Patent GB-810724-A (GB)
Title: Improvements in alternating current signal receiving apparatus
Inventor: HARRIS LIONEL ROY FRANK
IPC: H04Q11/04
Filed: 1954-04-08
Abstract: 810,724. Automatic exchange systems. POSTMASTER GENERAL. March 31, 1955 [April 8, 1954], No. 10412/54. Class 40 (4). An A.C. signal receiver is connected to a line only when an A.C. signal is on the line, and is so made to serve several lines. When A.C. signals are incoming on a plurality line an indication is given for each of the lines, by means of which a selection is made of one of the lines, which is then connected to the receiver. The receiver is released at the latest at the end of the signal. A pool of receivers smaller in number than the number of lines may be provided. As shown, leads H4, A4 are hold and audid leads forming part of a connection between a calling also circuit and a register as described in Specification 803,509. When a V.F. signal lying within a prescribed frequency range is received it passes via filter F101 and limiter LCA101 to a rectifier RU101 whose output opens gate PCG101 to admit a pulse train RP1, characteristic of the circuit, to common lead PL103. This is passed to gate PCG251 which is fed over lead PL109 with the pulses of circuits using V.F. signalling (as opposed, e.g. to dialled impulses). The output of this gate is then fed to gate PSG251 where the pulses of circuits already connected to V.F. receiver are removed. as also are pulses of circuits needing guarding against connection as indicated over lead PL653. The output of PSG251 is fed to selector SEL251 which selects one of these pulses and emits it on lead PL257. Each V.F. receiver is associated with an input DCIL252 to the multiplex MX252 which emits pulses in positions indicative of the free receivers. Selector SEL252 selects a free receiver and indicates it by D.C. on a combination of two leads DCIL258, DCIL259 out of a plurality of such leads. The marked leads open gates PG258, PG259 to admit the selected circuit pulse train on lead PL257 to corresponding circulating systems TD258, TD259, the outputs from which open a gate PCG254 individual to the V.F. receiver. The selected pulse is also applied to suppress gate PSG251 to release selector SEL251. The PGG252 output also enables a " modulator " or gate RRM252 which receives the circuit pulse train modulated by the incoming V.F. signal at RRM101 and passes the modulated pulses to the amplifier and demodulator RDA252. The resulting V.F. signals are filtered at F258 (to allow only those signals in the prescribed range to pass) amplitude limited at LCA252 and then applied to filters F250 ... F255 individual to the signal frequencies in use. Their outputs operate triggers TG250 ... TG255 to open gates PG256, PG251 ... PG255 to pass the circuit pulse train received from gate PCG254 to a lead or a combination of leads PL256, PL251 ... PL255 indicative of the signal received. The operation of the amplifier detector unit RDA252 causes the emission of a hold signal to the rectifier RU252 to mark the lead DCIL252 to suppress the corresponding pulse output from the multiplex MX252. The selector SEL252 is thereupon released. The arrangement is used in a system in which, when a signal on PL251 ... PL256 is registered (in equipment not shown) a " stop send " signal is sent as described in Specification 804,691. At the same time a guard signal consisting of the circuit pulse train is fed over lead PL653. This inhibits in gate PSG252 the hold pulses on lead PL103 so that the pulse train is deleted from TD258, TD259 so releasing the V.F. receiver. The signal also inhibts the input to the selector SEL251 to prevent the same incoming V.F. signal from re-seizing a V.F. receiver and so re-registering the signal. The guard signal is of such a duration as to give the distant sender time to stop sending in response to the stop send signal. If the distant sender is manually controlled however, the guard signal may be maintained (by means not shown) until the termination of the incoming V.F. signal. The equipment may be used to receive digital information as combinations of two out of five frequencies. Also for receipt of such a signal as " fee not known at originating exchange," instructions for alternative routing, or of the identity of the originating exchange. Similar equipment is described for use in the called circuit to register connection and may be used to receive such signals as " stop sending," " ring," " answer indication," " N.N." and busy " signals. Specification 722,179 also is referred to.