GB580167A - Improvements in or relating to secret transmission systems - Google Patents

Abstract

580,167. Secret transmission. STANDARD TELEPHONES & CABLES, Ltd., and LEVY, M. M. July 9, 1941, No. 8643. [Class 40 (iv)] [Also in Group XL] In a secrecy system in which separate channels are employed for the transmission of two sets of signals one of which conveys the message in distorted form while the other conveys the " key " to the distortion, one set of signals is applied directly to one channel and through a distorting network to the other, and a similar or an inverse network is used at the receiver. The two channels may be provided by frequency or phase modulation and amplitude modulation of a single carrier. In Fig. 2, speech is masked by mixing it with noise, the noise currents N being applied directly to channel 2 and through a network # to channel 1, to which the speech currents S are also applied. The noise currents N pass through a similar network at the receiver before being combined with the currents from channel 1 to give the original speech. In Fig. 4, the speech is inverted by the use of a wobbling carrier, the inverted speech currents IW being applied directly to channel 2 and through a network # to channel 1, which also transmits the carrier W. At the receiver, the currents from channel 2, after distortion, are combined with those from channel 1 to provide the wobbling carrier for restoring the inverted speech to its original form. In both these arrangements, the noise or inverted speech may be applied directly to channel 1 and through a distorting network to channel 2, the latter then being provided with an inverse or restoring network at the receiver. In a further arrangement in which speech is inverted intermittently under the control of an electronic multi-vibrator, the resulting speech currents are applied directly to one channel and after distortion to the other channel which also carries synchronising signals. The distorting network may take the form of a low-pass filter terminated in a resistance different from its characteristic impedance. When inverse networks are required, one of them may consist of an amplifier with a feed-back path.