GB1016284A - Privacy transmission systems - Google Patents

Abstract

1,016,284. Secret transmissions. WESTERN ELECTRIC CO. Inc. Dec. 13, 1962 [Dec. 18, 1961], No. 47063/62. Heading H4R. A message signal is rendered secret by marking it with a number of spaced echoes of the signal itself. Preferably, reverse reverberation is employed, e.g. a number of spaced echoes of increasing amplitude preceding the signal. The reverse reverberation may be obtained by the circuit of Fig. 3 comprising a delay line 31 having a series of taps 32 ... 35 which are taken through amplifiers 37 . . . 40 having progressively increasing gain, the outputs being combined in an adder network 42 with the signal itself fed through a negative gain amplifier 41. The contributions of the several amplifiers are indicated in Fig. 2 (not shown). A number of such circuits may be employed in series, each having different parameters, and if desired, normal reverberation may be added. A somewhat similar circuit may be employed at the receiver for cancelling the reverse reverberation, or use may be made of the normal reverberator having a feedback loop described in Specification 1,001,518, Fig. 5 (not shown). In practice the delay line may be realized by a rotating magnetic drum having a single recording head and a number of spaced pick-up heads, Figs. 8, 10, 12 (not shown).