phase difference modulation (PDM)
Phase difference modulation( PDM) is a special form of phase modulation( PM) in which the phase difference between two consecutive AC signals is transmitted in encoded form.
In phase difference modulation, several binary symbols are combined to form an AC signal. Examples are V.26 with a two-bit transmission and V.27 with a three-bit transmission.
In V.26, 2 bits are combined to form dibits 00, 01, 11, and 10 and transmitted with quaternary signals that differ in phase differences of 45 degrees, 135 degrees, 225 degrees, and 315 degrees.
In V.27, 3 bits are combined into tribits 000, 001, 010, 011, 111, 110, 100, and 101 and transmitted in octonary signals differing in degrees 0, 45, 90, 135, 180, 225, 270, and 315.