GB1123180A - Improvements in or relating to communication systems - Google Patents

Abstract

1,123,180. Pilot controlled A.V.C. systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 17 April, 1967 [20 April, 1966], No. 17432/67. Heading H4R. In a two channel communication system using the pilot operated level control system of Specification 1,084,651, in which the gain of the repeaters is controlled by a frequency modulated control signal transmitted along with the information signal, and in which the frequency of the control signal transmitted from one terminal to control the gain of repeaters in one channel is varied in accordance with signals, transmitted over the other channel, which depend on the received level at the other terminal, a comparison circuit is provided at one or both terminals to comprise the frequencies of the control signals in each channel and to give an alarm if the frequencies differ by more than a predetermined amount, indicating a fault in one channel. Fig. 1 shows a transmission system substantially identical to that shown in Fig. 2 of Specification 1,084,651, with the addition of the comparator circuits 46 and 46<SP>1</SP>. Each comparator circuit comprises two discriminator circuits, 47 and 48, connected to receive the control signal from a respective channel, each including a capacitor 49 or 50 charged by capacitors 54 or 55 and discharged by capacitors 56 or 57. The frequency discriminators 47 and 48 are connected in a bridge circuit with zener diodes 51 and 52 and it is shown that the current through the contact ammeter 53 is approximately proportional to the frequency difference between the control signals. Therefore if the frequency difference exceeds a predetermined amount the contact ammeter makes one or other of its contacts to operate an appropriate alarm. With two comparators, one at each terminal, the operating points of the contact ammeters are preferably displaced so that the alarm is given first at one particular terminal for an increase in damping in a respective direction of transmission.