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Digimodes summary

AMTOR

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PSK31

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PSK31

- an overview, summary or tutorial about the basics of PSK31 a form of digimode used on the HF amateur bands.

PSK31 is a digimode used on the amateur radio bands that derives its name from the fact that it uses phase shift keying (PSK) rather than frequency shift keying, and it transmits data at a rate of around 31 baud. The mode is aimed at providing a greater level of performance for keyboard-to-keyboard, conversational-style data communications than is available with other data modes. This makes it a particularly attractive form of communications for radio amateurs.

The aim is to provide an efficient yet straightforward system which does not use the complicated ARQ processes, and with only enough error correction to match the typical error rates that are encountered. Also, by using phase shift keying and a low data rate, it is possible to narrow the bandwidth, which considerably reduces the effects of interference and noise. Bandwidths of 31Hz can be used, making this an extremely narrow-band mode, and one capable of operating under severe conditions. Accordingly it is able to operate within the narrow band modes sections that are set aside in the amateur radio band allocations.

PSK31 basics

PSK31 is widely used, particularly on the HF amateur radio bands where its up to date features enable it to offer advantages over modes such as RTTY. It incorporates error correction techniques along with the use of a full ASCII and ANSI character set which enables it to fit in with modern day computer and telecommunications technology.

PSK31 uses a form of modulation known as phase shift keying (PSK). This is rather different to the frequency shift keying that is used for modes such as RTTY. PSK involves reversing the polarity, or phase, of the signal (180° phase shifts), and has been likened to reversing or swapping over the two wires in an antenna connection. However, in reality the phase reversals are not achieved in this way; instead they are generated and detected in the audio sections of the SSB transceiver being used.

There are various forms of PSK and the form of PSK described is actually known as binary phase shift keying (BPSK) and it is more efficient than either frequency shift keying, which has a greater bandwidth, or on/off keying which does not use the power as efficiently.

PSK31 uses a novel form of data encoding. When sending asynchronous ASCII data, systems use a fixed number of data bits as well as start and stop bits. However, when sending a long run of data it is possible for the receiver to lose synchronisation. Additionally, improvements in speed can be gained from adopting variable-length codes with those codes that are used most often being the shortest. This is used to good advantage in Morse where the character 'e' (which is the most common in English) is a single dot. By analysing the occurrence of different ASCII characters a code called Varicode was devised. The shortest code, '00', was allocated to the space between two words.

It is possible to add error correction to the system. However, to achieve this it is necessary to use a form of keying called quadrature phase shift keying (QPSK). Instead of two phase states 180° from one another, QPSK uses four phase states, each 90° from one another. However, in operation on the bands it has been found that error correction with the use of QPSK only sometimes gives improvements over ordinary BPSK. Accordingly it is possible to use either system, dependent upon the conditions.