Patent GB-1170284-A (GB)
Title: Improvements in or relating to Time Division Multiplex Communication Systems.
IPC: H04J3/06
Filed: 1966-11-04
Abstract: 1,170,284. Multiplex pulse signalling. NIPPON TELEGRAPH AND TELEPHONE PUBLIC CORP. 4 Nov., 1966 [10 Nov.. 1965], No. 49549/66. Heading H4L. In a time division multiplex pulse communication system wherein a plurality of switching centres each with its own clock pulse generator are interconnected, means are provided for controlling the frequency of the clock pulse generator at each centre to compensate for variations in the propagation delays. General arrangement, Fig. 1.-Switching centres A and B are provided with controllable oscillators Ga, Gb, respectively, transmission from station A to station B being via channel 1 and that from station B to station A being via channel 2. A phase detector Pa at station A provides a signal on line 3 representing the timing difference between the signals received from station B and the timing at station A to apply appropriate correction to generator Ga, and also via line 4 sends a signal of opposite sense to control generator Gb. A similar phase detector Pb is provided at station B controlling both generators Gb, Ga. The total delay between switching centres is adjusted to be an integral multiple of a frame repetition period. The system may be extended to include more switching centres. The arrangement is described in relation to a P.C.M. system in which each channel consists of 8 bits, there are 24 channels, and an additional bit in each frame provides a frame synchronizing signal consisting of alternate 1 and 0. Details of the control circuit. Fig. 2.-Incoming signals at d are supplied via a fixed delay DLI to a pulse regenerator and bit timing pulse extractor RA, bit timing pulses being provided at output b and the pulse signal at output a. Gates G3, G4, G14 in a synchronizing device A of which there is one for each incoming link, are opened sequentially by the counter C1 controlled by pulses b to pass the P.C.M. signal to respective stores M1 to M12, the number of these stores (twelve) being determined by the maximum amount of compensation likely to be required. The output of one of the stores M1 to M12 is read out at bit rate via gates G15 to G26 controlled by the local clock pulse generator MC through a counter C3, so that the store selected corresponds to the timing of the local clock pulse generator. A circuit FC includes gates G27, G28 which are responsive to the alternate 1 and 0 of the frame sync. signal to provide a sequence of digit 1 at the frame repetition frequency. If synchronization is lost, digit 0 will appear and will be detected at FED to supply a signal via gate G29 to set a flip-flop FFI. This causes a signal to be fed via gate G2 and one bit delay DL2 at the time when a signal is being written in to store M1 which inhibits gate G1 and stops the counter C1 for one clock pulse period, the operation being repeated until synchronization is obtained and the flip-flop FFI is reset. Each switching centre also has a timing circuit B for controlling the frequency of the local oscillator MC and assuming that the system has been synchronized the gates G47 to G49 are opened by a scanner SCAN which connected the synchronizing devices A in sequence. In order to measure the delay after a signal is written into store M1 until it is read out, when the gate G3 is opened a flip-flop FF10 is set via gates G49, G71 to open a gate G72. The output frequency of the local oscillator MC is multiplied by ten at FM and supplied via. gate G72 to a counter C4 which is stopped by a pulse from counter C3 resetting flip-flop FF10 at the time gate G15 is opened. This count, which indicates the above delay, is made once per frame, the gate G71 being controlled by a flip-flop FF11 which is set at time slot 161 and reset at time slot 172. The counter C4 is reset at time slot 160. A count indicating the corresponding delay at a co-oparating switching centre is received from that centre using seven time slots of the 24th channel, i.e. time slots 186 to 192, and is extracted from point c via sequentially opened gates G50 to G63 and is registered by flip-flops FF19. To check that the synchronization is correct at this time an output is provided by gate G31 co-operating with flip-flop FF2 and gate G30 at time slot 185. If it is incorrect, gates G48 and G49 are inhibited. The count registered by C4 and that registered by flip-flops FF13 to FF19 are converted into corresponding currents which are fed to respective control windings of a saturable reactor controlling the frequency of the local oscillator MC. In order to transmit the count registered at C4 to the remote switching centre it is transferred to flip-flops FF3 to FF9 in time slot 185 and from time slot 186 to time slot 192 the count registered by the flip-flops is transmitted via gates G32 to G38. A signal is also sent in time slot 185 via gate G40 to show that the system is synchronized. A gate G39 inhibits unwanted signals from the switching apparatus during this period.