deaf (TDD) is an electronic device for text communication via a telephone line, used when one or more of the parties has hearing or speech difficulties. Other names for TDD include TTY (telephone typewriter or teletypewriter), textphone (Common in Europe), and minicom (United Kingdom).

Miniprint 425 TDD. The coupler on the top is for use with telephone handsets. The printer records the conversation. The specific GA & SK keys allow for speedier use of common  abbreviations.
Miniprint 425 TDD. The coupler on the top is for use with telephone handsets. The printer records the conversation. The specific GA & SK keys allow for speedier use of common abbreviations.



The typical TDD is a device about the size of a small laptop computer with a QWERTY keyboard and small screen that uses light-emitting diodes or an LCD screen to display typed text electronically. In addition, [1]TDDs commonly have a small spool of paper on which text is also printed — old versions of the device had only a printer and no screen. The text is transmitted live, via a telephone line, to a compatible device, i.e. one that uses a similar communication protocol. In certain countries there are Telecommunications Relay Services, so that a deaf person can communicate with a hearing person on an ordinary voice phone using a human relay operator. There are also "carry-over" services, enabling people who can hear but cannot speak ("hearing carry-over"), or people who cannot hear but are able to speak ("voice carry-over") to use the telephone.


APCOM (Applied Communications) located in the San Francisco Bay area developed the acoustic coupler, or modem. Couplers were cabled to TTYs enabling the Bell Telephone company standard "500 handset" to couple, or fit, into the rubber cups on the coupler, thus transmitting and receiving a unique set of tones generated by the different corresponding TTY keys. The entire configuration of teletype machine, acoustic coupler, and telephone set became known as the TTY. The acoustic coupler modem was the invention of deaf physicist Robert Weitbrecht in 1964. The actual mechanism for TTY communications was accomplished electromechanically through frequency shift keying (FSK) allowing only one-way (simplex) communication. In 1973 the MCM (Manual Communications Module), which was the world's first electronic portable TDD (Telephone Device for the Deaf) allowing two-way telecommunications, premiered at the CAD (California Association of the Deaf) convention in Sacramento, California.


The battery-powered MCM was invented and designed by Michael Cannon in conjunction with physicist Art Ogawa and deaf interpreter Kit Patrick Corson. It was manufactured by Michael Cannon's company, Micon Industries, and initially marketed by Kit Corson's company, Silent Communications. In order to be compatible with the existing TTY network, the MCM was designed around the five-bit Baudot code established by the older TTY machines instead of the ASCII code used by computers. The MCM was an instant success with the deaf community despite the drawback of a $599 cost. Within six months there were more MCMs in use by the deaf and hearing impaired than TTY machines. After a year Micon took over the marketing of the MCM and subsequently concluded a deal with Pacific Bell (who coined the term "TDD") to purchase MCMs and rent them to deaf telephone subscribers for $30 per month. After Micon formed an alliance with APCOM, Michael Cannon, Paul Conover (Micon), and Andrea Saks (APCOM) successfully petitioned the California Public Utilities Commission (CPUC) resulting in a tariff that paid for TDD devices to be distributed free of cost to deaf persons. Micon produced over 1,000 MCMs per month resulting in approximately 50,000 MCMs being disseminated into the deaf community. Before he left Micon in 1980, Michael Cannon developed several computer compatible variations of the MCM and a portable, battery operated printing TDD, but they were never as popular as the original MCM. Newer model TDDs could communicate with selectable codes that allow communications at a higher bit rate on those models similarly equipped. However, the lack of true computer interface functionality spelled the demise of the original TTY and its clones. During the mid-1970s other so-called portable telephone devices were being cloned by other companies, and this was 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 prefers "TTY."


There are many different textphone standards. 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. Baudot is a common protocol in the US. In Europe, different states use different protocols. For example, V.21 is found in the UK and several Scandinavian countries. Other protocols used for text telephony are EDT, DTMF, V.23, etc.

The TDD/TTY protocols are generally incompatible with standard Hayes-compatible modems. In 1994 the ITU approved the V.18 standard. V.18 is a dual standard. It is both an umbrella protocol that allows recognition and interoperability of some of the most commonly used textphone protocols, as well as offering a native V.18 mode, which is an ASCII full- or half-duplex modulation method.

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.

In the UK, a virtual V.18 network, called TextDirect, exists as part of the Public Switched Telephone Network, thereby offering interoperability between textphones using different protocols. The platform also offers additional functionality like call progress and status information in text and automatic invocation of a relay service for speech-to-text calls.

In addition to regular Baudot, the UltraTec company implements another protocol known as Enhanced TTY, which it calls "Turbo Code," in its products. Turbo Code has some advantages over Baudot protocols, such as a higher data rate, full ASCII compliance, and full-duplex capability. However, Turbo Code is proprietary, and UltraTec only gives its specifications to parties who are willing to license it.


In addition to TDD, there are a number of pieces of additional equipment that can be coupled to telephones to improve their utility. For those with hearing difficulties the telephone ring and conversation sound level can be amplified or pitch adjusted, ambient noise can also be filtered. The amplifier can be a simple addition or through an inductive coupler to interact with suitable hearing aids. The ring can also be supplemented with extension bells or a visual call indicator.


There are some etiquette rules that users of TDDs must be aware of. Because of the inability to detect when a person has finished speaking, the term "Go Ahead" (GA) is used.

Commonly used abbreviations:

CA Communications assistant (another term for a relay operator)
GA Go Ahead
GA OR SK Goodbye
SK Stop Keying
SKSK Now hanging up
Q, QQ, QM Question Mark (?)
PLS Please
RO Relay Operator
OIC Oh, I See
OPR Operator
NBR Number
TMW Tomorrow
THX Thanks
XXXX X's are often used to indicate a typing error instead of backspacing

Sample Conversation

Caller B: WILL DO SK

SK is used to allow the users to say their farewells, while SKSK indicates an immediate call hang-up.

Note: TDDs only deal in capital letters, and is used above to authentically recreate the experience (and prepare potential users of the service for the "culture shock").


One of the most common uses for a TDD is to place calls to a Telecommunications Relay Service, which makes it possible for the deaf to successfully make phone calls to regular phone users.

The use of voice recognition systems is in limited use due to technical difficulties. However, a new development called the captioned telephone, now utilizes voice recognition to assist the human operators. Newer text based communication methods, such as short message service (SMS), Internet relay chat (IRC), and instant messaging have also been adopted by the deaf as an alternative or adjunct to TDD.

External links

  • Assistech A large selection of TTY devices, both printing and non-printing.