What Is a TDD Phone?

by Andy Joseph

TDD is an abbreviation for "telecommunications device for the deaf." It is an electronic device that was invented to make it easier for people who have hearing or speaking disabilities. In some instances, the TDD is also referred to as TTY, which can stand for "typewriter" or "teletypewriter."


The modern-day TDD is designed as a sort of mini-computer, consisting of a keyboard and an LED or LCD screen to display the electronic text being typed. Also, it can include a printer to print text; some TDDs resemble typewriters that have spools of papers. For this reason, the abbreviation TTY is occasionally used. However, in most cases, TTY is used for manual devices, while TDD is used for computer-assisted or digital devices.


James Marsters (1924-2009) and Robert Weitbrecht (1920-1983) are credited with inventing the TDD in 1964. Marsters had gotten through college and dental school despite his deafness. However, he still had difficulty using devices such as the telephone. He eventually teamed up with Weitbrecht, then a physicist at Stanford University who was also deaf and shared his thoughts about relaying a typed conversation via a telephone line. Noting that Marsters had Teletype machines stored in his garage, Weitbrecht came up with the acoustic coupler (or modem) to connect two of the machines. What the coupler did, through a telephone wire, was change electrical signals from one machine into tones, which then changed back into electrical signals at the other machine for the message to be printed. Marsters and Weitbrecht approached an electrical engineer, Andrew Saks, for financing, and together they found a company called the Applied Communications Corporation to promote the new technology.


According to Karen Peltz Strauss, author of "A New Civil Right: Telecommunications Equality for Deaf and Hard of Hearing Americans," there were only 18 TDDs in existence in 1966. That number ballooned to about 30,000 in 2006 according to the Blue Book, a national TDD-user directory.

Protocols and Types of TDDs

Although there are several different TDD standards, the original one---the Baudot code---is perhaps the most popular one. Other teleprinter codes include V.21, V.23, EDT and DTMF. Some TDDs are portable, eschewing the bulkier design akin to landline phones. Others can actually be hooked up to landline phones, or can enable the callers to make a telephone call as well as send text.


A TDD caller takes turns during a conversation by typing "GA" ("go ahead"), and types "SK" ("stop keying") to hang up. "SK" typed twice means "signing off." Putting someone on hold warrants the "HD" command. It is considered bad etiquette to type a message at the same time that the other caller is doing the same. Also, typing "SK" more than twice is tantamount to expressing anger or slamming the phone.

About the Author

Based in the D.C. area, Andy Joseph works full-time as a data analyst and technical writer. He has been writing articles about technology, health, politics, music, culture and automobiles since 2007. His work has appeared in The Express, Congressional Report and Road & Track. He has a master's degree in journalism and technology management.

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