Patent GB-581679-A (GB)
Title: Improvements in telephone or like systems
IPC: H04Q3/00
Filed: 1945-04-21
Abstract: 581,679. Semi - - automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., GILLINGS, C., and BEALE, C. E. April 21, 1945, No. 10079. Addition to 563,504. [Class 40 (iv)] In the mixed system described in the parent Specification which includes some junctions equipped for VF signalling by means of converting relay sets at each end and some for DC signalling and has provision for the straightthrough transmission of VF signals on calls involving two or more VF junctions in tandem, facilities are provided by which an originating operator may transmit a control signal in order to effect trunk offering or forward transfer. In the former case, the same signal may also be used for re-ringing. The system is otherwise similar to that described in the parent Specification. Each incoming relay set sends forward after each DC impulse train a VF testing signal which when received at a succeeding outgoing relay set initiates a backward signal consisting of a battery reversal, the offect of the two signals being to provide a straight-through path for subsequent VF signals. Thus, in the connection shown in Fig. 1, the testing signal sent from IRSI after the second DC impulse train is received at ORS2 and further conversion is prevented in these relay sets. If, however, the repeater A-A REP associated with the junction DCT includes an impulse regenerator, the testing signal does not get through and conversion at IRS1 and ORS2 continues for the succeeding digits. In the last-mentioned case, the reply signal is transmitted in cascade and the acknowledgment signal from ORS1 sets up 'first at IRS1 and then at ORS2 the conditions for end-to-end transmission : this feature is described in the parent Specification with reference to Figs. 9, 10. Actuation of a control key by the operator after receiving busy tone energizes a relay (OA) in ORS1 which is connected to the tip conductor after dialling and which causes the transmission of a long X frequency pulse followed by a short X pulse as distinct from the clear forward signal in which a long X pulse is followed by a Y pulse. By means of alternative connections at IRS2 the signal may control the application of earth to both speaking wires in order to unbalance a differential relay in the final selector in the customary manner for trunk offering, or it may cause the operation of a switching relay to transfer the call to an operator's position or to another train of switches which may be controlled to extend the call to another subscriber in the same exchange. In the former case the control signal is also available for re-ringing after the called party has cleared. In the case where end-to-end transmission of VF signals has not been established, the control signal is sent over the junction DCT by the application of earth to both speaking wires, and the initiating relay (OA) in the outgoing relay sets has differential windings connected in the incoming DC loop for responding to this condition, these windings being cut off on the receipt of a reply signal. The repeater A-A REP may be arranged to repeat the unbalance signal in the manner disclosed in Specification 351,550 (Fig. 7).