Patent GB-730955-A (GB)
Title: Improvements in or relating to automatic telephone exchange signalling and control systems
Inventor: FLOWERS THOMAS HAROLD
IPC: H04Q11/04
Filed: 1951-05-01
Abstract: 730,955. Automatic exchange systems. POSTMASTER GENERAL. Oct. 7, 1952 [May 1, 1951], No. 10246/51. Class 40 (4). In a pulse multiplex automatic exchange system using both forward and backward hold signals arrangements are provided to suppress a pulse series belonging to a calling channel if a forward hold signal but no backward hold signal is present. As shown, a calling channel 111, 112, 121, 122 comprising forward and backward paths 112, 122 are connected to the multiplexes MX1, MX2. The incoming pulse train passes from the highway H1 to a gate circuit comprising rectifiers MR51 . . . MR53 whose output consists of a train of rather narrower pulses which are applied to the highway H3. These pulses, set by means such as described in Specifications 694,092 or 722,179, a marked channel pulse generator CPG associated with an outgoing circuit BS, BH, FS, FH, which thereupon produces pulses in the same time position and applies these pulses to a gating circuit comprising rectifiers MR54 . . . MR56 to admit the pulses from H3 to a pulse stretcher and amplifier V53. This last valve feeds a demodulator circuit LPF to provide the forward speech signals and also a rectifier circuit MR511 which enables an oscillator V54 to provide a forward hold A.C. signal on the leads FH. When a backward hold A.C. signal is provided on lead BH from a subsequent switching stage, backward speech signals are permitted to modulate pulses from the channel pulse generator CPG for transmission over multiplex MX2 to the calling subscriber. The channel pulse generator is subsequently held in continuous operation if either forward or backward hold signals are present on leads FH, BH. The forward and backward hold signals energize two rectifier circuits which provide opposed biasing voltages for rectifier MR83. If a forward hold signal, but not a backward hold signal is present then MR83 conducts to pass a channel pulse from generator CPG to valve V81. A delay line DN is included in the forward hold bias circuit to give succeeding equipment time to revert the backward hold signal. The channel pulse is then restricted in width and accurately timed by a pulse K1 operating in the gate circuit MR84 . . . MR86 and then charges condenser C86 to render valve V82 conducting. The condenser C86 is discharged shortly before the next K1 pulse is due by a pulse K2. During the conducting period of V82 the network MR801 . . . MR804 passes A.C. from generator G81 to the delay line MDL which delays it by an amount slightly less than the channel pulse interval so that it emerges in time to straddle the K1 pulse corresponding to the next channel pulse. The delayed pulse is applied to a coincidence circuit MR807 . . . MR810 which operates on coincidence of the delayed pulse, a K1 pulse, and a pulse incoming over the multiplex MX1 to produce a pulse to cause V82 to conduct again. Thus as long as a pulse having this time position is received over MX1 a circulating pulse is maintained through the delay line MDL. This pulse is applied to lead L57 to close the gate MR51 . . . MR53 thereby preventing incoming pulses in this time position reaching the highway H3. The forward hold signal over FH is therefore suppressed and the channel pulse generator thereupon disabled so that the outgoing circuit is now available for further cables. The backward hold signal may be derived at a subsequent stage from the forward hold signal by rectifying the forward signal and using it to enable an oscillator providing the backward hold signal, Fig. 2(a) (not shown). Such an arrangement would be provided when using calling party release. Alternatively the backward hold oscillator could be under the joint control of the forward hold signal and a backward hold signal from a later stage, Fig. 2(b) (not shown). Such an arrangement would be used for last party release. Alternatively the following arrangement could be used: Signalling and ringing control bridge with means for preventing " called party hold " condition, Fig. 3. The forward hold signal passes to a rectifier MR71 whose output enables the oscillator V71 whose output passes through a normally open gate circuit MR72, MR73 to the backward hold circuit BH. The rectified output of this oscillator also provides anode voltage for gas tubes V72 . . . V76 and is applied via condenser C75 to strike V72. A gate MR77, MR78 is thereby opened to pass interrupted A.C. current from generator G1 over the speech wires FS to the called subscriber's line circuit, which responds as described in Specification 722,174 to transmit ringing current to line. The striking of V72 also opens the gate MR79, MR711 to pass ringing tone from generator G2 to the rectifier MR714 for gating to the backward speech leads BS. As described in the above-mentioned Specification, a backward hold signal is reverted to leads BH1 from the called subscriber's line circuit, the said signal being present only during alternate half-cycles of ringing current until the called subscriber answers whereupon the hold signal becomes continuous. The backward hold signal rectified at MR712, MR713 is passed to condenser C712 during the charging periods of which rectifier MR714 becomes conducting to revert ringing tone over leads BS. When the hold signal becomes continuous consequent on the called subscriber answering the condenser C712 becomes fully charged and the rectifier MR714 is blocked to suppress the ringing tone. The output of MR712, MR713 is also applied via a time-constant circuit to the striker of V73, the said time constant being such that the valve only strikes when the said continuous backward hold signal is received. The striking of V73 extinguishes V72 to cut off generators G1, G2. During the conversation the rectified output of the backward hold signal obtained from MR716, MR717 prevents V74, otherwise primed by V73, from firing. When the backward hold signal ceases due to the called subscriber hanging up however V74 fires, enabling the tube V75 to fire to the next " S " pulse received, thereby enabling V76 to fire when the next " Z " pulse is received approximately one minute later. If the called subscriber lifts his handset during this period V73 restrikes and extinguishes V74, V75. Otherwise, however, V76 strikes and closes the gate MR72, MR73 to suppress the backward hold signal to the line finder which thereupon operates to suppress the forward hold signal on FH and the signalling bridge circuit is restored to normal. Whilst the above description has assumed that the switch of Fig. 1 is used as a line-finder, the switch may also be used as a selector in the manner described in Specification 622,179. Specifications 722,172 and 722,174 also are referred to.