What are ASK, FSK, and PSK digital modulation schemes?

ASK (Amplitude Shift Keying) varies carrier amplitude to represent digital data; simple but noise-susceptible. FSK (Frequency Shift Keying) uses different frequencies for different symbols; good noise immunity but requires more bandwidth. PSK (Phase Shift Keying) varies carrier phase; efficient bandwidth usage and good noise performance. BPSK uses two phases (180 degrees apart), QPSK uses four phases encoding 2 bits per symbol. PSK and its variants are widely used in modern digital communications.