Patent GB-738587-A (GB)
Title: Synchronising arrangements for pulse communication systems
Inventor: GIBBONS PAUL
IPC: H04J3/06
Filed: 1952-08-06
Abstract: 738,587. Pulse code modulation systems. MINISTER OF SUPPLY. Aug. 13, 1953 [Aug. 6, 1952], No. 19851/52. Class 40 (5). In a pulse communication system there is inserted into the series of the informationbearing signals a synchronizing pulse train consisting of alternate mark and space signals repeated for longer than a predetermined time and terminating at the synchronizing instant with two consecutive signals of the same type. In the pulse code system described, the digitfrequency is 420 kc/s. and a synchronizing signal lasting 60 digit periods, i.e. 143 microseconds, is inserted ten times per second. The synchronizing signal is generated by a multivibrator V2a, V2b, Fig. 1, controlled through valve V1 by a 420 kc/s. oscillation applied through transformer T1. The multivibrator produces two consecutive pulses or zeros once every 143 microseconds when valve V1 is cut off by a negative pulse from terminal t1. The communication signal pulses are normally fed from terminal t3 through valve V6 to the output terminal t4. When the synchronizing signal is to be inserted valve V5 is rendered conducting to feed the pulses at the anode of valve V2b to the terminal t4 and to cut off valve V6 at its control grid by the drop in potential at the screen grid of valve V5. The pulses at terminal t1 control the gating of valves V5, V6 after delay in a 3.6 microsecond network D1. At the appropriate time the negative pulse from the end of network D1 cuts off valve V3a of a trigger circuit V3a, V3b. The resultant positive anode pulse, after differentiation in circuit C5, R5 is applied to diode V4a to charge condenser C7 and render valve V5 conducting at its control grid. At the end of the 143 microsecond period the next pulse in network D1 is applied, from a tapping, to diode V4b which conducts to discharge condenser C7 and render valve V5 non-conducting. The time constant of the trigger circuit V3a, V3b, is 0.1 second so that it only operates to render valve V5 conducting ten times per second as required. At the receiver, the incoming pulse train, Fig. 4 (a), is applied from terminal t6, Fig. 3, to the valve V10 in the anode circuit of which is a short-circuited network D2, having a delay of one digit period, so that the voltage applied to the auto-transformer T2 is a threelevel waveform, Fig. 4 (b) which is the combination of the incoming waveform with itself inverted and delayed by two digit periods. Alternatively the network D2 may be opencircuited and of half the delay period. The synchronizing signal produces a constant voltage at the middle level, so that valves V11a, V11b remain cut off for the whole of the synchronizing interval, during which condenser C2 continues to charge through resistance R2, Fig. 4 (c). At other times, one or other of valves V11a, V11b is intermittently rendered conducting to discharge condenser C2. After a predetermined time from the commencement of the synchronizing interval, the voltage on condenser C2 rises to a value which renders conducting valve V12a of a long-tailed pair V12a, V12b to produce the leading edge of a positive pulse at the anode of valve V12b, Fig. 4 (d). This pulse is terminated at the synchronizing instant when condenser C2 is discharged by valve V11a or V11b due to the occurrence of the two consecutive signals of the same type. The output pulse is passed to a differentiating circuit C6, R6, and clipper W6 to produce the negative trailing-edge synchronizing pulse, Fig. 4 (e).