GB759269A - Improvements in or relating to electrical signalling apparatus - Google Patents

Abstract

759,269. Automatic exchange systems. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd. July 20, 1954 [Aug. 7, 1953], No. 21889/53. Class 40 (4). [Also in Group XL (c)] A substation is connected to an automatic telephone exchange by a frequency-modulated radio link. Delay circuits in the receivers at both ends of the link prevent false operation by high-frequency signals of less than a predetermined duration. Dialling tone is provided. The substation modulates about a central frequency f1 and transmits impulses as pulses of frequency f2. The substation is called either by intermittent carrier or by frequency shift. Substation, calling, Fig. 1. A conventional subset includes switch hook contact C1, offnormal dial contacts C2, C3, and impulsing contacts C4. Line relay TA is followed by TB which starts the transmitter 13. Dialling tone returned from the exchange is fed to line through receiver 14 and hybrid 11. At the same time, relay TRS pulls up to provide a second hold on TB in response to the output from a limiter 42 after a delay introduced by circuit 44 (see later). Relay TA follows the dialled impulses and the transmitter shifts the frequency from f1 to f2 when TA is back, during each impulse. Exchange called, Fig. 2. In the receiver 16, a discriminator 19 produces no output for the central frequency f1. After a delay introduced by resistor 27 and capacitor 29, the output of limiter 18 renders a pentode 28 conductive in circuit 45 (see 44 before). The pentode is suppressed by the discriminator in response to pulses of frequency f2. When the pentode conducts, relay RS pulls up to complete the loop to the exchange 22. Relay B follows RS and RG starts the transmitter 40, dial tone from the exchange being fed to the transmitter through hybrid 21. Dialled impulses from the substation are repeated to the exchange by relay RS, the hybrid 21 and a choke 6 being shortcircuited by relay D during each digit. Exchange calling, Fig. 2. Interrupted ring- ing current from the exchange operates relay RG and provides an interrupted carrier from transmitter 40. On reply, relays RS, B pull up and RG is held by battery 39. The transmission may be shifted in frequency instead of interrupted. Substation called, Fig. 1. Relay TRS responds to the interrupter carrier from the exchange and connects ringing current to the subset from terminals 48, 49. On reply, relay TA, TB pull up and the transmitter 13 is started.