Patent GB-790442-A (GB)
Title: Improvements in or relating to signalling means for use in an automatic telephone system or the like
IPC: H04Q3/42
Filed: 1955-10-24
Abstract: 790,442. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Oct. 24, 1955 [Oct. 25, 1954], No. 30383/55. Class 40 (4). An arrangement whereby a subscriber obtains a wanted number by sending tone signals to a register at an exchange comprises an impedance at the subscriber's instrument, D.C. over a twoconductor line from the exchange energizing the impedance which can be incorporated in a resonant circuit by one of a number of keys, each of which determines a particular frequency of damped oscillation that is fed on to the exchange line from across a condenser in the resonant circuit. In Fig. 1, on initiation of a call, hook switch contacts K make and line equipment LR causes a line finder F to connect a relay set R to the line a, b, and causes a line finder V to connect a register M. A receiver MM in the register sends buzzer tone to the subscriber. R1 responds with test relay R4 over wiper Vc and feed relay R2 responds to the loop offered by the bell B and a resistor r1 and charges a condenser C1 by current through the microphone M. An inductor PC has ten tapped points so that if one of the keys P1-PO is pressed, part of the inductor is connected in parallel with capacitors C1, C3, and as C1 is charged, a damped oscillation of a given frequency is produced and passed to the line from across C1. The damped oscillation passes over wipers Va, Vb, to an amplifier G, and an appropriate filter T, to the receiver MM which actuates a registering mechanism RM that may be as described in Specification 691,237. When the actuated P key is released condenser Cl recharges and condenser C3 discharges through r2. When the wanted number has been fully indicated RM sets up the connection over Ve, Vf, and over Vd R3 is operated and locks up to disable R1 and complete the talking path. R2 remains operated over the loop, the polarity of which is reversed by R1 causing microphone M to be included in the connection. Microphone M is isolated by rectifier E during the setting up of a connection so as not to pick up tone signals. An alternative way of keeping tone signals away from microphone M is shown in Fig. 2, where the inductor PC forms part of the loop, in series with microphone M, fed by relay R2 which in consequence passes current through PC. Audio signals from the microphone M cannot operate the register RM due to an attenuator N before the amplifier G 1. When a key P is pressed the inductor PC is first connected to C1 and C3, and then a contact P is broken to remove PC from the loop circuit and short circuit M. A damped oscillation produced across C1 is fed on to the line. The loop is held by bell B and resistor r1 which causes a decrease in current through the windings of relay R2 and the production of a low frequency oscillation on the line which is passed by filter LF and an amplifier G2 to disable attenuator N so that the damped oscillation may pass filter HF to the register mechanism RM. To maintain audio signals of constant amplitude in face of varying line resistance a thermistor U is put in series with the feed relay R2. Specification 704,650 also is referred to.