Abstract: A filter for a telecommunication system, designed to suppress a signal frequency F.sub.a and to transmit an adjoining frequency F.sub.b without significant attenuation, comprises a resonant cavity of prismatic shape longitudinally traversed by a conductive coil wound on a hollow dielectric core into which an adjusting screw is inserted. One end of the coil is grounded at the cavity wall while its other end is left unconnected and forms with the adjusting screw a variable capacitance C.sub.d with distributed constants; a lumped-constant tuning capacitor C.sub.1 is connected between ground and an input lead connected to an intermediate tap on the coil, the inductance L.sub.2 " of that lead and the inductance L.sub.2 ' of the coil portion between the tap and ground forming an inductive voltage divider bridged by the lumped-constant capacitor C.sub.1. The remainder of the coil, lying between the tap and the unconnected end, forms an inductance L.sub.1 which together with the distributed-constant capacitance C.

Abstract: Two sets of line terminals in a pair of PCM head stations are interconnected by a set of transmission channels via a number of cascaded relay stations each including a line repeater for each channel. A service link extends from an address generator in one head station to a monitoring unit in each relay station, the monitoring unit including a regenerating circuit for throughgoing address codes and further including a discriminating network which, upon detecting an address code identifying that relay station, sends a sinusoidal priming signal to all the associated repeaters. A predetermined PCM test code, e.g. one containing a certain number of consecutive zeroes, is simultaneously transmitted from the same head station over the channel containing a selected repeater to be checked which, upon coincidence of the test code with the priming signal from the associated monitoring unit, closes a loop for feeding back the test code to the originating line terminal.