Patent GB-1265183-A (GB)
IPC: H04J3/06
Filed: 1969-10-23
Abstract: 1,265,183. Multiplex pulse code signalling. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 23 Oct., 1969 [26 Oct., 1968], No. 51927/69. Heading H4L. In a method of synchronizing a receiver to one of a plurality of bit streams, e.g. in a multiplex system having different patterns of synchronizing bits wherein each bit-stream is divided into blocks having a constant number of bits and each block having at least two different synchronizing bits arranged in a periodically repeated pattern, the synchronizing process is divided into two successive phases. In the first phase from an arbitrary bit position in the bit-stream a first divisional sequence consisting of a series of bits spaced apart by equal first distances is selected, temporarily stored and compared with a number of locallyproduced comparison sequences in turn until equality is found or in the event of lack of equality within a first time limit, the first phase is repeated with a different sequence et seq., until equality is found, after which the second phase of the synchronization process is commenced in which a second divisional sequence consisting of a series of bits spaced apart by equal second distances is selected and each bit of the second sequence is compared with those of the first sequence until inequality is found and in the event of lack of inequality within a second time limit, the synchronization process is repeated starting with the first phase and when inequality is found the first time within the second time limit the receiver is synchronized to a bit position which has a predetermined position in the second sequence. One of a plurality of bit-streams having a synchronization combination as illustrated in Fig. 1 (not shown) may be applied to a terminal 100 and passed to a shift register 102. A starting signal applied to terminal 103 sets an OR gate 104 and flip-flop 105 in the state 1 which opens an AND gate 106 which allows a pulse sequence having a repetition frequency of fb/d1 to be applied to register 102, where fb is the bit frequency and d1 equals 4 in this example. The pulse sequence is derived from a circuit pulse distributer 107 providing four pulse sequences shifted in time by one bit period the output being passed through a selected one of AND gates 109, 1-4 which also have an input from a cyclic pulse counter 110 having four positions. A pulse generator 108 having a frequency fb controls distributer 107. A device 112 produces a reference sequence which is compared in a device 111 with that in register 102. If the device 111 does not detect equality within a time limit set by a timer 117 previously set by flip-flop 105, the pulse sequence into register 102 is shifted one bit by gates 109, 14 et seq. until equality is found when flip-flop 105 is reset to 0 and timer 117 is disabled. This terminates the first phase. The second phase commences with the 1 output from device 111  also being passed to a selector 121 to which are applied two pulse sequences having a repetition frequency of fb/do (do =2) shifted in time by one bit period these being derived through a pulse distributer 122 from generator 108. The output from selector 121 is passed to an AND gate 123 which also receives the incoming bit stream from terminal 100 and to an AND gate 124 which also receives a 1 signal from block 134 which corresponds to the value of the bit in stage 0 of shift register 102 at the instant when comparison device 111 assesses equality. An AND gate 126 receives the output from gate 124 also the inverted output from gate 123 which starts a time delay circuit 127 which after the expiration of the delay advances the counter 110 by one step when the synchronization process is repeated.  When the AND gate 126 supplies a 1 signal prior to the time limit of circuit 127 this sets circuit 127 in the rest position so that the circuit cannot become operative. The 1 signal of AND gate 126 sets a counter 131 controlled by generator 108 and having a counting capacity of b (b=40) hence counter 131 indicates, for each incoming bit, the bit position in the block. The 1 output from gate 126 also resets selector 121 into the rest position through OR gate 132 when phase 2 of the synchronizing process is terminated by delay circuit 127 becoming operative, i.e. gives a 1 output. Selector 121 is illustrated in greater detail in Fig. 5 (not shown). It is stated that complete equality with the comparison sequence may be made more difficult in the presence of errors, hence equality at a determined number of places less than the maximum may be acceptable in such circumstances.