Modulation in which the instantaneous angle varies in proportion to the modulating waveform. When used in an analog manner, it is referred to as either frequency modulation (FM) or phase modulation (PM), depending upon whether the instantaneous frequency or instantaneous phase varies with the modulation. When used in a digital modulation format, it is referred to as either frequency-shift keying (FSK) or phase-shift keying (PSK). Both FSK and PSK can be used with either a binary or an M-ary alphabet, where M is an integer greater than 2. Indeed, the most common forms of PSK correspond to an input alphabet size of either two or four; while for FSK alphabets of two, four, or eight, symbols are typically employed.