Patent GB-482743-A (GB)
Title: Improvements in and relating to electric communication systems
IPC: H04M5/04
Filed: 1936-09-01
Abstract: 482,743. Manual exchange systems. TELEPHONE MANUFACTURING CO., Ltd., and PADDLE, L. H. Sept. 1, 1936, No. 23928. [Class 40 (iv)] [Also in Group XL] Calling and clearing signals between two operators comprise alternating current of the same frequency; response of a called operator disables the calling indicator and prepares the circuit whereby in the event that the called operator clears first, a clearing signal is transmitted from her position, but if the calling operator clears first a clearing signal is transmitted from the calling position, no clearing signal being transmitted from the position of the operator who clears last. Means are described for discriminating between the signals and the same frequency occurring in speech. Calling. When jack JS is plugged A pulls up followed by D, both of which prepare the clearing signal circuits of indicator Ind while D also operates N to prevent operation of L, K and E. The operator calls by applying 17# over jack JS whereupon G, shunted by a rectifier and tuned to 17#, connects a 1000# source to the outgoing line and a local 17# source to relay O which interrupts the 1000#, source. Relay F also pulls up to prepare the clearing signal relay L. At the distant end, the calling signal is fed to the input 10 of valve V1 which has two transformers in its anode circuit one of which, T1, is tuned to 1000# whereas the other, T2, is associated with a rejector circuit so that it only operates on frequencies other than 1000#. The calling signal is passed by both transformers, the rectified output of T1 being applied to the upper winding of relay K whereas the rectified output of T2 is applied across resistance 12 which varies the grid bias of V3 and also applies 17# pulses to the grid of V2 the output circuit of which comprises a transformer T3 tuned to 17#. The output of V2 is rectified and applied across resistance 14 in a direction to oppose the effect on the grid of V3 of potential variations across resistance 12. The net result is that the anode current of V<3> remains constant, or may be arranged to increase due to positive bias over 14 overcoming the negative bias over 12. Relay J having current in both windings pulls up and operates H. It will be seen that J operates on pure 1000# or 1000# interrupted at 17# but if there is no 17# component, transformer T3 applies no bias over resistance 14, and other frequencies present, e.g. in speech, cause negative bias to be applied over resistance 12 to reduce the anode current of V3 and prevent operation of relay J. Relay H prepares the clearing signal circuit and operates E which connects a 17# source over normal jack contacts to B which completes the circuit of indicator Ind. Relay K also pulls up and locks. The called operator on plugging jack JS, releases B and operates A followed by D, N and F as at the calling position to prepare the clearing signal circuits. Relays K and E fall back, the latter completing the speech circuit. Clearing. One or other of the operators withdraws releasing relay D followed by N which operates L followed by M to connect the 1000# source to line for the release period of M the circuit of which is opened by the relapse of F. The signal is also interrupted at 17# as previously described by relay O operated in this case by M. At the distant end, relay J pulls up followed by H which connects the 17# source to C (D is operated at both ends of the line) whereupon the indicator Ind is operated, C being held over the jack until the operator withdraws and all operated relays relapse. A backward clearing signal is not transmitted since the operation of H has released F so that L cannot operate. Modifications. The signals may be purely of 17# in which case relay J is connected directly in the line and the terminals marked 17# and 1000# are connected together, the 1000# source and interrupting relay O being omitted. A normally closed contact of G may be connected in the circuit of H to prevent this relay from being operated irregularly by 17#. Uninterrupted 1000# may also be used and in this case the thermionic valve receiver is modified by omitting valve V2 and arranging that the output of transformer T2 due to frequencies other than 1000#depresses the anode current of V3 to prevent operation of J. Separate calling and clearing indicators may be used.