GB836037A - Improvements in or relating to electric communication systems - Google Patents

Abstract

836,037. Automatic volume control. GENERAL ELECTRIC CO. Ltd. Nov. 22, 1957 [Nov. 27, 1956], No. 36287/56. Class 40 (4). [Also in Group XL (c)] A terminal station of a communication system has a receiver the gain of which is controlled by the amplitude of a received signal, discontinuation of the received signal disconnecting power from equipment at the terminal other than that required for listening out and also disconnecting the automatic gain control of the amplifier and substituting a fixed gain. The arrangement avoids spurious operation due to amplification of noise since with no received control signal the fixed gain of the receiver is below maximum. The system envisaged utilizes both-way multichannel carrier working over open-wire lines. Transmission is effected over a converter 4, in which a relay S may be used to interrupt the carrier frequency of a particular channel, a compressor 6, a modulator 7 with channel oscillator 8, amplifier 9, a filter, and a coupling and matching unit 12 to line 11. Reception is made over a filter, amplifier 18, demodulator 19, amplifier 16, and expander 15. Demodulator 19 also provides an input for a receiver 20 which recognises the presence of a carrier signal incoming on the line 11 and controls relays A and R. Relay R repeats interruptions in carrier frequency used for code signalling. Relay A connects power to compressor 6, oscillator 8, amplifier 9, expander 15 and amplifier 16, and connects up the thermistor 21 in the automatic gain control of amplifier 18, in response to the received carrier signal. Power is also connected to the above-mentioned circuits when relay S is operated for transmission. The amplifier 18 may comprise the three-stage transistor amplifier as shown, in which feed-back from winding 51 of the output transformer 36 in the push-pull stage 33, 34, is rectified and applied to resistor 48. A decrease in output will lower the potential across resistor 48 and increase the numerical value of the bias to the base of transistor amplifier 39, thereby increasing its gain. A thermistor 21 acts in opposition to the voltage feed-back and limits the variations in gain control. Contacts A2 disconnect the feed-back loop and connect up a fixed bias of intermediate value in the absence of a received carrier signal.