How Does a TTY Phone Work?
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TTYs Modulate Data
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A Tele-Type (TTY) phone, also known as a Telecommunication Device for the Deaf (TDD), works in much the same way as an analog computer modem. When a deaf or hard-of-hearing person needs to make a call, he simply places the handset from a standard telephone into an acoustic coupler on the TTY (some TTY units bypass the telephone and plug directly into the wall). When the user types text, the device uses standards set by the American National Standards Institute, or ANSI, to modulate the text data into a set of sounds that can be transferred across the telephone line.
TTYs Demodulate and Display Data
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When a TTY user receives a TTY call, she simply picks up the telephone receiver and places it in the acoustic coupler on the TTY, or presses an "Answer" key on a device connected directly to the telephone line. When the user on the other end sends modulated data as described in the previous section, the recipient's TTY device uses the same ANSI standards used to modulate the text to decode (or, in technical terms, demodulate) the incoming sounds. As the sounds are converted back into machine-readable data, the incoming text is sent to a small screen on the TTY for display. After reading the text, the user can type a response; the response is modulated into telephone sounds as described above and sent to the remote user.
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Some TTY Users Rely on Relay
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While the modulation/demodulation routine of TTY phones works very well when facilitating a conversation between two TTY users, a hard-of-hearing person sometimes needs to communicate with individuals who do not have access to a TTY phone. To communicate with non-TTY users, a hard-of-hearing person uses a special state-sponsored service known as a relay center. Available 24 hours per day, 365 days each year, operators at relay centers accept calls from TTY users on their own TTY devices, then verbally relay the information to a hearing person using a standard telephone connection. When the hearing person replies, the relay operator types the response into her TTY phone and sends the response to the TTY user at the other end. In cases where a hearing person needs to initiate a call to a TTY user, this process is performed in reverse (instead of the TTY user calling the relay center, the hearing person initiates the conversation and the relay center operator dials the hard-of-hearing person's TTY device).
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