Abstract: A receiving station of a data-transmission system, designed to decode an incoming pulse train in the shape of a differentially biphase-coded carrier wave whose cycles represent respective message bits, includes a phase detector responsive to a presynchronization oscillation preceding an incoming message. The presynchronization signal, in the form of a square wave of half the frequency of the carrier wave, is fed to a phase detector along with a locally generated square wave of carrier-wave frequency. The phase detector, comprising several cascaded flip-flops, determines the relative time position of the local square wave and the presynchronization oscillation with the aid of an Exclusive-OR gate in the input of the first flip-flop and emits a corrective signal shifting the local square wave by half a cycle whenever the leading edges of the latter wave do not coincide with the beginnings of half-cycles of the presynchronization oscillation.