PSK31
Encyclopedia
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
, 31 Baud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
" is a digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
radio modulation mode, used primarily in the amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
field to conduct real-time keyboard-to-keyboard informal text chat
Synchronous conferencing
Synchronous conferencing is the formal term used in science, in particular in computer-mediated communication, collaboration and learning, to describe online chat technologies. It has arisen at a time when the term chat had a negative connotation...
between amateur radio operators.
History
PSK31 was developed and named by EnglishEngland
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
amateur radio operator Peter Martinez (G3PLX) and introduced to the wider amateur radio community in December 1998.
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 and/or the use of high power.
Use and implementation
A PSK31 operator typically uses a single sidebandSingle-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
transceiver connected to the sound card of the 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 is transmitted.
From the perspective of the transmitter, this 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 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.
The use of PSK31 does not require exclusive use of a dedicated computer. When it is not running the PSK31 program, the station 'utility' computer can still be used exactly as before. PSK31 is a 'soundcard' mode, and many programs have since been created to utilise the same technology for other interesting modes such as RTTY, Hellschreiber
Hellschreiber
The Hellschreiber or Feldhellschreiber is a facsimile-based teleprinter invented by Rudolf Hell. Compared to contemporary teleprinters that were based on typewriter systems, the Hellschreiber was much simpler and more robust, with only two moving parts...
, Olivia MFSK
Olivia MFSK
Olivia MFSK is an amateur radioteletype protocol designed to work in difficult conditions on shortwave bands. The signal can still be properly copied when it is buried 10 dB below the noise floor...
etc. So, once it has been set-up to run PSK31, the same set-up can also be used to explore a variety of these other modes at the click of a mouse.
In addition to a standard radio transceiver, very little equipment is required to use PSK31. Normally, an older PC and a few cables will suffice; the software is both free to download and runs happily on old, slow computers from the early Pentium era or even earlier. 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 soundcard's output to be injected into the microphone input, the radio's audio output to be sent to the soundcard's input, and also handle the radio's transmit-receive switching. Recently introduced interfaces also incorporate their own soundcard, and can therefore be powered and run from the PC via one single USB connection.
Resistance to interference
PSK31 can often overcome interference and poor propagationRadio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
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, is not intended for the transmission of large blocks of data or text, or critical data requiring high immunity from errors. After encoding into varicode
Varicode
Varicode is a Huffman code for use in PSK31. It supports all ASCII characters, but the characters used most frequently have shorter codes. The space between characters is indicated by a 00 sequence, a variation of Fibonacci coding. Originally created for speeding up real-time keyboard-to-keyboard...
, the binary signal is further transformed into a quaternary
Quaternary numeral system
Quaternary is the base- numeral system. It uses the digits 0, 1, 2 and 3 to represent any real number.It shares with all fixed-radix numeral systems many properties, such as the ability to represent any real number with a canonical representation and the characteristics of the representations of...
set of phase shifts. A sliding window of five bits is used to select one of the four possible phase shifts, providing a means of error correction by spreading each bit of data across adjacent bits. To successfully decode an input bit requires a large number of phase shift sequences to be received, causing a 20 bit, 640 millisecond latency in the output of the decoder.
The idle sequence is a continuous sequence of zeros, since no varicode word may contain two adjacent zeros, which results in a continuous reversals of phase as in BPSK.
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
Throughput
In communication networks, such as Ethernet or packet radio, throughput or network throughput is the average rate of successful message delivery over a communication channel. This data may be delivered over a physical or logical link, or pass through a certain network node...
to provide even greater resistance to noise and other interference.
Technical information
PSK31 is created by switching the polarity of the signal used to key the computer's sound card. In the most-commonly-used variant (BPSK31), binary information is transmitted by either imparting a 180-degree polarity shift (a binary 'zero') or no polarity shift (a binary 'one') in each 32ms symbol interval. As shown in the figure above, a "raised cosine" filter is used to limit the occupied bandwidth, resulting in both amplitude and phase modulation of the carrier. The 180 degree phase shift for a "zero" bit code occurs during at an amplitude null. Subsequent amplification of the signal must be linear, preserving the modulation waveform, to ensure minimum occupied bandwidth.The boundaries between character codes are marked by two or more consecutive zeros. Since no character code contains more than one consecutive zero, the software can therefore instantly detect the 'space' between characters. Martinez arranged the character alphabet so that, as in Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
, the more frequently occurring characters would have the shortest encodings, while rarer characters used longer encodings. He gave the name 'varicode
Varicode
Varicode is a Huffman code for use in PSK31. It supports all ASCII characters, but the characters used most frequently have shorter codes. The space between characters is indicated by a 00 sequence, a variation of Fibonacci coding. Originally created for speeding up real-time keyboard-to-keyboard...
