A digital mode for radio transmission of text that seems to be on its way to supplant RTTY as the mode of choice for keyboard conversations among radio amateurs. It has many advantages, such as great readability with low power or poor band conditions, low bandwidth, and ease of use. It was created by Peter Martinez, G3PLX.

A PSK31 transmission is based on an SSB signal that is modulated by an audio tone. The changes in phase of the audio tone conveys the data bits of the protocol; no change means 0, while a phase change of 90 degrees means 1. PSK31 can transmit 31 bps, thence the name. The nominal bandwidth is also 31 Hz, provided there is no transmitter overdrive. (Yes, there is a relationship here, but I lack the mathematics competence to explain or even understand it.) Compare this to a typical SSB signal of 2.4 KHz, or a well-shaped CW signal of 50 Hz.

To minimize the transmission time, PSK31 uses the Varicode alphabet, which has bit sequences of variable length. As morse code, it is based on letting the more often used characters being represented by a shorter bit sequence. Segregation is guaranteed by having "00" as a character separator, and having no character with "00" in its bit sequence.