Abstract: A phase shifting circuit includes: a pair of input terminals; a pair of output terminals; a variable gain amplifier and an active alternating current integrator, connected in series between the input terminals and the output terminals; and a control circuit for controlling the gain of the amplifier in dependence on the voltage between the input terminals and the voltage between the output terminals so as to maintain the amplitude of the voltage between the output terminals substantially equal to the amplitude of the voltage between the input terminals. The phase shifting circuit exhibits low variation of phase shift with input signal frequency.

Abstract: A multi-link intercom system having a large number of stations selectively connected to one of eight links by respective station controllers. When a calling station goes off-hook, its station controller connects the station to the first available link during the one of four link access periods corresponding to that link. The calling station then applies appropriate signals to the link identifying the called station. These signals are decoded by a link controller for the link which generates called station address signals during its corresponding link access period. All of the link controllers generate called station address signals on the same bus, but the signals are time multiplexed to occur during a portion of each link access period corresponding to each link. The station controller for the called station recognizes its address and connects its station to the link corresponding to the portion of the link access period in which the station address was received.