Patent GB-856549-A (GB)
Title: Improvements in or relating to communication systems comprising echo suppressors
IPC: H04B3/20
Filed: 1959-01-30
Abstract: 856,549. Telephone transmission systems. WESTERN ELECTRIC CO. Inc. Jan. 30, 1959 [Feb. 4, 1958], No. 3370/59. Class 40 (4). In a switching system when two lines, each having echo-suppressors, are interconnected, arrangements are provided to disable one echo - suppressor and to adjust the release or hangover time of the other echosuppressor in accordance with the characteristics of the line on the other side of the connection. As shown, four-wire lines T 3 , R 3 , T 4 , R 4  and T 1 , R 1 , T 2 , R 2  may be interconnected over switches SN, TN, the upper and lower pairs being for West-East and East-West speech respectively. Each circuit comprises an echo suppressor, shown in block on the left-hand side and in detail on the right-hand side. Blocking of the speech paths is controlled by hybrid transformers ES or OS which when balanced, block transmission, but which may be opened up by short-circuiting the balance networks EN, ON. Trunks on the east side which have echo suppressors have earth connected over coil 49 to the T 2  wire and when one is connected through to a west side trunk having an echo-suppressor, this earth brings up CR therein which short-circuits the balance networks EN, ON on the east side, thereby disabling the associated echo suppressor. C.R. also returns earth over R 4  to light lamp 52. Each west side trunk is also provided with shunt inductances 71, 73 of whose inner terminals none, either or both may be earthed to signal to the east-side trunk the amount of hang-over time which is appropriate to the west side trunk. These earths operate none, one or both of relays PK, SK on the east side to switch in one of condensers C 1  ... C 4  in a time constant circuit which determines the hang-over time of the east side suppressor. The suppressor is governed differentially by the East-West and West-East speech currents. The former rectified at 3 reduce the cathode current of valve 10 to cause the release of the otherwise operated relay OM. This operates OH over its upper winding, thereby removing the short circuit from the east side EN balance network to close the West-East path. Relay OH also disables EM by short circuiting it. When the speech ceases and OM consequently re-operates, OH releases after a delay determined by the selected condenser C 1  ... C 4 . West-East speech currents rectified at 4 increase the cathode current of valve 10, thereby operating EM and EH follows to balance the East-West hybrid OS to disable the East-West path. Relay EM also energizes the lower winding of OM to keep it operated. When the West-East speech finishes EM releases, followed by EH after a time delay determined by the condenser T. The arrangement operates differentially the person speaking the loudest to breaking through. Finer graduations of time constant may be obtained by providing more condensers which are controlled in some cases by marginal relays, there being provision for connecting in resistance earth as well as direct earth to the innermost points of coils 71, 73. Alternatively A.C. signalling may be used for this purpose. Specification 245,554 is referred to.