Abstract: In a line network communication system coded data signals are transmitted on a base band carrier signal and are recovered in a receiver. The receiver includes an exclusive OR gate and a shift register which operate in conjunction to compare, during successive time intervals, a series of coded reference signals and a succession of received signals shifted in phase relative to one another by respective amounts to generate respective comparison signals forming a set. A control unit causes a change in the relative phases of compared signals until the value of a comparison signal in a set indicates a degree of correlation exceeding a threshold value. Then, in a tracking mode, the control unit monitors the values of comparison signals generated in successive sets to verify that a degree of correlation exceeding the threshold value persists. If the verification is successful the control unit, in a locking mode, adjusts the relative phases of the compared signals to maintain a desired degree of correlation.