Patent GB-1047639-A (GB)
Title: Improvements in or relating to time division transmission systems
IPC: H04J3/07
Filed: 1963-09-26
Abstract: 1,047,639. Multiplex pulse signalling. WESTERN ELECTRIC CO. Inc. Sept. 26, 1963 [Oct. 18, 1962], No. 37888/63. Heading H4L. In a time division multiplex pulse communication system the signals from a plurality of geographically separated non-synchronized pulse transmitters are retimed by a master clock source of slightly higher repetition rate than the highest pulse rate to be synchronized, by the insertion of control pulses and spaces into each pulse train, and the retimed pulse trains are combined in time division on a common transmission link. The control signals provide information at the receiver for reproducing the original pulse signals and repetition frequencies after demultiplexing. As shown in Fig. 1, signals from transmitters 10, 11, 12 are retimed by the insertion of control signals in circuits 14, 15, 16, controlled by a master clock 18, and supplied via a distributer 20 to the common link 19. Each frame of the transmitted signal consists of n successive time slots in which the first time slot always contains framing signals and the second time slot may contain control signals or information signals. At the receiver the respective pulse trains are selected by a distributer 22 controlled by a synchronization and framing circuit 24 and supplied to corresponding synchronizing receiver circuits 25, 26, 27 which provide outputs identical to and of the same timing as those from each transmitter 10, 11, 12. One of the synchronizing circuits 14 &c. is shown in Fig. 2, the input signal being applied via terminal 30 to a store 31 which allows pulses to be read out at a different rate and also supplies a voltage to a comparator 40 which is a measure of the difference in phase between the input and output signals. A master clock 18 supplies pulses via INHIBIT gate 32 to read out the data from the store at the higher pulse repetition frequency and after every nth pulse from the source 18 a divider 34 supplies a pulse via OR gate 35 to inhibit the gate 32 during that time slot. The output of the divider is also supplied to a circuit 36 and suitably altered to provide a framing signal, e.g. alternate pulses and spaces, which are supplied to the output via OR gate 37. The output of the divider is delayed by one time slot at 38 and applied via INHIBIT gate 39 and OR gate 35 to prevent the read-out of data from the store 31 during the time slot succeeding the framing signal, this time slot being termed the "variable time slot." After a predetermined number of frames an additional time slot of data will accumulate in the store 31 and this condition will be sensed by the comparator 40 which will supply a voltage to AND gate 42 together with the voltage from delay circuit 38. Thus a marker pulse will be supplied to the output during the variable time slot and this will indicate that in the next frame that time slot will contain a data signal. The marker pulse will also be delayed by one time slot at 43 to control a monostable device 44 whose period T is greater than one frame of n pulses but less than two frames. This provides an output to inhibit the gate 39 and allow information to be read from the store during the variable time slot succeeding the marker pulse. Figure 3 shows one of the timing recovery circuits 25 &c. at the receiver. A circuit 51 extracts the timing of the incoming signal to control via INHIBIT gate 56 the writing of the signal into store 52 which is similar to the store 31. A circuit 54 produces an output pulse on the receipt of each framing signal which is applied via OR gate 55 to inhibit the gate 56 during the framing interval. The output from 54 is also delayed by one time slot at 58 and supplied via INHIBIT gate 59 to inhibit the gate 56 during the variable time slot. When a marker pulse follows a framing pulse a gate 60 is actuated which controls a " phase-locked gate " 62 supplying a gate signal centred in time about the marker pulse and occupying a time interval of two frames which together with the marker pulse actuates AND gate 63 which via OR gate 64 triggers a monostable device 65. The output from 65 has a period T more than one frame but less than two frames and inhibits gate 59 to allow writing into the store during the variable time slot of the next frame. The output of device 65 also inhibits gate 60 so that the data pulse in the next variable time slot does not cause false operation of monostable device 65. The phase-locked gate 62 includes a pulse generator whose phase is controlled by the signal from the gate 60, Fig. 6 (not shown). Since a marker pulse must occur every mth or (m + 1)th frame, if a marker pulse is lost in transmission and does not occur within the gate interval, the gate 62 will generate an output pulse at the end of the gate interval (two frames). Thus if a marker pulse is lost in the mth frame errors will occur in the next frame, but if it is lost in the (m+1)th frame the next frame will be correct.