1. In a signaling system, a source of signal waves, a source of key waves, a transmission line or channel, means to allot periodic time intervals on said line to the transmission of signal indications thereover derived from said signals, means to produce signal indications for transmission and to encipher them comprising means to produce a pair of pulses of energy separated in time, and means to vary the absolute time positions of said pulses together under the conjoint control of said signal and key waves, one or the other of said two pulses always occurring within said time interval and the other always occurring outside said time interval, and means to transmit the former pulse over said line.