The TTY machine (a.k.a., Teletype/textphone/Minicom) is a communications device similar to a teleprinter that is used to send text messages over the public switched telephone network.
The TTY was developed in the 1960s to assist deaf and hard-of-hearing users in communicating over the telephone system. It consists of a keyboard, display, and acoustic coupler for a phone handset (some also included a small continuous roll printer or interface for external printers). The TTY converts typed characters to audio signals which can be sent over the phone system to TTY machine on the receiving end, where those audio signals would be converted back to text for display.
The TTY was supplanted in the 1990s by modern services such as instant messaging on computers and texting on phones (as well as video calls and video relay services for sign language use) but the technology is interesting to study, and in fact still works on some phone exchanges.
In this guide we'll take a look at how the TTY uses audio tones to communicate and build our own transmitter with a CLUE capable of sending messages to a TTY machine, both in standalone mode and in Bluetooth LE mode with messages being sent from iOS or Android.