Patent GB-729373-A (GB)
Title: Improvements in electric pulse selectors
IPC: H04J3/06
Filed: 1953-04-16
Abstract: 729,373. Multiplex pulse code signalling; pulse circuits. PHILIPS ELECTRICAL INDUSTRIES, Ltd. April 16, 1953 [April 19, 1952], No. 10430/53. Classes 40 (5) and 40 (6). A pulse selector for separating pulses of a constant recurrence frequency from other pulses comprises a coincidence mixer, which is normally cut off and which is controlled by the input pulses and pulses from an output feedback circuit containing a delay network, a starting circuit being provided for supplying initial pulses to the feedback circuit. The invention is shown applied to a pulse code multiplex communication system in which the synchronizing and channel code pulses are identical and selected from a series of equidistant pulses and is used to separate the synchronizing pulses which occur invariably at channel recurrence frequency. In one embodiment, incoming pulses are applied to a normally cut off coincidence mixer stage 27, Fig. 1, which also receives pulses fed back from its output through delay network 28, decoupling stage 36 and pulse widener 29, the delay of network 28'being equal to one synchronization cycle. At the start of operation, contacts 30 of relay 31 short-circuit circuit 27 and the first pulse across the latter energizes relay 31 after one cycle period from the output of widener 29 to open contacts 30. Thereafter pulses can reach the feedback circuit only through the coincidence stage 27. After a few cycles the only pulses which continue to produce the necessary coincidence are the synchronizing pulses, Fig. 2 (not shown), which are'taken, to control the channel distributer, from the output of widener 29. To increase the holding time of the selector, if one or more synchronizing pulses are not received, the output of delay network 28 is connected to the normally closed contacts 32 of a relay 33 connected in parallel to relay 31, relay 33 being arranged to respond to open contacts 32 only during the selection period when fed by more than one pulse per cycle The delay network 34 is thus normally connected to network 28, to feed pulses from points 35, 35<SP>1</SP>, through decouplers 36 to the widener 29 delayed by. a further one and two cycles respectively. At most two synchronizing pulses can thus be absent from the incoming pulse train before the selection process is again initiated by de-energization of relay 31. An increase in holding time may be provided by increasing the delay time and number of tappings of network 34. To avoid the circuit locking in to a free communication channel of which the pulses are alternately present and absent, a relay may be connected to the output of stage 27 which disconnects the holding circuit when the number of pulses reaching it per unit time is smaller than the number of synchronizing pulses. A detailed circuit of such a pulse selector comprises an input pentode coincidence mixer 37, Fig. 4, the negative pulses from which are fed to a delay network 45 supplying a normally conducting threshold and limiting pentode 49. The positive output pulses from the latter are fed back to the suppressor grid of the mixer 37 over conductor 38. The starting switch comprises a pentode 51, in parallel with pentode 37, and controlled by a cathode follower 52, supplied from conductor 38, a rectifier 53, an integrating circuit 59 and a normally cut-off amplifier 54, connected to the suppressor grid of pentode 51. The latter amplifies incoming pulses until cut off by its control circuit after more than one cycle period. The holding circuit is connected to the feedback circuit by a pentode 60, fed from the screen grid of pentode 49; through a differentiating circuit 61. Its output is connected to the delay network 63 which feeds a threshold and limiting pentode 64 connected to the feedback conductor 38, The switching valve 60 is controlled by connecting the output of integrating circuit 59 to a second normally cut-off amplifier 70, controlling the suppressor grid of valve 60. A limiting diode 68, connected to conductor 38, ensures a constant amplitude for the fed-back pulses. The output synchronizing pulses are derived from conductors 13, 38. In a modified circuit the coincidence mixer and starting valve are provided by a single pentode and a flip-flop pulse regenerator is provided in the feedback circuit, Fig. 5 (not shown). In a further modification, the coincidence mixer 95, Fig. 6, energizes the feedback conductor 99 through an amplifier 96, delay network 97 and pulse regenerator 98. The starting relay 100, 101 and holding relay 102, 103 are energized in a manner similar to those of the circuits of Figs. 1 and 4, but the holding relay completes a connection 104 between the output of pulse regenerator 98 and the input of amplifier 96. A local continuous pulse. generator is thus produced which is synchronized by the incoming synchronizing pulses. Alternatively the pulse regenerator 96 may be converted into the local pulse generator by a switch 106. In all the embodiments the delay period in the feedback circuit may be a whole multiple of the cycle period. Specifications 662,611, 684,318, 691,824, 698,707, 698,708 and 728,436 are referred to.