Patent GB-830087-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: CORNER ARTHUR CHARLES
IPC: H04Q5/00
Filed: 1956-09-17
Abstract: 830,087. Party-line systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd., and BRITISH TELECOMMUNICATIONS RESEARCH Ltd. Sept. 3, 1957 [Sept. 17, 1956], No. 28389/56. Class 40 (4). [Also in Group XL (c)] Selective calling of a wanted substation is effected by the transmission of low-frequency alternating current together with audio-frequency current of a frequency characteristic of the wanted substation, each substation having an " active filter " (defined in the penultimate paragraph below) tuned to the frequency characteristic of that sub-station. Fig. 1 shows one of a plurality of substations connected (by way of isolating transformers T2 if required) to a line L1, L2, which is normally extended over leads CL1, CL2 to a control station, possibly by way of an isolating transformer T1. To call a particular substation, an operator at the control station selects the appropriate audio-frequency signal and then by means of a ringing key operates contacts RR1, RR2, causing this signal superimposed on normal 17 c/s. ringing current to be applied to the line L1, L2 from leads SL1, SL2. The ring- ing current breaks down neon tubes NT1 at all the substations and, by means of rectifier bridges MR1, supplies power to " active filters " F1, which are described below with reference to Fig. 2. At the particular substation corresponding to the selected audio-frequency signal, the filter F1 is effective to operate relay RA which connects bell B1 into circuit to be rung by the 17 c/s. ringing current. The restoration of contacts RR1, RR2 completes the speaking circuit. A non-linear resistor NLR1 prevents overloading if an unduly high potential is applied to the line. In a modified arrangement, Fig. 3, mainsfrequency current of 50 c/s. provides the power for the " active filter " F2, the bell B2, and the microphone (by way of rectifier bridge MR4 and smoothing circuit SM). Low-voltage mains-frequency current over transformer T3 supplies microphone current, and this voltage is increased for ringing when contacts RR1, RR2 and RR3 are changed over by the operation of the operator's ringing key, contacts K1 and K2 being operated first and remaining operated throughout the call. Filters FL1-FL8 keep speech and mains-frequency currents to the right paths. A drop indicator I responds to A.C. from the magneto of any calling substation on the line. An " active filter " is a filter device incorporating thermionic valves or transistors so that a power supply in addition to the selective current is required. In a suitable circuit, Fig. 2, transistors V1, V2, V3 form an audio-amplifier having a bridged-T feedback network arranged so that the gain of the amplifier is very low except at the rejection frequency of the network. The output of the amplifier is applied by way of a shunt rectifier MR2 to a monostable circuit comprising transistors V4, V5, V6 having relay RA in its output circuit. The audio-frequency selective-ringing signals may be obtained from a circuit similar to the amplifier portion of the filter shown in Fig. 2, made to oscillate at the rejection frequency of the feed-back network.