A synchronous detector for demodulating and decoding a digital data signal modulated either according to a vestigial sideband or a quadrature amplitude modulation scheme is based upon the principle of recognizing that a vestigial sideband signal may be regarded and treated as an offset-keyed QAM signal. The detector comprises a tuner for tuning to a center frequency symmetrically displaced within the transmitted digital data signal frequency spectrum and a decoder circuit selectively operating to reconstruct the transmitted digital data stream according to the duration of a transmitted data symbol. A related method of demodulating and decoding a modulated digital data stream comprises analogous steps of tuning to a center carrier frequency and selectively switching between in phase and quadrature arms of the demodulator such that the switch resides in one position or the other for a duration equal to half the duration of a transmitted data symbol. A related modulator for modulating a digital data signal either according to a vestigial sideband or quadrature amplitude modulation scheme comprises a switch for selectively switching an input digital data signal between real and imaginary arms and a modulator for modulating at center frequency. In one modulator embodiment, a noise cancellation circuit may be coupled between the real and imaginary arms.