What type of modulation is used for digital modes like PSK31?

The choice of phase shift keying as the modulation type for digital modes like PSK31 is based on how the information is encoded and transmitted. In phase shift keying, digital data is represented by varying the phase of the carrier wave. This means that different phase shifts correspond to different symbols or bits in the transmitted information.

PSK31 specifically makes use of this modulation technique to transmit data efficiently over amateur radio frequencies. It is designed for low-bandwidth utilization while providing a robust method for data transmission, making it particularly effective in noisy conditions or where bandwidth is limited. Each symbol in PSK31 typically represents multiple bits of information, maximizing the efficiency of the communication.

In contrast, other modulation techniques like amplitude modulation or frequency shift keying do not operate on the same principle of varying the phase of the wave to encode data. Amplitude modulation changes the strength of the carrier wave, while frequency shift keying alters the frequency itself to represent information, neither of which correlates directly to the operation of PSK31. Therefore, phase shift keying is the correct modulation method employed for this digital mode.