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PSK31 - the Radio Teletype of Today

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PSK31 - the Radio Teletype of Today Amateur Radio Technology in Action PSK31 - the radio teletype of today In this monthly feature Hans van de Groenendaal ZS6AKV, executive chairman of the South African Amateur Radio Development Trust (SAARDT) looks at various technologies and activities that drive amateur radio. SAARDT is dedicated to the development of amateur radio in South Africa with a special interest in the youth. The organisation is funded from donations and supports the South African Radio League and SA AMSAT. For more details visit www.amateurradio.org.za or write to [email protected]. One of the main attributes of amateur radio For PSK31 Peter devised a new code that is that the sky is the limit. Actually not true! Let combines the best of RTTY and Morse. He called me re-phrase. Last month I told you about radio his creation the Varicode because a varying amateurs going to Mars - there is literally no limit number of bits are used for each character. in the technologies that radio amateurs can Building on the example of Morse, Peter allocated experiment. the shorter codes to the letters that appeared most often in standard English text. The idea was The arrival of the PC and the sound card during to send the least number of bits possible during a the 1980s certainly came at the right time to give given transmission. For example: In the English amateur radio new avenues to explore. One of language, E is a very popular letter so it gets a them was turning radio teletype into a digital Varicode of 11. Z sees relatively little use, so its mode called PSK31. It was developed by a UK Varicode becomes 111010101. The Creed 7B teleprinter from yesteryear. radio amateur Peter Martinez, G3PLX. He had some clear objectives when he approached his As with RTTY, however, we still need a way to signal design. He wanted to create a mode that was as the gaps between characters. The Varicode does after you momentarily lose the signal. PSK31 will easy to use as RTTY, yet much more robust in terms this by using “00” to represent a gap. The Varicode not always provide 100% copy; it is as vulnerable of weak-signal performance. Another criterion is carefully structured so that two zeros never to interference as any other digital mode. There was bandwidth. The HF amateur radio digital sub- appear together in any of the combinations of 1s are times, during a geomagnetic storm, for bands are narrow and tend to become crowded. and 0s that make up the characters. example, when ionospheric propagation will exhibit poor phase stability. (When you are trying Peter wanted to design a mode that would do all But how would the average radio amateur to receive a narrow-bandwidth, phase-shifting of its tricks within a very narrow bandwidth. generate a PSK31 signal and transmit Varicode? signal, phase stability is very important.) “PSK” is the acronym for phase shift keying, the Peter’s answer was to use the DSP capabilities of the common computer sound card to create an modulation method that is used to generate the From BPSK to QPSK audio signal that shifted its phase 180° in sync with signal; “31” is the bit rate. Technically speaking, the 31,25 bps data stream. In Peter’s scheme, a 0 Many people urged Peter to add some form of the bit rate is really 31,25, but “PSK31,25” isn’t bit in the data stream generates an audio phase error correction to PSK31, but he initially resisted the nearly as catchy. Peter used the Morse code as a shift, but a 1 does not. The technique of using idea because most error-correction schemes rely basis, after all it is a simple binary code expressed phase shifts (and the lack thereof) to represent on transmitting redundant data bits. Adding more by short signal pulses - dots and longer signal binary data is known as binary phase-shift keying, bits while still maintaining the desired throughput pulses - dashes. By combining strings of dots and or BPSK. If you apply a BPSK audio signal to an SSB increases the necessary data rate. If you double dashes, one can communicate the entire English transceiver, you end up with BPSK modulated RF. the BPSK data rate, the bandwidth doubles. As alphabet along with numbers and punctuation. the bandwidth increases, the signal-to-noise Morse uses gaps of specific lengths to separate Concentrating RF into a narrow bandwidth does ratio deteriorates and you get more errors. It’s a individual characters and words. wonders for reception. But when you’re trying sticky digital dilemma. How do you expand the to receive a BPSK-modulated signal it is easier When it comes to RTTY we’re still dealing with information capacity of a BPSK channel without to recognise the phase transitions - even when binary data (dots and dashes) but instead of significantly increasing its bandwidth? they are deep in the noise - if your computer on/off keying, we send the information by shifting knows when to expect them. To accomplish this, Peter finally found the answer by adding a frequencies - frequency shift keying - FSK. One the receiving station must synchronise with the second BPSK carrier at the transmitter with a 90° frequency represents a mark (1) and another transmitting station. Once they are in sync, the phase difference and a second demodulator at represents a space (0). If you put enough mark software at the receiving station “knows” when the receiver. Peter calls this quadrature polarity and space signals together in proper order to look for data in the receiver’s audio output. reversed keying - however better known as according to the RTTY code, you can send letters, Every PSK31 transmission begins with a short “idle” quaternary phase-shift keying or QPSK. numbers and a limited amount of punctuation. string of 0’s. This allows the receive software to The younger amateurs of today will not know what The RTTY code shuffles various combinations get into sync right away. Thanks to the structure a teleprinter is, but with PSK31 they can relive the of five bits to represent each character. For of the Varicode the phase transitions are also days of keyboard-to-keyboard communication example, the letter A is expressed as 00011. To mathematically predictable, so much so that the without a monstrosity on their desks. separate the individual characters RTTY must also PSK31 software can quickly synchronise itself when add “start” and “stop” pulses. tuning in during the middle of a transmission, or (Based on an article by Steve Ford, WB8IMY) 52 July 2007 - EngineerIT.
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