RTTY is an abbreviation of radio teletype, and in particular denotes a kind of FSK keyboard conversation mode used by radio amateurs.

Telegraphy is based on on-off keying of a single carrier (signal). While this is often very reliable, additional accuracy is obtained by sending a kind of "mirror image" of the signal a few Hz below, which is on when the original signal is off, and vice versa. Or put another way, the signal shifts back and forth between two different frequencies. The higher frequency is called the mark frequency, the lower is called the space frequency. The difference between the two frequencies is called the shift. The speed is measured in bauds, and is the same as the number of possible shifts (bits) per second.

This scheme can transmit any kind of binary data, not just morse code. In fact, FSK-keyed is very rare and almost non-existant. Radio teletype has been used and is still used by governments, military forces and commercial entities with a variety of different charset encodings, such as ASCII. The de facto standard, not least among radio amateurs, is still the ancient 5-bit Baudot code from the days of landline telegraphy.

The Baudot character set is commonly regarded as a little bit nasty. The reason is that when only 25 = 32 codepoints are available, and 26 are taken for the English alphabet (uppercase only, of course!) there are only 6 left. To be able to transmit numbers and punctuation, a not particularly clever hack was introduced. Two non-printing characters were created, figures shift and letters shift. After a figure shift character had been received, the same bits that make up a letter will be displayed as a number or a punctuation sign, depending on which letter it is. The same continues until a letter shift is received.

Other relevant properties of the amateur variant of RTTY is that it has a shift of 170 Hz, and a baud rate of exactly 45.45.

RTTY remains one of the most popular modes in amateur radio, and probably still is the most prevalent of the digital modes. This is attested by the fact that ARRL's DX Century Club, an award for contacting 100 or more different countries, has the three categories CW (telegraphy), Phone (you know, talking into the microphone) and RTTY (as the only digital mode). RTTY is on the rise now that free sound card software has come to replace the clunky electromechanical RTTY machines of the old days, but so are many other digital modes, including, but not limited to, PSK31.

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