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
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.