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TDD (Telecommunications Device for the Deaf)

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A telecommunications device for the deaf (TDD) or teletypewriter (TTY) enables people who are deaf, hard of hearing, or speech-disabled to utilize a telephone. The TDD is about the size of a laptop computer with a standard QWERTY keyboard and LED or LCD screen to display electronic text. In addition, TDDs often have a small spool of paper on which the text is printed to record the conversation.

The term TTY derives from "Teletype", which is a registered trademark of the Teletype Corporation. TDD, a later term, stands for "Telecommunications Device for the Deaf". Many people use the two terms interchangeably, while others use TTY for mechanical teleprinters and TDD for the modern electronic gadgets which perform the same function in a fraction of the size and weight. More recently, the Federal Communications Commission has come up with yet another term for the same thing. In one of their recent orders, they call it a "text telephone". Other names for TDD include textphone (Common in Europe and the UK), and minicom (United Kingdom).

TDD with modem and LCD screen. The acoustic coupler on the top is for use with telephone handsets.
TDD with modem and LCD screen. The acoustic coupler on the top is for use with telephone handsets.

This technology has been available since 1964. Both personal and some public phones can be equipped with a TDD. TDD and TTY’s can be used to communicate between two deaf individuals as long as they both use a compatible device with a similar communication protocol. The devices can also allow an individual who is deaf to communicate to a hearing person through a human relay operator (see links below for services).



The TTY was first developed by Robert Weitbrecht, a deaf physicist, with James C. Marsters, a deaf dentist and private airplane pilot, and Andrew Saks, an electrical engineer and grandson of the founder of the Saks Fifth Avenue department store chain. The concept was based on the teleprinters that ham radio operators used to communicate over the air.

In 1964, Weitbrecht received a patent for the acoustic coupler, or modem. That year, Weitbrecht, Marsters, and Saks founded APCOM (Applied Communications Corp.), located in the San Francisco Bay area, to develop a product named the PhoneType. [1][2][3] A unique set of tones representing different TTY keys were transmitted and received across phone lines, through a standard Bell Telephone handset, then through an acoustic coupler, to a teletype machine. The entire configuration of teletype machine, acoustic coupler, and telephone set became known as the TTY.

The first electronic portable TTD, the MCM (Manual Communications Module), was released in 1973. 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.


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 International Telecommunication Union (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. Same can be done with such software using a computer's soundcard, when coupled to the telephone line.

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.

Related Technology

Text / voice combinations

While TDDs typically involve the sending and receiving of text, some individuals prefer to use a combination of text and voice. People who can hear but cannot speak might use a "hearing carry-over (HCO)" device in which they hear the conversation and type responses. People who can speak but cannot hear might use a "voice carry-over (VCO)" device in which they view the typed conversation of the caller and respond by speaking. This latter approach is sometimes preferred by people who developed a hearing loss later in life.

Instant messaging / texting

TDDs, a specialized technology, are being supplanted by the use of instant messaging and texting, which provide a means for people to communicate through text, in real time, but using mainstream technology.

Use with messaging systems

TDD devices can be used as stand-alone phones or can also be addressed using automatic phone messaging systems such as Voice broadcasting technology. Both voice and text messages can be broadcast during an emergency or community alert scenario [1].

In-coming call alerts

A TDD user can be alerted to an oncoming call through audio, visual, or tactile means. For individuals who are hard of hearing, the telephone ring and conversation sound level can be amplified or the pitch adjusted to suit the needs of the user. A visual call indicator (e.g., flashing light) can alert the user to an in-coming call. Finally, a vibration feature built into a portable phone or an external vibrating pad (e.g., bed shaker) can provide a tactile alert.


One of the most common uses for a TDD is to place calls to a Telecommunications Relay Service (TRS), 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.

TDD & TTY Etiquette

TDD communication is one-way, and thus, requires callers to take turns. Since it is difficult to determine when a person is finished speaking the term "Go ahead"/"GA" is used. Some common abbreviations used are as follows. More can be located at

Commonly used abbreviations:

BRB Be Right Back
CA Communications assistant (another term for a relay operator)
CU See You (be seeing you)
GA Go Ahead
SK Stop Keying (used to say farewells)
SKSK Now hanging up (indicates immediate call hang-up)
HD Hold (transmitted before a voice intercept message)
Q, QQ, QM Question Mark (?)
PLS Please
RO Relay Operator
OIC Oh, I See
OPR Operator
NBR Number
TMW Tomorrow
THX Thanks
WRU Who are You? (or Where are You?)
XXXX Xs are often used to indicate a typing error instead of backspacing

The following is a sample conversation between two deaf individuals. Note that TDDs use only capitals.

Caller B: WILL DO SK
Caller A: BYE SKSK



Products and Relay Services

  • IVR Interactive Voice Response

External Links