Friday, June 06, 2008

Digital Mode of the Week: PSK31 (Part 2 - Phase-Shift Keying)

PSK31 was developed in 1998 by Peter Martinez, a British ham with the call G3PLX. It was intended to improve on RTTY for the purpose of direct, keyboard-to-keyboard contacts and rag chews. PSK stands for Phase-Shift Keying, and 31 refers to the baud rate of 31.25. This sounds slow, but it is perfect for hand typing, and it was also easy to count down from the 8000-Hz sample rate common for sound cards of the era.

Unlike many digital modes, which continued to be somewhat esoteric for ham use, PSK31 caught on immediately. From very early on, it was extremely well suited for generating and decoding with user-friendly, sound card software on ordinary personal computers. In use, the mode proved to be very efficient in its use of power and spectrum, allowing hams to communicate on HF without the equipment taking over their lives (and pocketbooks).

Right now, one can tune to 14070 kHz any time the band is open and find multiple signals all warbling away. Often it sounds like an attack from extraterrestrial science fiction insects. Less active frequencies are 10138-40 and 7035.

While PSK31 can have different modulation schemes, the one most commonly used is Binary Phase-Shift Keying (BPSK). Data is transmitted by commanding the sound card to shift the phase of the baseband audio signal by 180 degrees, in effect inverting the polarity.

Ordinarily, this would create a tremendous key click, but the amplitude is synchronously shaped by cosine modulation at the same time. The result is a best-case bandwidth for a PROPERLY MODULATED signal that is theoretically equal to the baud rate - yes, 31.25 hertz!! That is narrower than "hard" on-off keyed CW at a comparable 50 WPM.

Of course, things in the real world are somewhat more complicated. As can be easily seen on the waterfall, many signals have varying amounts of clicking or IM audio distortion, and they are maybe 40 or 50 Hz wide. This gets worse if the operator is hitting it too hard and causing more sidebands to appear. For the most part, though, PSK31 is one of the narrowest modes ever designed for hams.

The narrow bandwidth and low data rate make PSK31 practically immune to selective ionospheric fading (that Moog whooshing sound you hear on wider modes like HFDL and STANAG 4285). It performs less well under phase and Doppler distortion such as from aurora.

In this keying system, any phase flip signals a 0 bit. If there's no shift at the right time, it's a one bit. As we've seen, the state doesn't matter, it's the change in state. Again, it can be tuned in USB or LSB, though USB is traditional.

A character space is two zeroes - 00. This is why all the characters end in 1.

The PSK31 idler consists of all zeroes, which flip the phase at the baud rate. This and the cosine shaping produce what is basically double-sideband suppressed-carrier emission (J2B). ITU refers to it as 60H0J2B, in the long international designators. This means suppressed-carrier modulation for automatic reception using a maximum channel width of 60 hertz.

The transmission starts with a distinctive railroad track pattern on the waterfall. Bursts of hand typing create something looking more like a DNA molecule. At the end of the transmission, it all collapses back into the single tone.

Receiving consists of syncing to the baud rate, which is determined from the signal, and sending the resulting data to a Viterbi decoder. There are no connections, and no error checks. Stations take turns sending, just like RTTY. Character hits print gibberish, again like RTTY. However, the copy is usually much better on HF, everything else being equal.

There's not much else to know. BPSK31 is very uncomplicated to the user. You click on a signal and copy comes out.

PSK31 immediately gave rise to a bewildering array of phase-shift keyed, character-based, half-duplex modes. If you've used MultiPSK you know what I'm talking about. Each new version has more little buttons and more funny noises than the last one. Since decode sync is determined from the signal, there's also an autodetect mode.

An early variant that should be in just about all PSK31 software is QPSK31. This stands for quaternary phase-shift keying. On the standard computer phase constellation display, you will see four points instead of two. This is done through the use of a second BPSK carrier shifted 90 degrees (in quadrature), driving a second receive demodulator.

While this should double throughput, the extra capacity is used for an error check and more robust decoding. The performance of this mode on degraded circuits is impressive, but at the cost of more bandwidth and a tuning precision challenging even modern solid-state radios.

Other BPSK and QPSK modes are at 10, 63, 125, and 250 baud. There's also PSK220F, a 220-baud mode well adapted to broadcasting. It is used by the Cuban numbers operation at times.