Patent GB-381620-A (GB)
Title: Improvements in electric signalling systems
IPC: H04J1/14
Filed: 1931-07-14
Abstract: 381,620. Wired wireless systems. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, and WESTELL, E. P. L., Telephone Works, Stoke, Coventry. July 14, 1931, No. 20247. [Class 40 (v).] One or more special signals, such as alarm signals, are sent over a carrier wave telephone system by modulating the carrier to a greater depth than the speech currents and at a frequency higher than the calling or dialling impulses. The relay for receiving the special signal is unaffected by calling or speech currents and the calling relay is unaffected by the special signal. In the circuit shown in Fig. 1, telephone lines Tl .. T3 connected to the automatic equipment A, can each control the carrier wave transmitter T by a relay 1. When a call occurs, a relay in A1 earths the lead 2 and closes a contact 4 to energize relay 1. This opens a short-circuit 5 across the transmitter T which then sends carrier current into the line 56 for each calling impulse. These may follow each other at 10 per second. The special alarm signal is sent by closing a switch 17 which first operates relay 1 (if this is not already energized) and then energizes a self-interrupting vibrator 18, 22 which short-circuits the transmitter T, by connecting a large condenser 23 across the line, about 80 times per second. At the receiver, the carrier wave passes to a grid-leak detector 10 with relays 11, 12 in its plate circuit. The calling currents operate relay 11 and its contacts 13 to select the desired line T4, T5, T6 ; and also operate relay 12 and its contact 14. This contact in one position charges the condenser 15 from a battery, and in the other position discharges the condenser through a slow-release relay 16. The slow calling impulses do not result in sufficient current passing to operate the relay 16, but the more rapid alarm impulses operate the relay and give the alarm signal. On the other hand the highspeed alarm signal does not affect the line-selector equipment in A2. Speech signals which do not deeply modulate the carrier are taken off from the detector plate to the selected line T4, T5, T6 and do not affect the alarm signal receiver. In a modification shown in Fig. 3, the detector plate circuit includes one winding of a balanced relay 44, the other winding 44<1> of which is included in series with a second relay 50 in the plate circuit of a valve 49. Alternating components of the rectified signal pass through a condenser 47 and transformer 48, the secondary of which feeds the grid circuit of the valve 49 which is connected as a grid-leak rectifier. The valves 43, 49 are adjusted so that, with no signal coming in, the relay 44 is balanced. The 10-cycle calling signal reduces the plate current of the valve 43 but 'produces only a transient effect on the valve 49, so that the relay 44 is unbalanced and operates the line-selector contacts 55. The 80-cycle alarm signal reduces the mean plate current of the valve 43, but also affects the grid of the valve 49 and so reduces its mean plate current, thus leaving the relay 44 balanced. The tuned relay 50 is however operated at 80 cycles and closes its contact 52 and energizes the alarm signal relay 57.