RTTY ("radio teletype") is a very old digital mode. Amateur use started after World War II with surplus printing teletype units going into ham radio service. Digital modes could later be decoded in computer-controlled hardware with the AEA PK-232 - see https://www.eham.net/reviews/view-product?id=218 for some reviews on eHam. Modern software like fldigi from W8HKJ http://www.w1hkj.com decodes the tones into text using "sound card" interface into your computer.
Al K8WXQ organized a RTTY roundtable + Elmer session on Tuesday 2023-12-26 with Matt W8MAP and me (W8EMV). Al arranged for the use of the W8FSA 146.92 FM repeater. I only heard about it at the last moment - I was out and about on my bicycle and tuned in to 146.92 to hear the planning going on a few hours early - so it was good luck on my part.
The RTTY interface that I was able to put together on short notice that worked acceptably well as a start was to download the DroidRTTY application for Android from Wolphi. To use it, you hold up your phone near the handset of your favorite radio, key the mic with one hand, and transmit from the radio with the other. DroidRTTY will decode the RTTY tones, and will send messages that you select from macros or that you type in.
A few challenges here. You need two hands to do this, one to key the mic and one to send your message. There is a very tiny on-screen keyboard for composing messages which I don't have enough keyboard time in front of to feel like I can do an adequate job just yet. I had to select the "swap mark and space" option to be heard correctly.
An external Android keyboard of some kind looks like it would help for this.
I had also hoped to copy RTTY using OpenWebRX+ which has the option "RTTY-170 (45)" that corresponds to the 45.45 baud, 170 Hz spacing of amateur RTTY. Alas that didn't work right away and I'm looking to test it again later.
OpenWebRX+ distribution: https://github.com/luarvique/openwebrx
My receiver: https://rx.dunker-pentatonic.ts.net
More information about RTTY from the article "The origins and evolution of radio teletype", Ham Radio History, Ed Mins, W0YK