GB538807A - Electric signal transmission systems - Google Patents

Abstract

538,807. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.). Feb. 27, 1940, No. 3622. [Class 40 (iv)] In a carrier transmission system, a pilot current received at a main repeater station controls the strength of a second pilot current sent back to control the gain at the preceding repeaters. The second pilot current may also control the gain at the main station itself and at subsequent stations. In all cases gain is controlled by varying the heating of a substance having a high and preferably negative temperature co-efficient of resistance, e.g. silver sulphide or boron, in the beta circuit of the amplifier concerned. In one arrangement, Figs. 4 to 6 (not shown), the second pilot is controlled by the combined effect of two pilots received over separate lines and controls the gain of repeaters in a number of lines at each station. In a modification, Figs. 7 to 9, pilot current received over the uppermost line in the main station 5 is filtered out and rectified and control the amount of commercial frequency current supplied by a generator 71 to be passed on via an adjusting network 67 and hybrid coil to a control circuit 69 for the local amplifiers and via a network 83 and a further hybrid coil to a control circuit 85 which is coupled via a phantom circuit on the upper two pairs to the control circuit of the amplifiers at the preceding station 6. These control circuits are also fed with an adjustable amount of commercial frequency current via networks 68, 88 to determine the normal gain. The second pilot is also fed via a circuit 86 and a phantom circuit formed by the lower two pairs to similar adjusting networks and hybrid coils at the repeater station 8 for the control of gain there and at repeater station 7. At each station the ignition circuit of a three electrode cold-cathode discharge tube is fed with current direct from the commercial frequency source and with current from the gain-control circuit in opposition. Normally the tube remains inert but if the pilot current fails the tube breaks down and provides a circuit whereby the gain controlling elements are heated to an average extent. The circuit 87 may involve a phantom circuit on additional signalling pairs.