PSK31

Background

PSK31 was developed and named by English amateur radio operator Peter Martinez (G3PLX) and introduced to the wider amateur radio community in December 1998. The 31 baud BPSK modulation system was introduced by Pawel Jalocha (SP9VRC) in his SLOWBPSK program written for Motorola's EVM radio.

Instead of traditional frequency-shift keying, information is transmitted by patterns of polarity-reversals (180-degree phase shifts). PSK31 was enthusiastically received and its usage spread rapidly worldwide, lending new popularity to on-air digital communications. Due to its efficiency, it became especially popular with operators who cannot install large antenna systems or use high power.

PSK31 is typically created by software that generates an amplitude- and phase-modulated waveform converted to audio by a sound card. In BPSK31, binary information is transmitted by either a 180-degree phase shift (binary zero) or no phase shift (binary one) in each 32 ms symbol interval. The Varicode, a kind of Fibonacci code, marks character boundaries with two or more consecutive zeros, and more frequent characters have shorter encodings. The symbol rate of 31.25 Hz was chosen because a normal typing speed of about 50 words per minute requires about 32 bits per second, and because 31.25 Hz can be easily derived from the 8 kHz sample rate used in many DSP systems.