Briefly called a TDD, this is a piece of equipment used by deaf individuals to communicate over regular phone lines. Pressing a key on the TDD produces a tone or signal that is sent over the telephone lines to the receiving TDD, which interprets the signal and displays the corresponding letter on its screen. TDD is the name more commonly used by hearing people and usually refers to an electronic version, while TTY (from "teletype") is the term deaf people usually prefer because of historical precedent: the ancestor of the electronic TDD was a clunky machine that worked like a teletype. Almost all devices in use are now electronic, but "TTY" is still used by deaf people.

TDDs come in several forms, all with the same function. Some have printing capabilities, generally copying the conversation onto a narrow roll of paper like that used for cash register receipts. Others are compact and fold up to be taken along for use on the road, and a few of these have connections for cellular phones. Some have acoustic couplers, which require a telephone handset to be placed on the machine, and others are connected directly to the phone line. A few models have an integrated answering machine, which records the tones it receives and then plays them back to the interpreter so the message can be read on the screen. Pay phones sometimes have models built into a drawer that automatically extends when it receives TDD tones. TDDs usually use either Baudot code (45 bits per second) or ASCII to send the signals. Of course these just sound like noise to a hearing person, so some machines have a button for a voice annunciator that tells the person they're receiving a TDD call.

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