PSK31
From Wikipedia, the free encyclopedia
PSK31 or "Phase Shift Keying, 31 Baud" is a digital radio modulation mode, used primarily in the amateur radio field to conduct real-time text "chat" conversations between amateur radio operators.
Contents |
[edit] 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 who's circumstances do not permit the erection of large antenna systems and/or the use of high power. Also, very little in the way of additional equipment is required (normally just an old PC and a few extra interconnecting cables), and the software is both free to download and will run on older, slower computers.
As the computer is used simply as a 'means to an end', the mode has also attracted a new audience of 'mainstream' radio amateurs, not necessarily interested in computing or data communications per se. Nowadays on some bands the operating 'atmosphere' is less akin to the world of data, and more like that usually found in the morse code sections of the bands. Indeed many operators simply treat the mode as 'automatic telegraphy', even adopting long-established morse code conventions, language etc., but using a keyboard instead of a morse key.
PSK31 has also been heard in use on the Citizen's Band.
[edit] Use and Implementation
Put simply, a typical operator uses a Single-Sideband (SSB) transceiver connected to the sound card of a PC. When the operator starts typing, the software produces an audio tone. This sounds - to the human ear - like a continuous whistle with a very slight 'warble' superimposed onto it. This is then fed into the transceiver, either via the microphone socket (via a resistor to reduce the soundcard's output power to 'microphone' levels) or other auxiliary connection, and then transmitted.
From the transmitter's viewpoint, this amounts to little more than somebody whistling into the microphone. However, the 'whistle' contains a waveform, the phase of which is rapidly switched between 'normal' and 'inverted' by the software to form the 'varicode' (hence the slight 'warble'). These phase shifts serve the same function as the two separate alternating tones used in traditional radio-teletype systems such as RTTY.
To receive PSK31, the received audio 'whistle' from the transceiver's headphone socket is simply fed into the PC soundcard's 'Audio In' socket, and the software then decodes it in real time onto the screen.
The software also creates a visual control panel on the PC screen, which is used to operate the software by mouse and keystrokes.
[edit] Resistance to Interference
PSK31 can often overcome interference, signal fading etc. and 'get through' 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.
PSK31 works well with propagation paths that preserve phase, but can be adversely affected by those that do not, such as trans-polar paths, where 'auroral flutter' can disrupt the text.
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 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 the more frequently occurring characters had shorter encodings and the rarer characters used the longer encodings, a coding scheme similar to Morse code.
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.
Normal PSK31 uses no error control but an allied mode (QPSK31) uses four phases to provide a degree of 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. As well as making very efficient use of transmitter power, it also uses very little band space, as (with disciplined operating) at least twenty simultaneous PSK31 contacts can be carried out side-by-side in the bandwidth required for just one voice contact.
[edit] Media
- A sample PSK31 transmission (file info) — play in browser (beta)
- The text "Welcome to Wikipedia, the free encyclopedia that anyone can edit." sent as PSK31 (QPSK).
- Problems listening to the file? See media help.
[edit] External links
- PSK31 Info Page – Provides information and links to software implementations
- PocketDigi Introduction – An Open Source (GPL) digital mode application for PocketPC PDAs
- pskmail – an implementation to allow email over PSK63
- radioteletype
- shortwave
