GB1234074A - - Google Patents

Abstract

1,234,074. Automatic gain control. SOC. ITALIANA TELECOMUNICAZIONI SIEMENS S.p.A. 1 May, 1968 [2 May, 1967], No. 20561/68. Heading H4R. In a communication system in which a pilot signal is transmitted to control the gain of a receiver amplifier the pilot signal is transmitted at one or other of two discrete levels to convey signalling information, and the receiver, on detecting the signalling level, inserts the appropriate attenuation factor in the gain regulating [loop to maintain the system gain constant. Fig. 1 shows a system in which a speech signal from line m is fed via hybrid F 1 and band-pass filter PB 1 to a modulator M 1 for transposition to the appropriate transmission band. A pilot signal from generator G is added to the speech signal before modulation, the level being switchable between two values dependent on signalling information, e.g. calling, selection, replay, or end of conversation, to be transmitted. At the receiver the incoming signal is amplified in variable gain amplifier AR demodulated in M 2 and the pilot signal extracted and rectified by detector D. A trigger circuit T discriminates between the two signalling levels of the pilot signal to operate relay H when the higher level signal is detected and as well as feeding the appropriate information to line U 1 the gain of the amplifier, AC, in the gain regulator loop of amplifier AR, is varied to compensate for the increased pilot level. Alternatively the level of the reference voltage from generator Z may be changed by relay H to a value corresponding to the nominal high level of the pilot signal from the nominal low level. The gain of the control amplifier AC may be varied by switching in or out, by contacts of relay H, a negative feedback resistor in the emitter circuit of a transistor amplifier, Fig. 2 (not shown). Alternatively the gain may be controlled electronically by a transistor shunting part of a potential divider in the control signal path, the transistor being switched between saturation and cut-off by the control voltage variation between the two signalling levels, Fig. 3 (not shown). Similarly the level of the reference voltage may be changed by a relay contact operated by relay H, Fig. 4 (not shown), or by a transistor switched between saturation and cut-off, Fig. 5 (not shown). During periods when no speech signal is being sent signalling may be transmitted by switching between low level pilot and no pilot.