PSK31 - Phase Sifht Keying 31 Baud

PSK31 or  "Phase Shift Keying, 31 Baudis a digital communications mode which is intended for live keyboard-to-keyboard conversations, similar to radioteletype. Its data rate is 31.25 bauds (about 50 word-per-minute), and its narrow bandwidth (approximately 60 Hz at -26 dB) reduces its susceptibility to noise. PSK31's ITU emission designator is 60H0J2B. It uses BPSK modulation without error correction or QPSK modulation with error correction (convolutional encoding and Viterbi decoding). In order to eliminate splatter from the phase-reversals inherent to PSK, the output is cosine-filtered before reaching the transmitter audio input. PSK31 is readily monitored and the most popular implementation uses DSP software running on a computer soundcard inside a Personal Computer.

There is a preamble at the start of each transmission and a postamble at the end. The preamble is an idle signal of continuous zeroes, corresponding to continuous phase reversals at the symbol rate of 31.25 reversals/second. The postamble is just continuous unmodulated carrier, representing a series of logical ones. This makes it possible to use the presence or absence of the reversals to squelch the decoder so that the screen doesnt fill with noise when there is no signal.

History
PSK31 was developed and named by English amateur radio operator Peter Martinez (call sign G3PLX) and introduced to the wider amateur radio community in December 1998.
The 31 baud BPSK modulation system used in PSK31 was introduced by Pawel Jalocha (SP9VRC) in his SLOWBPSK program written for Motorola's EVM radio. Instead of the traditional frequency-shift keying, the information is transmitted by patterns of polarity-reversals (sometimes called 180-degree phase shifts). PSK31 was enthusiastically received, and its usage grew like wildfire worldwide, lending a new popularity and tone to the on-air conduct of digital communications. Due to the efficiency of the mode, it became, and still remains, especially popular with operators whose circumstances do not permit the erection of large antenna systems, the use of high power, or both.

Use and implementation
A PSK31 operator typically uses a single-sideband (SSB) transceiver connected to the sound card of a computer running PSK31 software. When the operator enters a message for transmission, the software produces an audio tone that sounds, to the human ear, like a continuous whistle with a slight warble. This sound 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, from which it is transmitted.
From the perspective of the transmitter, the sound amounts to little more than somebody whistling into the microphone. However, the software rapidly shifts the phase of the audio signal between two states (hence the name "phase-shift keying"), forming the character codes. These phase shifts serve the same function as the two tones used in traditional RTTY and similar systems.
To decode PSK31, the audio whistle received from the transceiver's headphone output is fed into a computer sound card's audio input, and software decodes it. The software displays the decoded text.
The use of PSK31 does not require exclusive use of a dedicated computer. When it is not running the PSK31 program, the computer can still be used for normal activities. Because PSK31 was developed for use through a computer's sound card, many programs have since been created to use the same technology for other modes, such as RTTY, Hellschreiber, and Olivia MFSK. So, once it has been set up to run PSK31, a computer can be used to explore a variety of digital message transmission modes.
Aside from a standard radio transceiver and a computer with a sound card, very little equipment is required to use PSK31. Normally, an older computer and a few cables will suffice, and many PSK31 software applications are free. Many operators now use a commercially available interface/modem device (or "nomic") between their computers and radios. These devices incorporate the necessary impedance matching and sound level adjustment to permit the sound card output to be injected into the microphone input, send the radio's audio output to the sound card input, and handle the radio's transmit-receive switching. Recently introduced interfaces also incorporate their own sound card and can be powered and run from the computer via a single USB connection.

Resistance to interference
Like other narrow band digital modes, PSK31 can often overcome interference and poor propagation conditions in situations where voice or other methods of communication fail. However, PSK31 was designed only for leisure use by amateurs, and due to its relatively slow speed and limited error control, is not suitable for transmitting large blocks of data or text, or critical data requiring high immunity from errors.
PSK31 works well over propagation paths that preserve phase, and resists fading (QSB) well. However, it can be adversely affected by propagation modes—such as transpolar paths—where auroral "flutter" or multipathing can disrupt the signal phase continuity. In such cases the use of QPSK (see below) is often beneficial.
Some software supports PSK10 and PSK05 variants, running at 10 baud and 5 baud, respectively. These slower speeds sacrifice throughput to provide greater resistance to noise and other interference. Conversely, PSK63 is increasingly used for faster exchanges, especially during contest operating.

Spectrum efficiency compared to other modes
PSK31's efficiency and narrow bandwidth make it highly suitable for low-power and crowded-band operation. PSK31 contacts can be conducted at less than 100 Hz separation, so with disciplined operation at least twenty simultaneous PSK31 contacts can be carried out side-by-side in the 2.5 kHz bandwidth required for just one SSB voice contact.

Common frequencies
The following amateur radio frequencies are commonly used for transmitting and receiving PSK31 signals. They normally occupy the lower edge of each band's digital modes section. PSK31 operators generally use upper sideband (USB), even on frequencies below 10 MHz where the convention normally calls for lower sideband. This is because (a) signals then spread upwards into the digimode section from the "base" frequency, and (b) using QPSK requires both stations to use the same sideband.

PSK31 Frequencies

FrequencyAmateur Band
1.838 MHz 160 meters
3.580 MHz 80 meters
7.035 MHz* 40 meters (regions 2,3)
7.040 MHz* 40 meters (region 1)
10.142 MHz 30 meters
14.070 MHz 20 meters
18.100 MHz 17 meters
21.080 MHz* 15 meters
24.920 MHz 12 meters
28.120 MHz 10 meters
50.290 MHz 6 meters

Software
There is much software for getting in the air on PSK31. You can choose based on the Operating System of your computer and after that based on the hardware you have.
psk31 digipan 01 

To download the introduction and interface PC please clik: IntroductionHandbook and Interface Schematic 
To download the software and user manual please click: Fldigi 3.23 & manualDigiPan 2.0 & manual, DM780 & manual.

Source :  

  • http://www.arrl.org/psk31-spec
  • http://en.wikipedia.org/wiki/PSK31
  • http://aintel.bi.ehu.es/psk31.html
  • http://psk31.software.informer.com/downloads/
 

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