Patent GB-1281934-A (GB)
Title: Arangement for voice-frequency signalling, e.g. for a telephone system
IPC: H04M1/50
Filed: 1969-11-04
Abstract: 1281934 VF signalling system TELEFONAKTIEBOLAGET L M ERICSSON 4 Nov 1969 [5 Nov 1968] 54083/69 Headings H4K and H4R  In a voice frequency signalling system for sending v.f. signals from a sub-set A to an exchange X a switching system in the exchange responds to a first line condition, e.g. interruption of line current at the sub-set, to produce a higher voltage on the line and to connect v.f. receivers to the line. The higher voltage causes breakdown of a diode in the sub-set, causing current to be restored and energizing the v.f. signalling oscillator in the sub-set. As shown in the Figure removal of the subscribers handset operates hook switch K to put the transmitter T across the lines L. The resulting current is detected in the line circuit LR and a free link LNK connected to the line via SLV selectors, under the control of marker M1. Subsequently the marker M2 is operated to control the connection of a register REG via selectors RS. During connection of the register, relays R5, R4 and R1 are operated and R5 and R4 held via the line current drawn through the sub-set via the transmitter T, and dialling tone Su is sent to the subscriber via the coil of relay RR3 in the register and the coils of relay R4 feeding the lines to the sub-set. When the subscriber pushes one of the buttons P1 to Pn to send a v.f. dialling signal, the transmitter T is disconnected from circuit and a connection made instead to the oscillator circuit OSC via contact k. The exchange battery voltage is normally not sufficient to cause the PNPN diode q 2  to break down and consequently after capacitor C3 is charged current flow through the sub-set ceases causing relays R4 and R5 in the link to be released. Opening contact 53 of R5 causes auxiliary battery + V to be connected in series with the exchange battery raising the line voltage to a level sufficient to break down PNPN diode q 2  and cause oscillator OSC to oscillate. Simultaneously contact 55 closes to cause energization of the v.f. receiver TM so that the v.f. signals are received. In addition relay RR2 is operated to disconnect dialling tone from the line and to self hold over its upper winding and preparing a circuit for operation of relay RR1  over contact 122. However before RR1 operates the restoration of current due to break down of q 2  causes relays R4 and R5 to be energized cutting off the positive battery supply to the lower winding of relay RR2 and the coil of RR1 via contacts 122. Energization of relay R5 also cuts the supply to the v.f. receiver TM but the blocking of the receiver is delayed sufficiently for recognition of the v.f. signal to be effected. Energization of R5 also restores normal battery voltage, by removing the auxiliary supply but the diode q 2  remains conductive and the oscillator active until the button Pn is released when the oscillator is stopped, the diode becomes non- conductive, but line current is maintained via the transmitter T to maintain relays R4 and R5 operated. Each time a pushbutton P1 to Pn is pressed a v.f. signal is passed to the receiver TM and when sufficient signals have been sent a communication path to another subscriber is established via means not shown. When the path is established the register is released by energization of RR1. After a path has been established further v.f. signals or pulse signals may be transmitted over the path, pulse signals by operation of key P0, the signals being repeated by relays R4 and R5 over lines c2 and d2. The marker can be reconnected and v.f. signals used to set up information calls, conference calls &c.