GB540233A - Electrical signalling systems - Google Patents

Abstract

540,233. Secret transmission. STANDARD TELEPHONES & CABLES, Ltd. May 7,1940, No. 8188. Convention date, May 19, 1939. [Class 40 (iv)] [Also in Group XL] In a system in which a signal wave such as a speech wave is transmitted in the form of pulses or trains of waves of short duration compared with the intervals between the pulses or trains, parasitics occurring in the intervals are balanced out at the receiver by parasitics of similar wave form received on a channel of adjacent frequency, the adjacent channels being received by a common H.F. or I.F. amplifier and subsequently separated by filters the outputs of which are applied to a differential detector. In system also providing automatic balancing of the parasitics, Figs. 2 and 3, the pulses marking the leading and trailing edges respectively of variable duration pulses are transmitted on two different frequency channels, from which balancing parasitics are derived at the receiver, and automatic correlated gain control of the two channels is provided at the transmitter and also at receiver. At the transmitter, Fig. 3, the speech wave is applied at 31 to a multi-vibrator 32 providing outputs of variable duration pulses depending on the amplitude of the wave. Sets of short unidirectional pulses marking the leading and trailing edges of a set of variable duration pulses are derived across resistances 35, 36 due to condensers 33, 34 and the sets are used respectively to key or shock excite oscillators 37, 38 of different frequencies coupled over 39 to the aerial. The outputs of the oscillators are also applied to a pair of differentially connected rectifiers 40 and the differential output is applied to bias the oscillators to vary their outputs in opposite senses, thereby equalizing them. At the receiver, Fig. 2, the output of the intermediate frequency amplifier 15 is applied to a pair of filters 16, 17 which separate the two channels, each of which comprises one set of pulses and accompanying parasitics. The outputs of the filters are amplified at 18, 19 and applied to rectifiers 20, the pulse outputs of which are applied over condensers 21, 22 to a double stability trigger device 23 from which the original wave is derived in known manner. If the two branches are balanced, the parasitics in the outputs of 20 cancel out. The balance is maintained automatically by applying the voltage developed due to lack of balance across resistance 25 shunted by condenser 28, to vary the bias of the intermediate frequency amplifiers in opposite senses to restore balance.