Sampling system for decoding biphase-coded data messages

Abstract

A receiving station of a data-transmission system decodes an incoming pulse train in the shape of a differentially biphase-coded carrier wave whose cycles represent respective message bits; the wave undergoes a phase reversal at the beginning of every cycle in which the transmitted binary signal has the logical value '1' but does not change in phase when that value is '0.' The received pulse train is differentiated to yield either one or two spikes during each cycle, one such spike invariably occurring in the middle of a cycle. Since only these periodically recurring spikes determine the instants at which the carrier wave must be sampled during decoding, a gate in the differentiator output is blocked during part of each cycle by a locally generated square wave which also controls the sampling. A phase detector, comprising several cascaded flip-flops, receives the unsuppressed spikes together with the local square wave and shifts the latter by half a cycle whenever the spikes clearing the gate recur irregularly because blocking takes place at the wrong time.