Patent GB-1179143-A (GB)
Title: Circuit Arrangement for the Dial Information Receiving Devices in a Telephone Exchange System.
IPC: H04Q1/453
Filed: 1967-10-19
Abstract: 1,179,143. Telephone exchange systems; V.F. receivers. INTERNATIONAL STANDARD ELECTRIC CORP. 19 Oct., 1967 [27, Oct., 1966 (2)], No. 47649/67. Headings H4K and H4R. A receiver, of the type described in Specification 1,065,540, in a telephone exchange to receive V.F. signals from a subscriber's set, is provided with means to prevent the " proceed to dial " signal from reaching the V.F. receiver circuits. Either the "proceed to dial" generator and the V.F. receiver are connected to the subscriber's link via a hybrid of which the balance network is automatically adjusted to provide minimum feed through from the generator to a circuit in the receiver tuned to the " proceed to dial " signal, or alternatively, a " proceed to dial " signal rejector circuit is provided in the feed to the V.F. receiver which is normally in circuit but is shunted out on reception of a valid code, which also inhibits the " proceed to dial " signal generator. Fig. 1 shows a first embodiment in which a subscriber's set TLN is connected via link circuit VS, register KF, and hybrid transformer Ga to the voice-frequency receiver circuits, GF1, GF2 &c., and to the " proceed to dial " signal generator WG via gate T. On initiation of a call " proceed to dial " signal is fed to the subscriber over gate T, hybrid Ga and link VS. Breakthrough of the proceed to dial signal to the V.F. receiver is detected by a circuit EW tuned to the " proceed to dial " signal and causes automatic adjustment of the balancing network N to minimize the breakthrough. On reception of a valid signalling code as recognised by decoder D the gate T is shut, terminating the " proceed to dial " signal, and the balance network setting is held until the V .F. code receiver is released. The receiver EW may be a normal code frequency receiver which is tuned to the " proceed to dial " signal until required for V.F. reception. The " proceed to dial " signal may be a tone or an announcement. The balancing network may be adjusted in discrete steps, as by switches K1, K2 &c., or continuously. Fig. 2 shows an alternative embodiment in which the link Vs is connected to a hybrid transformer U1, U2, which is connected on the one hand to the " proceed to dial " generator WG and forward extending line La via a diode gate including diodes D1 and D2, and on the other hand to the V.F. receiver T on E via a "proceed to dial" signal rejector U5,C1. On initiation of a call, diodes D1 and D2 are conducting, connecting the generator WG to the link circuit Vs, and diodes D3 and D4 are non conducting, so inserting the "proceed to dial" signal rejector between the hybrid and the V.F. receiver. On reception of the first valid tone signal by the V.F. receiver switch TS is closed, blocking diodes D1 and D2 to disconnect the generator WG from the hybrid, and switch W.T.F. is closed, making diodes D3 and D4 conductive to short out the rejector circuit U5,C1. If the " proceed to dial " signal is an announcement the speech immunity circuits of the V.F. receiver are arranged normally to be switched out of circuit and only switched in when a V.F. tone signal is detected, the tone signal only being regarded as genuine if its reception persists after restoration of the voice immunity circuits.