Patent GB-1486887-A (GB)
Title: Telecommunication signal frame alignment arrangements
IPC: H04J3/06
Filed: 1973-09-05
Abstract: 1486887 Signal frame alignment procedures POST OFFICE 30 Aug 1974 [5 Sept 1973] 41750/73 Heading H4M In an apparatus for reframing a frame structured communication signal, a signal detecting and generating arrangement 1 receives the frame structured signal and generates a line frame sync signal when the frame sync pattern is detected in the signal. A main local frame structure generator 8 produces local frame timing signals for timing the reception and decoding of the communication signal and a provisional local frame structure generator 20 generates provisional local frame timing signals. The local frame structure generator is reset by each occurrence of the line frame sync signals. In response to persistent non coincidence of local frame timing signals and the line frame sync signals and persistent coincidence of the provisional local frame timing signals and line frame sync, a decision arrangement (gate 12) resets the main local frame structure generator in alignment with the provisional local frame timing signals. The frame structured telecommunication signal is received on an input line 5, e.g. as a 60 k bit/s serial binary stream having an 800 bit frame containing a distributed 20 bit frame signal pattern, spaced at 10 bit intervals in the frame. The signal is entered into a frame store 2 comprising a 200 bit shift register under the control of a 60 k bit/s clock X derived from the line by conventional technique. The contents of the register at bit positions corresponding to framing bits are compared with the frame pattern contained in a permanent store 20. When the frame pattern is detected a line frame pulse appears on the output line 7. A local frame structure signal distributer (LFD) 9 provides channel pulses on lines 9A- 9N corresponding to the individual multiplex channels. It may be a shift register containing one binary "1" and shifted by the clock X. The LFD provides a logical "1" once per frame on output 14; in normal error free conditions this coincides with the line frame pulse on line 7. This coincidence is detected by AND gate 11 whose output is applied to cause a weighted bidirectional counter 10 to count DOWN 1 per frame. Lack of coincidence causes AND gate 16 to deliver a pulse once per frame and cause the counter to count UP 3. The count limits of the counter 10 are zero and nine; when the upper limit is reached a "1" is delivered on line 18 and applied to AND gate 12, if this is receiving a "1" on line 31 (see below), a signal is applied to reset the LED 9. Thus if the local frame is out of synchronism with the received frame more often than one frame in 3, the LFD will be reset. In the provisional frame generator circuit 20, a scale of 800 counter under the control of the 60 k bit/s clock X counts up steadily until reset to zero by the line frame pulse on line 7. These pulses will be separated by 800 bits under error free conditions. If a frame signal pattern other than in the position determined by the local frame structure is detected, the counter 21 resets to embody a provisional frame structure and provides a frame pulse on lead 28 once per provisional frame. This is applied to AND gate 23 receiving also the signal on lead 7. The gate output is applied to a counter 22 whose maximum count is 3. When provisional and detected frames coincide, the counter reaches its maximum and a pulse is delivered to lead 30. This passes AND gate 26 and is applied to AND gate 12 which however would be open under error free conditions. If an out of position frame signal is detected, counter 22 is reset by the line frame pulse on lead 7 passing via AND gate 25. The provisional frame structure signal changes to the newly observed frame signal and if three successive correspondences are obtained, a "1" on lead 31 will be delivered to coincide with a "1" on lead 18. A pulse will be delivered on lead 19 to reset the LFD 9. Thus a provisional frame structure must be determined and checked for persistence (circuit 20), while simultaneously another persistence check must establish that there is a true misframe condition (circuit 8).