PSK31 Totally Explained

Everything about Psk31 totally explained

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

History

PSK31 was developed by English amateur radio operator Peter Martinez (G3PLX) and introduced to the wider amateur radio community in December 1998. Martinez initially called his creation "varicode", because it uses variable length encodings (Huffman codes) to represent characters.
PSK31 was enthusiastically received, and has since quickly spread into worldwide use. Due to the efficiency of the mode, it has become especially popular with operators whose circumstances don't permit the erection of large antenna systems and/or the use of high power. Very little equipment, in addition to a standard radio transceiver, is required—normally an old PC and a few cables will suffice; the software is both free to download and runs on older, slower computers.
Because the mode is seen as such an effective means to communication via PC, its audience has grown like wildfire in the years since its release and lent a new tone to the on-air conduct of digital communications.

Use and implementation

To employ PSK31, an operator typically uses a single sideband transceiver connected to the sound card of a PC. When the operator enters a message for transmission, the software produces an audio tone, which sounds to the human ear like a continuous whistle with a slight warble. This is then fed through either a microphone jack (using an intermediate resistor to reduce the sound card's output power to microphone levels) or an auxiliary connection into the transceiver, where it's transmitted.
From the perspective of the transmitter, this amounts to little more than somebody whistling into the microphone. However, the software rapidly shifts phase of the audio signal between two states (hence "phase-shift keying"), forming the varicode. These phase shifts serve the same function as the two tones used in traditional RTTY and similar systems.
To decode PSK31, the received audio whistle from the transceiver's headphone output is fed into the sound card's audio input, and the software decodes it. The software also includes a user interface on the PC, which is used to display the decoded text and manage the software configuration.

Resistance to interference

PSK31 can often overcome interference and poor propagation conditions in situations where voice or other data methods of communication fail. However, PSK31 was only designed for leisure use by amateurs, and due to its relatively slow speed and minimal or no error control, isn't intended for the transmission of large blocks of data or text, or critical data requiring high immunity from errors.
PSK31 works well with propagation paths that preserve phase, and can be adversely affected by those that do not, such as transpolar paths, where auroral influence can disrupt the signal phase continuity.
Some software supports PSK10 and PSK05 variants, running at 10 baud and 5 baud, respectively. These slower speeds sacrifice throughput to provide even greater resistance to noise and other interference.

Technical information

PSK31 is a half-duplex mode of communication. Thus only one participant in the conversation can transmit at a time.
Technically, varicode is the encoding method, and PSK31 the transmitting method. Varicode was designed so that, as in Morse code, the more frequently occurring characters would have shorter encodings, while rarer characters used longer encodings.
PSK31's bandwidth of 31.25 Hz was chosen because a normal typing speed of about 50 words per minute requires a bit rate of about 32 bits per second, and specifically because 31.25 Hz could easily be derived from the 8 kHz sample rate used in many DSP systems, including those used in the 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 normally uses no error control, but an allied mode, QPSK31, uses four phases to provide a degree of forward error correction.

Spectrum efficiency compared to other modes

PSK31's efficiency and narrow bandwidth make it highly suitable for low-power and crowded-band operation. As a result, with discplined operation at least twenty simultaneous PSK31 contacts can be carried out side-by-side in the bandwidth required for just one voice contact.

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