GB702078A - Improvements in or relating to telephone systems - Google Patents

Abstract

702,078. Voice frequency signalling; twoway transmission. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. June 12, 1952 [June 28, 1951], No. 15343/51. Class 40(4) In a voice-frequency signalling system the receiver for incoming signals is rendered insensitive to outgoing signals and to locally produced speech, noise, and tones by associating it with a winding of a hybrid transformer which is also connected to the network employed for balancing the local 2-wire line. As shown in the 4-wire termination of Fig. 1, the V.F. receiver VR is connected in shunt with the balance network QA, YA which balances the local 2-wire line ES. Outgoing V.F. signals are applied directly to the pair OL by contacts SX1, SX2 under control of the outgoing relay set which connects the V.F. currents to leads SG. If desired, signalling may take place simultaneously in both directions. Due to an imperfect balance at the remote exchange part of the transmitted signal may appear on the incoming pair IL and if the same frequency is employed for signalling in both directions operation of VR may occur. This may be prevented by short-circuiting IL during outgoing signalling. Another method of preventing such false operation is shown in Fig. 2 in which the contacts SX1, SX2 disconnect the receiver during outgoing signalling and apply the V.F. currents to the balance winding of the hybrid. This has the advantage that access to the 4-wire side of the hybrid is not required. In the case of a 2-wire trunk the trunk is connected as is IL in Fig. 2'and the outgoing pair OL (Fig. 2) is replaced by a further balance network. The invention is particularly applicable in cases where it is desired to receive signals while transmitting dialling or N.U. tone. At a tandem exchange at which switching is on a 4-wire basis the hybrids may be connected in a " tail-eating " circuit, the V.F. receiver in the outgoing relay set being disabled by the incoming relay set and vice versa, as described in Specification 674,243, to prevent both receivers responding to one signal.