What type of modulation is used by JT65?

JT65 is a weak-signal digital protocol originally designed for EME (moonbounce) communications, though it became highly popular for terrestrial HF work. It utilizes a 65-tone Continuous Phase Frequency Shift Keying (CPFSK) modulation scheme, which is a form of Multitone Audio Frequency Shift Keying (AFSK). The 65 distinct tones allow the protocol to carry forward error correction and synchronize data efficiently under extreme noise and fading conditions. One tone is used for synchronization, while the other 64 carry the actual data. Other modes like PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) change the phase or amplitude of the signal, whereas JT65 strictly shifts between distinct audio frequencies. RTTY is a much simpler, two-tone FSK mode.