Telecommunications devices for the deaf

A telecommunications device for the deaf (TDD) is an electronic device used for telephone communications by deaf persons and those with other hearing difficulties. In Europe, the term textphone is more commonly used.

The typical TDD or textphone 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. The TDD allows for the transmission of input text via the telephone. Direct communication can only be between compatible devices, i.e. those that use a similar communication protocol. In certain countries there are telecommunications relay services, so that a deaf person can communicate with a person on an ordinary voice phone using a human translator. 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.

Original TDD devices were called TTYs, derived from teletype, and was invented by deaf-physicist Robert Weitbrecht in 1964. Communications was through frequency key shifting and single tone, allowing only one-way (simplex) communication at once. During the mid-1970s, portable TTYs were developed and was also 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 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.

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 (CapTel), 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.de:Schreibtelefon nl:Teksttelefoon