Abstract: During the reading of serial data signals, for example, from a so-termed magnetic floppy disc, a synchronized condition between the local oscillator of the reading device and the timing of the data stream must be reached as quickly as possible after the start of the read operation. When the initial synchronization has been reached, readjustment of phase and frequency may take place only slowly, because the instants at which the actual signal transitions occur may exhibit a large spread with respect to the nominal instants. Therefore, during a synchronization operation an element of a read clock pulse train is applied to the phase comparator at each status transition of the medium as well as at further instants. The data sample signal of the clock pulse generator is applied at twice the bit frequency to the other input of the phase comparator.