GB792664A - Improvements in and relating to electric signalling systems - Google Patents

Abstract

792,664. Automatic exchange systems. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd. July 20, 1954 [Sept. 21, 1953], No. 26036/53. Class 40 (4). [Also in Group XL (c)] Automatic exchanges are interconnected by a frequency-modulated radio link. A frequency shift from the normal speed carrier frequency f1 to frequency f2 constitutes the calling signal. A dialled impulse is transmitted as a frequency shift from the signalling frequency f2 to the speech carrier frequency f1 and back to f2. Reply is signalled by a called exchange to the called terminal as a battery reversal and is transmitted over the link as quickly following frequency shifts from f3 to f4 to f5 to f3 when f4>f3>f5 and f3 is the central frequency normally transmitted by the called terminal. This signal is repeated to the calling exchange as battery reversal and the transmitter's frequency is automatically restored to f1. Calling exchange and link terminal, Fig. 1. In a call from exchange 10, line relay A is followed by B which starts the transmitter 14. With battery 18 connected to the transmitter, the frequency is shifted from the normal f1 to f2(f2>f1). Relay A responds to impulses and shifts the frequency back to f1 for each impulse. Called exchange and link terminal, Fig. 2. The receiver 24 is tuned to a band on a central frequency f and just including f2. After a delay, a trigger 32 operates and relay VS pulls up. Marginal relay AA follows if the received frequency is f2, in which case, the Foster- Seeley frequency discriminator 27 gives a maximum positive output within the pass-band of the receiver and the loop to the exchange 19 is closed. The negative output of 27 is cutoff by rectifier 33. Relay BB follows AA and starts transmitter 23 at frequency f3. Relay AA repeats impulses to exchange 19 and dialling relay CD shorts the hybrid during each digit. Relay D pulls up when battery is reversed on reply and by connecting terminal 49 to the transmitter raises the frequency from f3 to f4 until relay DB pulls up after a delay. Terminal 51 is then connected to the transmitter to drop the frequency to f5. The frequency rises again to f3 when relay SD4 pulls up, after a delay, and holds. Calling exchange, reply. Relay RS, like VS in Fig. 2, responds to the transmission after a delay. Relay FP responds to frequency f4 which produces a positive output from the discriminator 39. When frequency f5 follows and produces a negative output, relay FN pulls up and, as FP is slow to release, DD pulls up, reverses battery 16 on the line to exchange 10 and restores the transmitting frequency at 14 from f2 to f1. Battery reversal may operate a meter at exchange 10.