' to this encoding scheme.
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
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
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).
Colloquial usage of the term 'PSK31' in amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
usually implies the use of the most commonly used variant of PSK31: binary phase shift keying. BPSK uses no error control, but an allied mode, QPSK31, uses four phases instead of two, to provide a degree of forward error correction
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
. It is very simple to switch from BPSK to QPSK if difficulties arise during a contact.
Spectrum efficiency compared to other modes
PSK31's efficiency and narrow bandwidth make it highly suitable for low-powerQRP operation
In amateur radio, QRP operation means transmitting at reduced power levels while aiming to maximize one's effective range while doing so. The term QRP derives from the standard Q code used in radio communications, where "QRP" and "QRP?" are used to request, "Reduce power," and ask "Should I reduce...
and crowded-band operation. PSK31 contacts can be conducted at less than 100Hz separation, so with disciplined operation at least twenty simultaneous PSK31 contacts can be carried out side-by-side in the bandwidth required for just one SSB voice contact.
Common Frequencies
The following amateur radioAmateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
frequencies are commonly used for transmitting and receiving PSK31 signals.
| Frequency Frequency Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency... |
Amateur Band Amateur radio frequency allocations Amateur radio frequency allocation is done by national telecommunications authorities. Globally, the International Telecommunication Union oversees how much radio spectrum is set aside for amateur radio transmissions... |
|---|---|
| 1.838 MHz | 160 meters 160 meters Just above the mediumwave broadcast band, 160 meters is the lowest radio frequency band allocation available to amateur radio operators in most countries. Seasoned operators often refer to 160 meters as the Top Band... |
| 3.580 MHz | 80 meters 80 meters The 80 meter or 3.5 MHz band is a core amateur radio frequency band, allocated frequencies from 3.5 to 4.0 MHz in IARU Region 2, and generally 3.5 to 3.8 or 3.9 MHz in Regions 1 and 3 respectively. The portion of the band used for phone communications is sometimes referred to as 75 meters... |
| *7.040 MHz | 40 meters 40 meters The 40-meter or 7-MHz band is an amateur radio frequency band, spanning 7000 to 7300 kilohertz , allocated to radio amateurs in all countries worldwide.40 meters is considered one of the most reliable all-season DX bands.... (regions 3) |
| *7.080 MHz | 40 meters 40 meters The 40-meter or 7-MHz band is an amateur radio frequency band, spanning 7000 to 7300 kilohertz , allocated to radio amateurs in all countries worldwide.40 meters is considered one of the most reliable all-season DX bands.... (region 2) |
| 10.142 MHz | 30 meters |
| 14.070 MHz | 20 meters 20 meters The 20-meter or 14-MHz amateur radio band is a portion of the shortwave radio spectrum, comprising frequencies stretching from 14.000 MHz to 14.350 MHz. The 20-meter band was first made available to amateurs in the United States by the Third National Radio Conference on October 10, 1924... |
| 18.100 MHz | 17 meters |
| *21.080 MHz | 15 meters 15 meters The 15-meter band is an amateur radio frequency band spanning the shortwave spectrum from 21 to 21.45 MHz. Almost all countries permit amateur communications on the entire band.... |
| 24.920 MHz | 12 meters |
| 28.120 MHz | 10 meters 10 meters The 10-metre band is a portion of the shortwave radio spectrum internationally allocated to amateur radio and amateur satellite use on a primary basis. The band consists of frequencies stretching from 28000 to 29700 kHz.-History:... |
| 50.290 MHz | 6 meters 6 meters The 6-meter band is a portion of the VHF radio spectrum allocated to amateur radio use. Although located in the lower portion of the VHF band, it nonetheless occasionally displays propagation mechanisms characteristic of the HF bands. This normally occurs close to sunspot maximum, when solar... |
There is no authoritative list as the frequencies are determined by common convention.
Further Reading
- Peter Martinez. PSK31: A new radio-teletype mode with a traditional philosophy. http://det.bi.ehu.es/~jtpjatae/pdf/p31g3plx.pdf
- Meltz, Steve "The New HF Digital Modes - PSK31", QST, April, 1999, pp. 50-51
- Martinez, Peter. http://www.arrl.org/tis/info/pdf/x9907003.pdf "PSK31: A New Radio-Teletype Mode". RadCom, December 1998, updated February 1999