Teletypewriter (TTY) & Telecommunications Device for the Deaf (TDD)

Teletypewriter (TTY)

Teletypewriter or Teletype, a typewriter with an electronic communication channel.

A teleprinter (teletypewriter, teletype or TTY for TeleTYpe/TeleTYpewriter) is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point through a simple electrical communications channel, often just a pair of wires.

The most modern form of these devices are fully electronic and use a screen instead of a printer. These teletypewriters are still in use by the deaf for typed communications over the telephone, usually called a TDD (Telecommunications Devices for the Deaf) or TTY (although TTY, as indicated in the previous paragraph, refers to teleprinters in general).

Telecommunications Device for the Deaf (TDD)

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, although TTY is also a term used for teletypes in general), textphone (Common in Europe), and minicom (United Kingdom).

The typical TDD is a device about the size of a small laptop computer with a QWERTY keyboard and small screen that uses LEDs or an LCD screen to display typed text electronically. In addition, TDDs commonly have a small spool of paper on which text is also printed. The text is transmitted live, via a telephone line, to a compatible device, i.e. one that uses a similar communication protocol. In many countries, including the United States 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.

Devices

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.

Relay

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.

Source: http://en.wikipedia.org/wiki/Teleprinter