GB149139A - Improvements in electric signalling systems, especially applicable to combined telegraph and telephone systems - Google Patents

Abstract

149,139. Western Electric Co., (Western Electric Co.). Sept. 1, 1919. Simultaneous telegraphy and telephony.-When a telephone trunk is also used for telegrsphy, the usual 16-cycle ringing-current cannot be employed, because it is too nearlv of the same period as the Morse impulses. The invention comprises means by which the 16-cycle current supplied to the trunk at its outgoing switchboard end is replaced at an intermediate point by, say, a 133-cycle current, which is again replaced, before arriving at the incoming switchboard end, by 16-cycle current. The higherfrequency current is of such small amplitude as not to be distorted by the amplifying- valves which may be used in long trunks. In the arrangement shown in Figs. 1 and 2, the trunk L1, L2 is divided by a filter, consisting of condensers 30, 31, 32, 33 and an inductive bridge 3, which prevents through transmission of currents of higher frequency than 200. Ringing-current of frequency 16 from the line L1 passes through the circuit 4, 5 to a filter 8, which only passes current of about this frequency, and thence through a resonant circuit 10, 11 to a valve A1. which it overloads so that harmonics are developed in the out-put circuit. One of these harmonics is in the neighbourhood of frequency 133, and current of this frequency is transmitted by a filter 12, which is opaque to other frequencies, to an amplifying-valve A2. The amplified currents are transmitted partly to a rectifying-valve A3, which energizes a direct-current relay 19, and partly through contacts of the relay 19 to the line L2. On the other hand, current of frequency 133 from the line L2 passes through the circuit 6, 7 and filter 20 to amplifying-valves A4, A5 and a rectifying-valve A6, which energizes a relay 27 to connect the line L1 to a set of valves A7, A8, A9 acting as a generator of 16-cycle current. The valve A7 is coupled and tuned to oscillate at this frequency, giving currents which are amplified by the valves A8, A9 and purified by a filter 30. The adjustable resistance shunts 9, 21 are provided to prevent interference by Morse and switch-hook impulses. Modifications are described in which the 16-cycle current directly energizes a tuned relay to connect the higher-frequency current to the line L2.