Just like it says on the tin, it’s phase shift keying, and a proper implementation uses a phase detection circuit or something similar. [Craig] thought it would be fun to build an analog BPSK31 ...

The goal was to decode the frequency-shift-keyed (FSK) signal into an 8-bit parallel output, and maybe drive a seven-segment display as the characters came off the tape at a screaming 300 baud.