Patent GB-830046-A (GB)
Title: Improvements in voice frequency signalling equipment
Inventor: DAVIDSON JAMES ALEXANDER
IPC: H04Q3/00
Filed: 1957-02-15
Abstract: 830,046. Automatic exchange systems. MUIRHEAD & CO. Ltd. Feb. 13, 1958 [Feb. 15, 1957], No. 5366/57. Class 40 (4). [Also in Group XL (b)] In a communication system, e.g. for picture telegraphy, using a voice-frequency carrier and a number of lines converging to a trunk receiving centre, all the incoming lines are connected together through individual attenuating networks to provide a start signal, and tappings on the networks are individually connected to linefinding equipment, and a single voice-frequency detector responsive to the calling tone over any of the lines energizes the selector drive of the line-finder to test each incoming line for the calling tone until the calling line is found whereupon relays are energized to operate a supervisory lamp or other call-indicating device. As indicated diagrammatically in Fig. 1 the incoming 2-wire (or 4-wire) lines are connected via pairs of resistors 12, 13 to ten pairs of terminals P1, P2 and are also connected via further pairs of resistors 14, 15 to terminals P3, P4. When the tone is received over any of the lines, V.F. detector 16 is operated and relay L/1 energizes so that at contact L1 the stepping magnet DM of the selector is operated to step the line finder over contact banks 17, 18, 19, 20. When the calling line is found, the received tone at terminals P1, P2 operates detector 22 so that relay K/3 operates and at contact K1 breaks the circuit of the magnet DM. At contacts K2, K3 the detector 16 is disconnected and relay L - de-energizes so that over the homing bank 21 1 of the selector and contact bank 19 the calling lamp 23 is illuminated. The call-answering action by the operator energizes relay S/3 which locks up over contact S1 and homing bank 21. Also the incoming tone ceases so that relay K/3 is de-energized. At contact S3 the circuit of the lamp 23 is opened and over contact S2 a circuit is completed for the magnet DM which steps the selector switch to its home position in which locking circuit of relay S/3 is opened, and the apparatus is ready to respond to another call. In the modification, Fig. 3, a number, for example, four, line finders are associated with all the incoming lines, for example, forty in number, and the line finders are allotted as required by a switch having contact banks R1, R2, R3, R5, R6 of which contacts 1, 5, 9 . . . are connected to the first line-finder, contacts 2, 6, 10 ... are associated with the second line-finder, and so on. When the calling tone is received over any line, the V.F. detector 25 energizes relay ST/1 which at contact ST1 completes a circuit through contact K1 and R5 to the stepping magnet 26 of the line-finder selected, but relay H/5 is not energized at this stage. When the calling line is found, V.F. detector 27 operates and relay K/1 energizes so that contact K1 opens the circuit of driving magnet 26 and allows relay H/5 to energize. Contact K1 also operates magnet RA25/8 to open contacts RAdm. At contact H2 the signal lamp 24 is illuminated and H3 extends a ground connection to the line-finder in use. Contact H4 short-circuits the incoming tone on LF1, LF2 and relay K/1 releases. At contacts K1, the circuit of magnet RA25/8 is opened, and contacts 8 RAdm close so that the allotter moves to the next contact. When the operator inserts the answering plug, relay S/3 operates and at contact S2 releases relay H/5 and extends a ground connection to magnet 26 which homes the linefinder and releases relay S/3, and S1 removes the line-finder busy condition at the bank R6. If the first step of the allotter finds the next linefinder engaged, the corresponding relay H/5 will be in the operated condition, and the allotter will step to the following line-finder by reason of the earth connection over contact H3.