Patent GB-1078946-A (GB)
Title: Improvements in or relating to communication systems for the transmission of signals
IPC: H04B3/10
Filed: 1965-02-09
Abstract: 1,078,946. Automatic volume control systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Feb. 9, 1965 [Feb. 12, 1964], No. 5533/65. Heading H4R. In a communication system in which a number of parallel transmission lines each include a number of repeaters at least one of which includes an adjustable level controlling impedance, the impedances are controlled collectively in the various transmission lines by an output signal derived from an averaging device connected to receive the pilot signals of each of the transmission lines. The final station in each line has a level control circuit which is controlled by the pilot signal individual to that line. Fig. 3 shows a preferred embodiment of the system which includes two lines 2 and 2<SP>1</SP> each including a number of repeater amplifiers and a terminal station. At the terminal station a pilot signal received over the line is applied via a selective amplifier 37, a rectifier 38, and a comparator 39, to an averaging device 36 which derives a signal corresponding to the mean level of the pilot signals in all the transmission paths. The mean pilot signal is applied to drive a motor 146 which adjusts a capacitor 48 in a capacitive voltage divider fed with an alternating voltage from an oscillator 50. The A.C. output from the voltage divider, whose output is proportional to the mean pilot level, is rectified and amplified and applied via D.C. converters 46, which may be of the type described in detail in Specification 1,078,945, to adjust the values of feedback resistors in those repeaters provided with level control circuits. Thus the gain of the repeater amplifiers is adjusted to maintain the mean pilot signal level at a predetermined value. In addition to the common control of the repeater amplifiers 6,8,10, ... 6<SP>1</SP>, 8<SP>1</SP>, 10<SP>1</SP> &c. an amplifier, i.e. 28, 28<SP>1</SP> in the terminal station is also provided with level control, but in each case by the signal appropriate to its own particular channel so as to bring the pilot level in each channel to its nominal value. Interaction between the two forms of control on each line is reduced by the provision of the motor adjusted level control which is slow acting in conjunction with the fast acting individual control at the terminal station. Fault conditions on any one line cause the operation of an over-voltage relay B or an under-voltage relay A which disconnects the faulty line from the averaging device 36 at a or b. In an alternative arrangement where the controlled repeaters are more widely spaced the arrangement of Specification 1,078,945 may be employed in which the common level control circuit is arranged to over-compensate for changes in mean pilot signal level which arrangement ensures that the mean level in the line remains closer to the nominal value, Fig. 4 (not shown). In a further embodiment the individual pilot signals are connected to the averaging device by a diode network so that only the highest pilot signal and the lowest pilot signal are applied to the averaging device 36, Fig. 6 (not shown).