Patent GB-516742-A (GB)
Title: Improvements in telephone systems
IPC: H04M15/10
Filed: 1938-07-06
Abstract: 516,742. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., TERRONI, T. B. D., and WRIGHT, D. July 6, 1938, No. 20023. [Class 40 (iv)] In a system wherein duration metering at one exchange is controlled by alternating current signals transmitted from timing equipment at another exchange, the signal receiver includes a guard circuit to prevent its operation if the signal is accompanied by non-signalling frequencies. In the system shown in Fig. 1 a connection from exchange X1 to X2 is set up by register-controller RC over switch FCS at X1 and incoming selector SR at a central exchange C where the connection involves A.C. impulsing and timing equipment R1, to an incoming selector at X2 having A.C. impulsing equipment R2. A call originating at C also utilizes equipment R1 but ordinary duration metering signalling, e.g., by booster battery is employed in this case, R1 sending such signals or A.C. metering signals in accordance with the path of access thereto as described in Specification 516,741. The A.C. metering signals are received by a circuit MRRG at the first code selector at X1 which repeats them in D.C. form to the calling subscriber meter. The circuits of Fig. 2 show diagrammatically the signalling arrangements at R1 which are fully described in the abovementioned Specification, and leads 11, 12 at exchange X1 connect to the circuits of delayed metering relays DA, DB described in Specification 475,419 also employing A.C. metering signals and to which reference should be made concerning the operation of relays DA, DB. When the called party replies, relay D operates and the next S pulse operates DA which prepares a winding of DB for operation by the subsequent Z pulse in series with another winding of DA. Relay E(i) is also operated by the Z pulse over lead 11 in parallel with the last-mentioned winding of DA and locks over P(ii) and lead 11 connected to the hold wire of the first code selector. During application of the Z pulse, a metering relay sends one or more impulses to the meter and at the end of the pulse DA falls back to prevent further operation of the metering relay, but DB holds. Relay P operates Q and connects the A.C. responding and guard relays AC, AR to the speaking circuit. Towards the end of the conversational period, R1 at the central exchange sends a warning signal of 800# from generator Y, and at the end of the period a low frequency pulse, preferably of supply mains frequency, is transmitted to operate AC whereupon Q falls back slowly and grounds lead 12 to short-circuit the holding winding of DB. When the A.C. pulse terminates, the relapse of AC removes the ground, from 12, Q pulls up again and repeat metering controlled by DA, DB occurs as before. Relay AC responds to all frequencies or only to the signal frequency, whereas AR responds to frequencies other than the signal frequency on account of the rejector circuit L, QD. The operation of AR on non-signalling frequencies holds Q and so prevents the release of DB and subsequent release metering. A variable resistance R is used to adjust signal level to a standard value at all exchanges receiving signals from exchange C, the operation of guard relay AR by heavy signal current being thus prevented. Relay Q is made slow so that it does not fall back unless a correct signal of minimum duration is received. -