So what is PSK31 all about? This page is not intended as a complete tutorial covering all facets of PSK. It's an overview of PSK, enough for you to understand the terminology and concepts, and give you a head start towards further research.
What is PSK?
PSK stands for Phase Shift Keying. In the context of amateur radio, it describes a family of operating modes (each of which has a slight variation in the signaling parameters) that are used for on-the-air keyboard-to-keyboard contacts.
Where did PSK31 come from?
Phase Shift Keying has been around as a digital modulation scheme in the commercial and military worlds for many decades. Hams (notably SP6VRC) realized in the late 1990s that computer technology, including the ubiquitous sound card, had progressed to the point where simple software could be used to modulate a PSK signal at an audio IF, which could then be upconverted to RF and transmitted using a conventional SSB transmitter. (The receive process is the reverse - the SSB receiver downconverts the incoming PSK signal to the audio IF, and the computer/soundcard demodulates it.)
Peter Martinez (G3PLX) established signaling parameters (bit rates, coding schemes, etc.) and called his product PSK31. This included two variants: BPSK31 and QPSK31. Binary PSK transmits one bit at a time, using one of two possible phase states. Quaternary (or Quadrature) PSK sends two bits at a time, using one of four possible phase states. Both send the same number of phase transitions per second, so QPSK can send twice as many bits as BPSK. Martinez used this extra capacity to include a rate-1/2 convolutional code to improve its efficiency. He selected 31.25 baud (31.25 phase transitions per second, rounded down to 31 for use in the name) as a reasonable rate that allowed text to be sent at approx. 50 wpm and could also be easily derived from the 8 kHz. sampling rate common in DSP systems (like a computer sound card).
Others have developed extensions of the PSK31 concept to enhance its capability for specific applications. For example, if you want higher text speeds, you could try PSK63 and PSK125. For operation at lower signal-to-noise ratios, there is PSK10. All of these, and even the original QPSK31 variant, are much less common on the air than BPSK31.
How does PSK work?
The generation of a PSK31 signal can best be described in the following sequence of four steps. Actual implementations may not follow precisely this sequence, but the result is the same.
1. Each individual keyboard character is encoded into a string of bits. This is done using a lookup table that implements what Peter Martinez called a Varicode, which is a variable-length coding scheme. Martinez examined the English language and the frequency of appearance of all the characters, and used shorter bit sequences for the most common characters and longer ones for the less-common ones. (Note that this is exactly what Samuel F. B. Morse did approximately 160 years earlier to develop the code that now bears his name.) Unlike RTTY (and Morse), Martinez included both upper-case and lower-case letters, and the lower-case ones get shorter codes because they are more common in written text. He also defined the possibility of "extensions" to the Varicode to cover additional character sets, like for non-English languages. Details of the Varicode can be found here.
2. PSK31 is defined to operate at 31 baud, which corresponds to 31 bps for BPSK. If the operator is not typing fast enough to create a stream of characters that generates bits at this rate, "fill bits" are inserted.
3. The 31 bps stream is then used to PSK-modulate an audio "carrier". (Technically PSK31 uses Differential PSK, where the "difference" between successive bits defines the transmitted phase. [For logic levels, if the two bits are the same, the difference is zero. If the two bits are different, the difference is one.] This allows for a simpler receiver, as it need not establish a phase reference other than the immediately previous bit.) The signal generated is then a carrier whose phase "shifts" 180 degrees when the next bit is the same as the previous bit, and doesn't shift at all when the next bit is different from the previous bit. (This may seem backwards compared to the definition, but it forces phase transitions even when the bit sequence is continuous "fill" bits, allowing the receiver to stay in synch.)
4. The final step is an amplitude modulation that reduces the amplitude to zero at the phase transitions. (The amplitude is not reduced if the phase is not shifted.) This has two benefits: First this serves as a timing reference for the receiver providing an easy way to determine when the transmitted signal is "shifting", and, secondly, the instantaneous phase shifts produce many harmonics; reducing the amplitude to zero at precisely that instant suppresses these sidebands.
Reception of the PSK31 waveform follows these steps in reverse:
1. The amplitude modulation is recovered to determine the timing reference.
2. The phase of the signal is sampled based on this reference and the phase shift sequence is recovered.
3. The fill bits are removed.
4. The remaining bits are translated into text using the Varicode lookup table.
How is PSK31 generated by an amateur radio station?
The classic PSK hardware configuration is a computer with sound card, an SSB transceiver and an interface between the two (for audio signals and sometimes the Push-To-Talk or PTT). The computer generates the PSK signal as an audio signal - effectively the audio serves as the first IF of the transmitter chain. The output of the computer sound card then feeds an SSB transmitter which is used only as an upconverter - the audio signal is simply translated to the RF spectrum. The translated frequency is equal to the sum of the transmitter frequency plus the "audio IF" for USB, and the transmitter frequency minus the "audio IF" for LSB.
PTT can be enabled in multiple ways, depending on how your interface is configured. Some radios can use their internal VOX functions, especially when using the MIC input. (This is not often available with DATA inputs.) Some interfaces receive "trigger" signals via a conventional serial port, or a USB port, and perform the level conversion necessary to "close" a PTT switch, either in the MIC jack or on a DATA jack. Some interfaces have internal VOX-type circuits, so the software doesn't have to do anything special, but the interface closes the PTT switch itself. And some software/interface combinations use CAT commands, the same type of signals used for computer control of the radio. Examples of these can be found on the PSK Hardware/Software page elsewhere on this web site.
What is the Audio Offset?
"Audio Offset" is another tem for the audio frequency at which the PSK signal is generated. It's called this because it's the "offset" between your transmitter dial frequency and the actual frequency at which your PSK signal appears in the RF spectrum. Additional information on this can be found on the PSK Frequencies page elsewhere on this web site.
So what's different about QPSK31?
The QPSK31 format transmits two bits per phase state, and transmits one of four phase states each time. (If you think about it, there are four possible combinations of two bits, and each of these combinations is assigned a unique phase.) This allows twice as many bits to be sent in the same period of time (and bandwidth) as with BPSK. Martinez coupled this with a rate-1/2, constraint length 5 convolutional code to improve the power efficiency of the modulation (defined as the signal-to-noise ratio required to deliver a specified bit-error-rate, or BER).
This adds a step in the transmission process - generation of the coded bitstream - after the insertion of fill bits. It also adds a corresponding step in the receive process. After the bits are recovered from the phase transition, a Viterbi decoder removes the redundancy inserted by the code, corrects bit errors (up to a point) and recovers the original bit stream for translation of the Varicode.
One interesting note is that while the BPSK31 signal is phase-symmetric, QPSK31 is not. This means that if you are using BPSK, it doesn't matter if you are using USB or LSB - the signal is the same. If you are using QPSK, both stations must be using the same sideband, or one must use the "invert" function in their software.
Randy, K7AGE, has produced numerous ham radio videos for You Tube. Four of these are an introduction to PSK31:
Many thanks to Steve, W3HF, for this page.