Radioteletype

Lore & Background

Radioteletype evolved from landline teleprinter operations that began in 1849 with a circuit between Philadelphia and New York City. Émile Baudot designed a five-unit code in 1874 still in use today. The US Navy successfully tested printing telegraphy between an airplane and a ground radio station in August 1922, and later that year RCA tested it via their Chatham, Massachusetts station to the RMS Majestic. Commercial RTTY systems were active between San Francisco and Honolulu by April 1932 and between San Francisco and New York City by 1934. The US military used radioteletype in the 1930s and expanded usage during World War II, with the Navy calling it RATT and the Army Signal Corps calling it SCRT (Single-Channel Radio Teletype).

A radioteletype station consists of a teleprinter, a modem (terminal unit), and a radio. The teleprinter generates a 5-bit character with a start bit and stop bit, using the Baudot code which requires letters and figures shift states to accommodate more characters. The modem converts digital signals to audio tones (e.g., 2295/2125 Hz in the US, 2125/1955 Hz in Europe) for transmission via SSB or direct FSK. On reception, the FSK signal is converted back to tones and demodulated. Electromechanical teleprinters were heavy and noisy, later replaced by electronic units and personal computers running emulation software from the 1980s.

Reader's Guide

Radioteletype's significance lies in its role as one of the earliest digital radio communication systems, bridging landline telegraphy with wireless transmission. It enabled reliable text communication over long distances, particularly for military and commercial use, and its FSK technology proved very reliable even over long distances. The system's legacy includes the widespread adoption of the Baudot code and asynchronous character transmission, which influenced later digital modes. In amateur radio, RTTY remains popular, with modern computers using sound cards and software like fldigi, MMTTY, or MixW to emulate the modem and teleprinter functions. The use of paper tape for message storage and the distinctive 'beedle-eeeedle-eedle-eee' sound of FSK signals are characteristic of the mode. The term 'radioteletype' now encompasses the entire family of systems connecting teleprinters or PCs over radio, regardless of alphabet, link system, or modulation.

Did You Know?

• Commercial RTTY systems were in service between San Francisco and Honolulu as early as April 1932.
• The military acronym RATT stands for Radio Automatic Teletype.
• The most common test signal for RTTY is a series of 'RYRYRY' characters.