GB283135A - - Google Patents

Abstract

283,135. Telefonaktiebolaget L. M. Ericsson, (Assignees of Laurent, T.). Jan. 4, 1927, [Convention date]. Manual exchange systems. - In systems in which currents of a broad range of frequencies such as voice frequencies and other signals of a narrow range within the broad range such as calling and clearing signals are transmitted over the same long-distance line, the receiving arrangements comprise a blocking device responsive to frequencies other than the signal frequencies to prevent incorrect operation of the signalling means by speech currents. In one arrangement, Fig. 1, the anode circuit of an amplifier 10 associated with the incoming end 11, 12 of the line contains filter circuits 15, 16, the former being non-permeable to signal currents so that these pass through transformer 6 to operate a relay 2, whilst the circuit 16 passes only signal currents so that all other frequencies pass to transformer 7 and operate relay 3. Signal currents thus operate signalling device 1 whilst other voice frequencies hold open the signalling circuit. The construction of filters 15, 16 is such that if voice currents of signal frequency arrive at the commencement of operations, relay 3 will act more quickly than relay 2 to prevent its falling back in response to momentary speech-currents of signal frequency. In another arrangement, Fig. 2, the signal 1 is controlled by a single relay 19 in the anode circuit of a valve 20 to the grid of which are connected neon tubes 25, 26. Signal currents passing through transformer 6 render tube 25 conducting and increase the positive potential on the grid, thus operating relay 19. On the other hand, voice currents render tube 26 conducting and make the grid more negative. In a modification, the secondary of transformer 6 is directly connected to the grid of valve 20, but a neon tube is connected directly between grid and filament, The invention is stated to be applicable also to wireless telephone systems, to systems in which a carrier current of voice frequency is employed and to telegraph systems.