A Telecommunication Devices for the Deaf (TDD, aka Text Telephony (TTY)) was an early modem capability to allow deaf people to use telephones by including a keyboard and output device (such as teletype, printer, line of LED text, etc.). There were devices available as early as 1964, communicating at about 50 baud using 5-bit characters.
In the late 70s or early 80s I remember wondering why mass market analog modems did not include support for TDD devices. Was there a technical reason, or did they just feel the potential market was too small for this feature? I thought it would have been a good idea because then anyone with a modem could communicate with deaf people.