GB768917A - Signalling in telephone systems - Google Patents

Abstract

768,917. Automatic exchange systems. SKILLMAN & CO., PTY., Ltd., T. S. April 26, 1955 [June 10, 1954], No. 12033/55. Class 40 (4). In a system in which signalling is effected by raising the level of an alternating current to one approximately equal to that of average speech currents, filter means are provided to separate the signalling and speech currents, said filter means being inadequate to reduce peak speech currents to below the operate level of the receiver which is therefore provided with delay means to prevent spurious operation by such peaks. The invention as described is applied to the reversion of the called party answer signal. As shown, an outgoing circuit OE is connected to an incoming circuit IE over a carrier circuit of the usual type. On completion of a calling loop, relay A operates B and DB, DA follow, but the latter shortcircuits DB which releases again. Relay A operates OHS which injects tone into the modulator OM. After demodulation at IDM in the incoming circuit the tone is filtered out at IHF and operates the receiver ISR to bring up relay IHR. The latter operatse HH and M follows. Relay HH completes a calling loop to an operator's position or to an automatic exchange. In the latter case dialled impulses are repeated by A, OHS, IHR and HH, relay N coming up during the train to provide resistive loop to the exchange. When the called party answers, battery reversal over IE operates F and IHS follows to revert tone over the carrier link. In the outgoing circuit the tone is separated by filter OHF to operate relay OHR. The latter short-circuits DA which releases slowly, permitting DB to operate and release DD which reverses the polarity over OE as an answering signal. A backward clear releases OHR and DA operates to release DB by short circuiting. DB re-operates DD. The release and operate times of DA, DB, DD may be so arranged that DD may faithfully repeat backward impulses of prescribed duration, e.g. multi-metering pulses. False operation by speech or like peaks breaking through filter OHF is prevented by the slow release trains of DA and DD. The tone used may be the carrier frequency itself.