(Courtesy of Don Veckarelli, W4AWP/AFA4IX )
What is PSK31?
Essentially PSK stands for phase shift keying. It is a digital communications mode which is intended for live keyboard-to-keyboard conversations. When PSK is transmitted it involves the modulation of a PSK signal in an audio IF and then converting it to RF and transmitting it using SSB. When receiving the reverse takes place, i.e., the SSB receiver changes the incoming PSK signal into audio tones and the computer soundcard decodes or demodulates them. Peter Martinez, G3PLX, is the one credited with establishing the signaling parameters, such as, bit rates, coding schemes, etc, and has given the name to this mode of PSK31. If you break down the name it stands for “phase shift keying at a 31 baud rate”. The actual baud rate is 31.25 but Peter shortened the title name to use 31. The bandwidth of the signal is very narrow, in the neighborhood of 60 Hz or less. For this mode a single tone is used. PSK31 has two variants: BPSK31 and QPSK31. BPSK31 stands for binary and QPSK31 for quarternary. The binary variant is what one will hear mostly on the amateur bands. It is phasesymmetric, i.e., it doesn’t matter if you are using USB or LSB – the signal is the same. For QPSK31, however, both stations must use the same sideband. BPSK31 involves the sending of one bit at a time with two possible phase states where as QPSK31 involves the sending of two bits at a time with four possible phase states. Note: both variants are sending the same number of phase transitions per second but QPSK31 is sending twice as many bits. The 31.25 baud rate used correlates to about 50 words per minute. PSK also comes in other variants, such as, PSK10, PSK63 and PSK125. PSK10 is slower than PSK31where as both PSK63 and PSK125 are faster.
How does PSK work?
Each individual keyboard character is encoded into a string of bits. These bits are based on a varicode which is a variable-length encoding scheme. Different characters are represented by variable-length combination of bits. Shorter ones are used for the more common letters thus improving efficiency in terms of average character duration. This varicode is self-synchronizing, i.e., no separate process is needed to define where one character ends and the next begins. PSK31 adds fill bits, inserted when the operator is not typing fast enough to create a string of characters. This 31.25 bps character stream created is then used to phase shift modulate an audio carrier. This created signal represents a carrier whose phase shifts 180 degrees when the next bit is different and remains unchanged when the next one is the same as the previous on. At phase transitions the amplitude modulation is reduced to zero. So in conclusion, PSK is a phase shift mode that has several variants. As you remember Pactor was FSK or frequency shift keying. The common variants are referred to as BPSK31 and QPSK31. This digital mode is a keyboard to keyboard one that uses a narrow bandwidth providing typing speeds of about 50 WPM. To use it one requires a computer with a soundcard and a program such as FLDIGI. If you are already set up to send and receive MT-63 then you are set up to use this mode too.
For a good overall view of all the Digital Modes including examples of what they sound like go to:
.. below is a specific more technical explanation of PSK31..