TTY Phones Use Modems
The primary purpose of TTY telephones---short for Tele-Typewriter, or just TeleType phone---is to allow hard of hearing persons to communicate via written text rather than spoken sound. To facilitate this task, TTY phones accept text input through a keyboard much like the one attached to a personal computer or electric typewriter. As the text is entered, it is sent to a display screen on the TTY phone; if the user approves the text, a processor in the device sends the text to a modem. Much like a modem in a personal computer, the TTY modem modulates the text into sound suitable for transmission across the telephone network. When the user makes a call using the TTY device, he can either initiate the call using a standard voice telephone and then place the phone handset into an acoustic coupler on the TTY, or simply initiate the call from the device itself (some TTY phones offer a built-in telephone connection and handset, altogether eliminating the need for an acoustic coupler). Once the call is in progress, the TTY modem sends the modulated data across the telephone connection to a remote machine.
TTY Phones Demodulate Data
When a TTY phone receives a call, either through a direct telephone connection or an acoustic coupler, it sends a tone to indicate that a machine has answered the call. The calling TTY phone responds with its own tone, and the two machines engage in a short protocol-setting session known as a "handshake." After the handshake is completed, the calling machine begins sending modulated data, which the called device demodulates into electronic text. This text is sent to a display screen on the TTY phone, where the user can read it and respond using an attached keyboard. The exchange of modulated data continues until one user terminates the call, at which time the modems simply disconnect from one another.
Some Users Rely on Relays
Since not everyone who has a telephone also has a TTY device, TTY users sometimes must communicate with hearing persons who use standard telephones. In order to communicate with non-TTY users, hard of hearing persons can dial into a state-funded, independently operated communications office known as a "Relay Center." An operator at the relay center receives the incoming TTY connection, then dials the hearing person using a standard voice telephone. Once both connections are made, the relay operator reads the text from the TTY user to the hearing person, and enters responses into her TTY device. Every state has its own relay center, and this process has effectively served the hard of hearing community since 1975.
Tags: acoustic coupler, hard hearing, hearing persons, modulated data, telephone connection, across telephone, call using