Patent GB-1407892-A (GB)
Title: Frame synchronization system
IPC: H04J3/06
Filed: 1973-04-30
Abstract: 1407892 Multiplex pulse code signalling INTERNATIONAL STANDARD ELECTRIC CORP 30 April 1973 [10 May 1972] 20365/73 Heading H4L A frame synchronization system is provided for a time division multiplex binary data signal having a superframe with M midframes, each of the M midframes including m subframes, and the data signal including a first sync. signal having a first predetermined pattern disposed in each of the M midframes (short sync.) and a second sync. signal (long sync). having a second predetermined pattern composed of M bits, each of the M bits being disposed in a different one of the M midframes of a superframe, M and m being integers greater than one. As shown in Fig. 1, a superframe A comprises 64 midframes B and a midframe consists of 1 subframes C, the odd subframes in each midframe having 9 bits and the even subframes 8 bits. The first 8 bits of each subframe are assigned one bit at a time to the channel groups and the 9th bit, if any is assigned to supervizing, synchronizing etc., as shown for V, C, V, SO, V, D, L, SI, Fig. 1. The short sync. code consists of alternate binary 1 and 0 and the long sync. is in the form of a 64 bit pseudo-random code. The last short sync. bit (SI) of each superframe is deleted. Synchronization system details.-The bit rate is recovered at 2, Fig. 4, from the incoming data at 1 and supplied to a divide by two counter which may be enabled or disabled for a 48 channel or a 96 channel mode respectively and controls a subframe counter 5 including a divide by eight counter 6. Pause logic 7 stops the counter 6 for one clock period whenever enabled by the midframe and superframe counters 8, 12 respectively causing the subframe to be either 8 or 9 bits long. Midframe counter 8 is a divide by fifteen counter formed by a divide by sixteen counter 9, 10 with logic 11 causing it to skip the 16th count and generates the short sync. code at counter 10. Superframe counter 12 includes a divide by sixty-four counter 13, 14 and a further divide-by-sixty-four counter consisting of a six-bit shift register 15 with feedback logic 16 arranged to produce the pseudo-random 64-bit sync. sequence. One pulse per cycle from this long sync. code generator resets counters 13, 14 to keep them synchronized. The data on input 1 is compared continuously at 18 with the short sync. code generated at counter 10 and when a mismatch is present the output of bi-stable 19 in search logic 20 causes an up-down counter in decision circuit 21 to count down one, a match causing it to count up one. Logic 20 may be as described in U.S.A. Specifications 3,597,539 or 3,594,502. Logic 20 when enabled by circuit 21 via AND gate 24 generates a HALT pulse whenever timing logic 35 indicates that a short sync. bit should be received and a mismatch is indicated, to inhibit counters 5, 8 via gates 26, 27, a " down " pulse being counted. A succession of mismatches causes a continuous HALT condition but no more " up " or " down " pulses are generated until the next short sync. bit arrives. When logic 20 is disabled, " up " or " down " pulses are counted according to the comparison obtained but no HALT pulses are generated. Decision circuit 21 is shared by both the short and long sync. signals and its operation depends on whether the count is above or below predetermined thresholds, Fig. 5. The " up " pulses are disabled in a region above threshold 28 and " down " pulses are disabled below threshold 30. Above threshold 32 the decision circuit responds only to " up " and " down " pulses from the long sync. circuit and below this threshold it responds only to the short sync. pulses. Below threshold 33, HALT pulses are generated to change the frame phase for the short sync. pulses and eventually correct the midframe phase. In the case of the long sync. signal, below threshold 35 a frame outof-sync. alarm is operated. When after the midframe phase has been corrected the count reaches threshold 32 and continues upward, further changes of the frame phase are stopped and the alarm is disabled. The long sync. framing circuit includes shift register 15 and feedback logic 16 and a feedback circuit via switch 36 which generates the local long sync. code. This is compared at 37 with the received long sync. code from the demultiplexer. A bistable 38 generates " up " pulses if there is a match and "down" pulses if there is a mismatch. Mismatches cause the count to decrease until an ENABLE signal changes over switch 36 to pass the received long sync. code to shift register 15. This is called the " load mode " and occurs below threshold 34, Fig. 5, and enables the shift register to be filled with error-free long sync. bits whereupon the matches detected at 37 cause "up" pulses to be generated until the switch 36 reverts to its previous position. The shift register 15 then operates independently of the received long sync. code and continues in this mode while synchronism is maintained.