Patent GB-960388-A (GB)
Title: Improvements in or relating to electrical signalling systems
Inventor: CROFT GEOFFREY FRANCIS
IPC: H04J3/06
Filed: 1960-12-02
Abstract: 960,388. Multiplex pulse code signalling; telegraphy. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. Nov. 28, 1961 [Dee. 2, 1960], No. 41577/60. Headings H4L and H4P. Relates to a time division multiplex pulse code or like signalling system in which one channel of each frame is assigned to a characteristic synchronizing signal which is tested at the receiver during each frame and treated as acceptable only if it comprises a predetermined minimum number of consecutive signalling elements of one kind followed by an element of the other kind. The system described has 24 speech channels, making a total of 25-channels and uses a 7-digit binary code, a further element at the beginning of each binary series being used for other signalling purposes. The synchronizing signal consists of seven 1's followed by a 0. As shown in Fig. 1, an 8-stage counter C1 at the receiver is operated continuously by clock pulses S and a channel switching counter C2 is also operated by the clock pulses when coincident with a pulse from the eighth stage of the counter C1. Normally, the counter C1 is reset to position 1 but when the counter C2 is on position 24 a pulse is supplied via gate G4 to reset the counter C1 to position 2, and the counter C2 moves to the synchronizing position S. In the position S an output TS is supplied to gates G1 and G2 so that the incoming signal IN may be tested for the synchronizing signal. If the correct synchronizing signal 11111110 is being received, the counter C1 is driven to position 8 while six 1's are received and the following 1 (and any further 1's) cause the gate G2 to be opened so that further movement of the counters C1, C2 is prevented by the inhibiting gates G3, G5. When a 0 is received normal stepping is resumed. Thus six or more 1's followed by a 0 is recognized as an acceptable synchronizing signal. If a 0 appears on the input before counter C1 reaches position 8, a pulse is supplied via inverter I1 to open gate G1 to inhibit gate G3 and reset the counter C1 to position 1, the inverter 12 ensurng that the gate G1 receives an input in positions 1 to 7 of the counter C1 The counter C1 can now only be advanced to position 8 by at least seven successive 1's followed by a 0. In a modification, Fig. 2, means are provided whereby any failure during the first six 1's of a synchronizing signal will not initiate a hunting operation. As before, counter C1 will cycle continuously during information channels and counter C2 will be advanced until it reaches the synchronizing position S, and supplies the output TS to gates G1,G2 and G9. If the correct synchronizing signal is received the system functions normally, but if a 0 appears in what would normally be a sequence of 1's gate G1 will be opened to apply an input to gates G6 and G8. On the next timing pulse S gate G8 is opened to operate bi-stable device M and supply a second input to gate G6 via 16-bit delay line DL. The delay line DL prevents immediate operation of the gate G6 and if the disturbance is not repeated, the bi-stable device M is reset via gates G5 and G9 at the end of the synchronizing signal. If the incorrect operation is repeated before M has been reset an output is obtained from gate G6 which resets counter C1 to position 1 and via gate G3 inhibits the stepping circuit. Synchronization can then only be obtained by seven 1's followed by a 0. The remaining gates G2, G4, G5 operate as in Fig. 1. In a modification of Fig. 2, the gate G4 is omitted and the counter C1 always reset on the 1 position. A further modification, Fig. 3 (not shown), is similar to Fig. 2, but does not include the feature whereby a synchronizing signal comprising a smaller number of 1's than the maximum is acceptable.