TDD

A telecommunications device for the deaf (TDD) is a terminal that converts typed text into electrical signals for transmission over standard telephone lines, and converts incoming signals back into visible text. The user types a message on a QWERTY keyboard, which is encoded and sent to another TDD or compatible device on the receiving end, where it appears on a small liquid crystal or light-emitting diode display. The device bridges the gap between deaf and hard-of-hearing persons and hearing telephone users when relay services are involved.

The hardware typically consists of a keyboard, a modem, an acoustic coupler or direct telephone connection, and a visual display. Older models used 300 or 1200 baud modulation standards; many modern TDDs operate at 2400 baud. The acoustic coupler design, where the TDD speaker and microphone couple directly to the telephone handset, was the original architecture and remains common in portable units. Direct-connect TDDs plug straight into the telephone jack and are faster and more reliable than acoustic-coupled devices.

Common operational scenarios

In one-to-one TDD communication, both parties must have compatible equipment. More commonly, a deaf user calls a relay service where a hearing relay operator reads the incoming text aloud to a hearing party and types the hearing person's spoken response back to the TDD user. The relay operator types GA (go ahead) to signal turn-taking, and SK (stop keying) to indicate the call is finished. This asynchronous workflow is essential because the TDD user cannot hear voice while typing.

TDD reliability depends on clean telephone lines and correct baud rate synchronization between devices. Line noise, caller ID systems, and digital telephone networks can degrade signal quality. Bell 103 frequency shift keying (FSK) at 300 baud is the minimum standard for TDD compatibility, ensuring that older and newer equipment can interoperate. Video relay services and IP-based text solutions have expanded options, but TDDs remain the legal standard in many jurisdictions for emergency access and public accommodation requirements.

The term TDD has become somewhat dated; TTY (teletypewriter) is now used interchangeably in North America, and textphone is preferred in Europe. However, TDD remains the legal term in the Americans with Disabilities Act and related regulations, making it essential vocabulary in telecommunications infrastructure planning and customer service policy.