Patent GB-399494-A (GB)
Title: Improvements in or relating to electric signalling systems
IPC: H04J1/14
Filed: 1932-04-04
Abstract: 399,494. Wired - wireless signalling. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, and WESTELL, E. P. L., c/o General Electric Co., Stoke, Coventry. April 4, 1932, No. 9687. [Class 40 (v).] In a carrier-current signalling system in which one signal is sent by modulating and another by a train of interruptions of the carrier, the second signal receiver is unresponsive to all signals which do not interrupt the carrier and also to a single interruption thereof. The invention is described as applied to a ringing system for carrier-current telephony systems, the apparatus for connecting a lowfrequency telephone line 2 to a carrier frequency line 8 being shown in Fig. 1. The signals are transmitted in one direction by a carrier frequency hereinafter referred to as " A " and in the other direction by a frequency " B." A source 13 of carrier A connected to a modulator 5 is connected to the line 8 through a carrier A direction filter 7, the voice frequency signals from line 2 being fed to the modulator 5 through a hybrid coil 14. Signals from line 2 are also applied to a polarized relay 15 adapted to respond to 17-cycle ringing current, whose voltage is normally above the speech voltages, and which is blocked from the modulator 5 by condensers 16. When relay 15 operates, relay 17 is energized to interrupt the carrier current from the modulator 5, 17 times per second. Carrier B received from line 8 passes through a carrier B directional filter 4 to a demodulator 3, signals from which are led to a low-pass filter 18, hybrid coil 14 and line 2. The demodulator 3 may comprise a metal rectifier or, as shown, a leaky-grid thermionic detector 6, in whose anode circuit is one winding 19 of a balanced relay 21, the other opposed winding 20 of which is supplied with a steady current tending to operate the relay. The relay operates when the anode current of valve 6 falls due to a carrier-current signal in its grid circuit. Intermittent operation of relay 21 at a frequency above a predetermined frequency, say 15 cycles per second, charges and discharges condenser 26 through resistance 25 and slow release relay 27 respectively in such a way that the average current through the latter is sufficient to operate its contacts 10 to connect line 2 to a line 12 connected to a source of 17-cycle ringing current. Interruptions of the carrier at a frequency of less than 15 cycles per second fail to operate relay 27, while voice modulation of the carrier produces insufficient change of current through relay 21 to operate it. In a modified system for producing ringing current in the line 2, Fig. 2, one of the fixed contacts 23 of relay 21 is connected to a slow release relay 29 and the other 24 through the contacts 30 of relay 29, a slow release relay 27 having contacts 10, as in Fig. 1, and the primary of a transformer 31 whose secondary is connected to the line 12. When relay 21 receives 17-cycle ringing impulses relays 29 and 27 remain operative, the latter connecting line 2 to line 12 and the transformers 31, which is supplied with 17-cycle impulses through the vibrating contact 22. Specification 381,620 is referred to.