Patent GB-547468-A (GB)
Title: Alternating current signalling systems
IPC: H04Q1/453
Filed: 1941-04-04
Abstract: 547,468. A.C. signal receivers. STANDARD TELEPHONES & CABLES, -Ltd. April 4, 1941, No. 4510. Convention date, April 5, 1940. [Class 40 (iv)] Several arrangements are described for receiving A.C. signals of the kind in which the actual signal is preceded by a prefix signal of the same or different frequency or combination of frequencies, the effect of the prefix signal being to condition the circuits for a predetermined period for the reception of the actual signal. All the arrangements include a volume-limiting amplifier VL which is adjusted so that the current reaching the responding relay is only slightly above the minimum operating value, thus reducing the risk of operation by extraneous currents. In Fig. 1, filters FA, FB tuned to the signalling frequencies A, B have substantially the same current build-up and decay characteristics, the build-up time being equal to the decay time, and these filters feed relays RA, RB through detectors DA, DB. It is assumed that frequency A is applied when the line is .seized and that each dialled impulse is represented by the substitution of frequency B. Relay RC, which is connected up by RA, remains operative after the latter relapses during the charging period of condenser CA and removes a short-circuit from relay RB so that it can respond to frequency B. Relay RD is connected up by RB and remains operative after the latter releases during the charging of condenser CB, which is accelerated when RC again pulls up by the cutting out of resistance WD. The chart in Fig. 3 shows that the impulses are repeated with a delay but without distortion. In a modification, Fig. 2, relay RD is under the control of two auxiliary relays RE, RF which are inserted in the two signalling circuits by the operation of relays RC, RG, these being delayed-release relays controlled by RA, RB respectively. The arrangement in Fig. 4 responds first to a seizure signal comprising frequency B followed by frequency A, and then to dialled impulses in which frequency B is substituted for frequency A. The former results in the operation of relay RM, which closes the outgoing circuit OG and holds during impulsing, and each impulse produces an operation of repeating relay RN. When the line is used on an outgoing call, supervisory signals of similar character are received and transmitted over the circuit OG. A disconnect signal comprises frequency A followed by an impulse of frequency B that is long enough to release RM. Relay RL serves to disconnect the line during signalling from any trunk to which it has been extended. In Fig. 5, relays RA, RB, RI, RK are biassed to operate quickly, while relays RH, RJ, which control the outgoing circuits, are normally biassed against operation by RA, RB respectively. On the response of RA to frequency A, for example, RI pulls up and reduces the bias on RJ so that it operates when RB responds to frequency B. Relays RI, RK are provided with delayed-release circuits, as in the previous arrangements. In Figs. 6, 7, the prefix is of the same frequency as the actual signal. In both arrangements, on the relapse of relay A, which responds to the prefix, relay B is operative in the charging circuit of condenser K and prepares relay C for operation by the signal, either by substituting it for A or by transferring an adverse biassing resistance Q from C to A. Specification 270,817, [Class 40 (v)], is referredto.