A .pi./4-shift DQPSK modulator modulates a digitized voice signal and other information. An FM modulator modulates the analog voice signal and other information. The FM modulator is coupled to the quadrature mixers (109 and 110) of the .pi./4-shift DQPSK modulator. When an FM modulated signal is required, the mixers (109 and 110) are biased (114) to allow carrier feedthrough by applying a fixed, non-zero DC signal to one or both mixers (109 and 110). The carrier is then FM modulated using conventional methods such as voltage-modulation of a phase locked loop (PPL) (113). When .pi./4-shift DQPSK is to be generated, the conventional baseband I and Q vector-length signals (101 and 102) are applied to the mixers (109 and 110), and the carrier is left unmodulated by switching (115) out the input signal to the PLL (113). The PPL (113) will then generate only the carrier frequency to be mixed with the I and Q vector-length signals (101 and 102).