Telecom Devices for the Deaf

An electronic device used for telephone communications by people with hearing impairment is known as telecommunications device for the deaf (TDD). In Europe, it is commonly called textphone.

Typically, a TDD is the size of a small laptop computer with a QWERTY keyboard and a small screen with light-emitting diodes or an LCD screen to display typed text electronically. Input text can be transmitted via telephone and direct communication is possible only between compatible devices. In certain countries, telecommunications relay services allow a deaf person to communicate with another person on an ordinary voice phone using a human translator. Carry-over services facilitate the telephone communication of people with either hearing or speaking impairment.

APCOM (Applied Communications) developed the acoustic coupler, which, when connected to TTYs, enables a standard telephone to transmit and receive a unique set of tones generated by the different corresponding TTY keys. Originally, the mechanism for TTY communications was accomplished electromechanically through frequency shift keying (FSK), which allows only one-way (simplex) communication.

The ephemeral MCM (Manual Communications Module), invented by Kit Patrick Corson, called for a change, rather progress, from TTY. The MCM designers continued using the five-bit Baudot code of older TTY machines. Latest TDD models could communicate only with selectable codes that allow communications at a higher bit rate. However, the lack of true computer interface functionality put an end to original TTY and its replicas. During the mid-1970s the term “TDD” began being used largely by those outside the deaf community.

Besides TDD, there are many types of equipment that can be connected to telephones to improve their utility. Amplification and pitch adjustment of the telephone ring and conversation sound level could be useful for people with hearing problems. Ambient noise filtration can also be useful. The amplifier can simply plug to the phone and extension bells can be used to enhance ring.

TDDs widely used to place calls to a Telecommunications Relay Service, which enables the deaf to successfully make phone calls to people using traditional telephones.

Technical difficulties restrict the use of voice recognition systems. However, captioned telephone (CapTel), a new development, utilizes voice recognition to assist human operators. Nowadays text-based communications 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.