Abstract: A vestigial-sideband transmission system for synchronous data signals. The transmitter is provided with a filtering and modulating circuit for generating a vestigial-sideband channel signal which at the location of its carrier frequency and at the location of a frequency which is spaced from this carrier frequency by a distance equal to one half of the clock frequency of the data signals is a double-sideband modulated signal. In the receiver the reference carrier for coherent domodulation and the reference clock signal for regeneration can be simply recovered from the transmitted vestigial-sideband channel signal itself.