Where did PSK31 come from?
Phase Shift Keying has been around as a digital modulation in the commercial and military world for many decades. Hams (notably SP9VRC) 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 reversethe SSB receiver downconverts the incoming PSK signal to the audio IF, and the computer/soundcard demodulate it.) Peter Martinez (G3PLX) established the 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 twic
