Level control circuitry for two way communication system

Abstract

Signals carried over a multiplexed transmission medium between first and second stations suffer transmission losses such that the signal one station generates has a substantially greater magnitude than the signal it receives from the other station. A differential amplifier is utilized to extract and amplify the weaker signal thereby establishing signal balance. Following the differential amplifier is an automatic gain control stage which includes novel sample and hold and activity checker circuitry. The activity checker responds to the frequency of input signals to either allow variable gain control action or, when the frequency is representative of noise, to hold the gain control at its instantaneous value prior to the onset of noise.