23 Nov 18

PSK31 derived from the 31 baud BPSK modulation system used in PSK31 was introduced by Pawel Jalocha (SP9VRC) in his SLOWBPSK program written for Motorola’s EVM radio. Instead of the traditional frequency-shift keying, the information is transmitted by patterns of polarity-reversals (sometimes called 180-degree phase shifts).

For more:

Wikipedia… Click Here

QST Article April 1999 … Click Here