Software is now available for a bewildering variety of modes, including many developed specifically for amateur use. These included PSK (phase shift keying), and amateur implementations of commercially used modes such as MFSK (multi-frequency shift keying). The 1990s saw the development of an ever-increasing number of new modes coming into use. Packet became extremely popular and quickly became the most widely used amateur digital mode. Packet allowed faster data rates than RTTY, while using a narrower bandwidth (500 Hz for a 300 baud packet signal, as opposed to 1.2 kHz for a 50 baud RTTY signal). The most popular new mode was Packet Radio, which was adapted by amateurs from a commercial computer networking protocol. One of the earliest of these new modes was AMTOR (AMateur Teleprinting Over Radio). Then in the 1980s, RTTY received a further boost from the development of home computers and software that could generate and decode FSK signals using the computer's sound card.Īs more amateurs explored the uses of computers, this led inevitably to the development of new digital modes, all of them derived from RTTY but offering faster speeds and more up-to-date features such as error correction. These techniques brought RTTY within the reach of all amateurs. But RTTY was not a mode for the faint-hearted, because these machines were very bulky and noisy, and they needed a constant supply of paper.īack in the 1970s there was a major revolution in RTTY operation, as amateurs replaced the old teletype machines with computer keyboards and monitors, connected to small terminal units that generated and decoded the FSK signals. The two most popular teleprinters used by Australian amateurs were the Model 15 Teletype (made in USA), and the Creed teleprinter, which was made in England. Nowadays the decoding can be done by a computer sound card, so there is no need for a separate terminal unit.Īmateur RTTY operation originally depended on the availability of surplus ex-commercial or military teletype machines.
The terminal unit would decode the incoming audio from the receiver and convert the two-tone signal to a series of pulses that were then sent to the teleprinter. On the receiving end, RTTY stations originally used decoders known as "terminal units". This technique is known as AFSK (audio frequency shift keying).
Nowadays FSK transmission is normally achieved by feeding an audio tone generator (or, more usually, a computer sound card) into an SSB or FM transmitter. This technique is known as "frequency shift keying" or FSK. To send teletype signals on the air, the transmitter generates a continuous carrier that is shifted slightly between two different frequencies that correspond to the mark or space states.
This code was later superseded by the ASCII code, which remains standard today. The 5-bit code was later modified by Donald Murray, and the resulting "Murray Code" became the standard code used in commercial teletype links. The way around this problem was a "shift" key that allowed the machine to print either letters or figures. This was not enough to allow each character - letter, numeral, or punctuation sign - to have its own unique code. The 5-unit Baudot code allowed the transmission of 32 different characters. When these pulses reached the receiving machine, they would determine which character was printed. The first pulse in the Baudot code was a "start" bit, then five data bits, followed by a "stop" bit which marked the end of each character. The "on" state was referred to as the "mark" state, and the "off" condition was called the "space" state. When a key was pressed, the teletype machine would generate a series of pulses by switching a DC voltage on or off. (click picture above for a larger view)įrom the late nineteenth century, the standard coding system for teleprinters was the 5-unit Baudot code (named after its inventor, Emile Baudot). These machines were capable of sending data at a much faster rate than was possible using Morse, and messages could be printed out automatically at the receiving end. For printing, teletype machines used a motor-driven mechanism that controlled a printing head or a set of typehammers like those used in ordinary typewriters. The original teleprinters included a keyboard that could generate a series of electrical impulses corresponding to whichever key was typed. This required a qualified Morse operator at each end of the circuit, and the data rate was slow because the speed was limited by the ability of the operators to copy Morse signals by ear. Before that time, all telegraph communications had used Morse Code. Teleprinter machines were first used for landline communication in the mid nineteenth century. RTTY (short for RadioTeleTYpe) is the first digital mode to be used by radio amateurs.