GB780866A - Improvements in or relating to secret telephone systems - Google Patents

Abstract

780,866. Secret transmission. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 10, 1955 [Nov. 18, 1954], No. 33397/54. Class 40 (4). A transmission system is made secret by periodically inverting the phase of the transmitted audio waveform which is synchronously re-inverted at the receiver, phase inversion being achieved by passing the audio wave form through a transformer having a centre-tapped secondary and periodically and alternately taking output from between one or other end of the secondary and the centre tap. The switching between one end or the other of the transformer may be effected by relays but preferably, as shown in Fig. 2, square waves (a) and (b) of Fig. 4, are applied to the grids of switching diodes which in consequence supply a transformer T9 with a periodically phase-inverted wave form. At a receiver square waves (a) and (b) are synchronized by any known means with those at the transmitter to re-invert the wave form. In Fig. 3 pentodes are used to cause phase inversion and comprise balanced gates operated by wave forms (a) and (b) the output being taken from the common anode terminal T8. A variable cathode leg resistance is used to obtain no output when the pentodes are switched in the absence of audio input.