The transmitter in a digital communication system sends a synchronization sequence that uniquely identifies the polarity of the received data. From the received data, the receiver produces non-inverted and inverted polarity data streams that couple to like synchronization detectors. Only the detector to which the correct polarity data stream is coupled responds. Logic circuitry determines which detector responds and, accordingly, selects the polarity of received data to be further processed by the receiver.After polarity is established, both detectors monitor the received data stream of the selected polarity to determine whether synchronization shifts after having been initially established. Monitoring continues until terminated by other receiver circuits.