The Beginnings of Digital Modes
Before the computer there were several modes of transmitting text messages by Radio. The first was the use of Morse Code or CW (Continuous Wave) where the signal was sent letter by letter in a series of coded long and short signal bursts. This was done by hand and an expert could reach a speed of up to 60 words per minute.
CW speeds were increased and skill needs reduced by pre-recording the CW signal on paper tape and playing it at higher speeds to be recorded by the receiving station. Then came the inevitable demand for even faster and more accurate text message transfer, and the teletype machine came into being.
A teletype machine, or teleprinter, was essentially an electric typewriter sending signals down a wire using a five bit code, the Baudot Code. This code restricted the machine initially to 32 characters but with the use of shift keys this could be increased. When a key on the radio teletype keyboard was operated, a specific code was transmitted. At the receiving end, each 5-bit code sequence would cause its corresponding character to be printed.
Teletype came into commercial use in the 1920s. By the 1940s it was the major method of distributing text in the newspaper world.
Radio Teletype (RTTY)
Radio Teletype, or RTTY as it is commonly called today, was in commercial operation in 1932 and is still actively used by amateurs and others today. Developments include the wider use of the 7 bit ASCII Code (American National Standard Code for Information Interchange) to replace the more limited 5 bit Baudot Code. Most HF amateur radio transceivers are equipped to transmit and receive RTTY mode.
Earlier RTTY systems consisted of a a teletype machine, a tone generator and decoder, and a radio. The tone generator converted the digital signal from the teletype machine into audio tones which were then transmitted by the radio. At the receiving station, the signal was fed from the receiver to a decoder, or Terminal Unit, which converted the received tones into DC pulses that were then sent to the teleprinter.
Today such functions can be performed by computers. The bulky and noisy electromechanical systems have been replaced by the computer. The sound card performs the functions of the tone generator and terminal unit, and the computer screen replaces the teleprinter. The use of computers has also allowed the introduction of software able to produce more accurate and speedier communication.
Packet radio is a development of RTTY, invented in the 1970 by workers at the University of Hawaii. Like RTTY, it is an FSK (frequency shift keying) system, but it differs from RTTY in two respects. The speed of data transfer is much faster than traditional RTTY, and the data is sent in blocks, or "packets" rather than in a continuous stream.
Sending the data in packets allows stations to take turns transmitting, and each is able to check received packets for errors and to send the other station an acknowledgment that each packet has been received error free.
Packet also has the advantage that it can store messages to allow retrieval at a later time.
The mode became very popular in the 1980s, and it is still widely used for transmitting messages and maintaining bulletin boards.
A major application today for amateur packet radio is Automatic Position Reporting System (APRS), where stations use a GPS (Global Positioning System) receiver to transmit their precise geographic location.
Packet radio systems originally used a terminal node controller (TNC), which is a small modem that performs the same functions as an RTTY terminal unit, plus various logic functions such as sorting the message data into packets. Nowadays it is increasingly common to replace the separate TNC with PC software emulation that uses the computer sound card.
PSK and related modes
As is inevitable, experience with packet radio has led to further advances designed to further improve the speed and reliability of data transfer.
Systems such as Pactor and Clover were designed to improve the quality of data sent through potentially noisy radio channels such as are found in long distance HF transmissions.
At present an increasingly popular mode is PSK31, developed by amateur Peter Martinez G3PLX for real time keyboard to keyboard contacts. PSK means Phase Shift Keying, and it is a computer based mode that uses sound cards to generate and decode the signals.
Other digital modes include MFSK (multi frequency shift keying). Unlike RTTY and packet, MFSK modes transmit and decode more than two audio tones (the use of 16 or more tones is quite common). For weak signal work, newer modes like WSJT are able to decode signals that are far below the noise.
With the use of computer sound cards for signal generation and decoding, new digital modes are being developed very rapidly. We can now choose between a variety of different modes to suit a wide range of operating conditions. Different digital modes can be optimised for data speed, reliability, or sensitivity to weak signals.
The development of digital modes has now come full circle, with modes such as QRSS, which is capable of sending CW signals in extremely narrow bandwidths and allowing communication to take place when signals are far too weak to be heard.
Digital Modes for Image Communication
Digital modes have been used for many years for image transmission. The first method used was Facsimile (FAX) which used a mechanism with a rotating drum to scan an image and convert it to a series of audio tones representing the black, white and grey elements of the picture. At the receiving station, a similar machine would use a stylus to burn the image onto a sheet of sensitised paper.
As with other digital modes, the use of mechanical scanners and printers has been replaced by computerised image generation and decoding.
Image transmission became popular in the amateur world back in the 1970s, with the invention of slow-scan television (SSTV). The earliest SSTV systems used video cameras, modified to produce the slow scanning rates that were required to allow pictures to be transmitted within the narrow bandwidth occupied by an SSB transmission. Modern SSTV systems use computer techniques to allow the transmission of full colour images from video cameras, webcams, or digital cameras. As with RTTY and packet, SSTV stations use computer sound cards to generate and decode the audio data that is fed to and from the transceiver.
There is also a long tradition, going back to the 1930s, of amateur fast-scan television activity. The earliest TV transmissions in Australia were made by amateurs, and the mode is still popular today. ATV systems use the same technical standards as broadcast television, so amateur ATV signals can be received on any TV set that is able to tune to their frequencies.
The most recent development in ATV is the use of digital transmission systems such as DVB-T, which is the same system used by broadcast TV stations. As with analog ATV, there are a number of digital set-top boxes on the market that can tune to the frequencies used for ATV.
Digital Voice Communication
Digital audio transmission is the newest aspect of amateur digital transmission. The best known system is D-Star, which allows digital audio and data to be exchanged either directly between simplex stations or via repeaters. Amateurs are also making use of surplus radios using the APCO digital voice system.
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