PSK31

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PSK31 or "Phase Shift Keying, 31 Baud" is a digital radio modulation mode, used primarily in the amateur radio field. It is usually used to conduct real-time text "chat" conversations between two amateur radio operators.

Several adjacent PSK31 signals, at least six, taken from the waterfall display of DigiPan. Frequency is plotted horizontally, and time is plotted vertically, with the earliest at the bottom. Strength is indicated by colour, increasing from blue to yellow.

[edit] History

PSK31 mode was developed by English amateur radio operator Peter Martinez (G3PLX). Martinez initially called his creation "varicode", because it uses variable length encodings (Huffman codes) to represent characters.

Technically, varicode is the encoding, and PSK31 the transmitting method. Varicode was designed so that the more frequently occurring characters had shorter encodings and the rarer characters used the longer encodings, a coding scheme similar to Morse code.

[edit] Use and implementations

In practice, PSK31 is usually used to exchange relatively short, conversation style messages between two radio operators. PSK31 is a half-duplex mode of communication. Thus only one participant in the conversation can send a message at any given time.

[edit] Resistance to interference

PSK31 can often overcome interference where voice or other data methods of communication fail. However, due to its relatively slow speed and minimal or no error control, PSK31 is not suited for the transmission of large blocks of data.

PSK31 works well with propagation paths that preserve phase, but poorly for those that do not, such as trans-polar paths.

The radio operator uses computer software to generate and decode an audio signal containing the encodings; the encoded audio signal is used to modulate the RF carrier of a transmitter.

Some software supports PSK10 and PSK05 variants which run at 10 baud and 5 baud respectively, for even greater noise resistance, at the cost of lower throughput.

[edit] Technical information

PSK31 audio bandwidth is very narrow (31.25 Hz), making it highly suitable for low power and crowded band operation.

The 31.25 Hz bandwidth was chosen because transmitting Varicode at a normal typing speed of about 50 words per minute needs a bit-rate of about 32 per second.

The bit-rate of 31.25 was chosen because it could easily be derived from the 8 kHz sample-rate used in many DSP systems such as those used in computer sound cards commonly used for PSK31 operation: 31.25 Hz is 8 kHz divided by 256, and so can be derived from 8 kHz by halving the frequency eight times.

PSK31 uses no error control but QPSK31 uses four phases and to provide forward error correction.

[edit] Spectrum efficiency compared to other modes

PSK31 bandwidth is very narrow (31.25 Hz), making it highly suitable for low power and crowded band operation.

RTTY and Packet spectrum efficiency compared with respect to {bits/second} throughput

[edit] Media

[edit] External links

• PSK31 Info Page – Provides information and links to software implementations
• PSK31 – PSK31 resources, from software to psk31 documentation
• pskmail – an implementation to allow email over PSK63
• radioteletype
• shortwave