GB1252740A - - Google Patents

Abstract

1,252,740. Secret transmission. PLESSEY CO. Ltd. 7 Feb., 1969 [9 Feb., 1968], No. 6612/68. Heading H4R. In a speech secrecy system the speech signal is differentiated and fed to a phase splitter 3 the two outputs, 4 and 5, of which are selected alternately for connection, via a filter 8, for transmission. The selection is controlled by a digital code generator, 7, comprising a shift register with two stage coupled, via an "exclusive or" gate, to the register input. At the receiver the incoming signals are fed to a phase splitter with its outputs selected by a switch controlled by a similar digital code generator to that used at the transmitter, the selected signals being integrated before reproduction. In order to remove the syllabic rhythm of the speech a speech detector may be employed to switch a constant frequency bias tone to the coder input during inter word gaps in the speech. At the receiver a similar speech detector may be employed to remove the bias tone from the decoded speech, or a notch filter tuned to the bias tone may be used. Distortion at the receiver due to the band limiting characteristic of the transmission channel on the switched signal can be reduced by the circuit shown in Fig. 5 in which the incoming signal is sampled at a point immediately before the switching takes place, using an appropriate shift register tapping and delay 28. The sample is then gated to a pulse forming network 32 and added to the incoming signal applied to the phase splitter.