Patent GB-1033069-A (GB)
Title: Improvements in or relating to code pulse receiver apparatus
IPC: H04J3/06
Filed: 1963-07-04
Abstract: 1,033,069. Pulse code signalling systems. WESTERN ELECTRIC CO. Inc. July 4, 1963 [July 9, 1962], No. 26484/63. Heading H4L. A receiver for reconstituting a message from code groups of pulses of an incoming pulse train comprises a clock pulse generator operative in response to the received pulse train to generate clock pulses, means for determining an incorrect time relationship between the clock pulses and the incoming pulse train, and means operative upon the determination of an incorrect time relationship to change the relative phase of the clock pulses in one time direction by a predetermined number of pulse periods and thereafter to change the relative phase of the clock pulses intermittently in the other time direction one pulse period at a time until a correct time relationship is reached. As described, Figs. 2 and 3, in a time division multiplex system the framing signal consists of the repetition of 1010... with the 1 bit (F 1 ) appearing at the beginning of each frame and the 0 bit (F 0 ) at the middle, each frame also containing various control and supervisory bits P 1 , P 2 , and S 1  to S 4 . The incoming signal is supplied via receiver 10 to control a clock generator 13 at the bit frequency which when the apparatus is correctly framed, supplies pulses to a counter 11 via gate 21 which is inhibited for an out-of-frame con. dition. The counter 11 supplies timing pulses via a translator 12 in known manner. The signal from the receiver 10 is also fed to a flipflop 31 via a logic circuit 32, 33, 34 so that a binary 1 sets the flip-flop to its " 1 " state or causes it to remain there and a binary " 0 " sets it to its " 0 " state. The " 1 " and " 0 " output of the flip-flop 31 are supplied to AND gates 41, 42, respectively, of a frame loss detector, together with respective timing pulses F 0 , F 1  from the translator 12, an output from either gate 41 or 42 indicating loss of frame. Any output from gate 41 or 42 is supplied via an OR gate 44 to advance the count of a forward-backward counter 43. The F 0  and F 1  clock signals are also supplied via an OR gate 46 to a binary counter 45 which continuously counts to three and resets in response to the F 0 , F 1  signals so that a " count-down " pulse is applied to the counter 43 via a gate 47 every fourth clock pulse applied to the counter 45. This prevents the start of a frame search operation until a true out-of-frame condition exists. Short noise bursts or signal fade lasting one or two frames will cause the counter 43 to count up but upon termination of the noise it will return to zero in response to the countdown signals. For a true out-of-frame condition the count-up signals are applied at a faster rate than the count-down signals so that the counter 43 quickly reaches a predetermined number (e.g. six). When this predetermined count is reached a signal from flip-flop 48 via an inverter 50 results in the " inhibit " signal being removed from gates 51, 52, which receive clock pulses F 0 , F 1  and respective outputs of the flip-flop 31, and a signal is supplied at the output of OR gate 54 if an out-of-frame condition exists. The signal from flip-flop 48 is also supplied after differentiating and clipping at 62 to set the flip-flop 61 which supplies an inhibiting signal to gate 22 and also a signal to the AND gate 63 together with the signal from the gate 54. This sets flip-flop 64 and after a short delay at 66 resets flip-flop 61. The signal from flip-flop 64 inhibits gate 22 and partially enables gate 65. When counter 11 returns to zero this is detected at 67 and a signal is supplied via gate 65 to advance the counter 11 by n bits. After a short time delay the output of gate 65 is applied to reset the flip-flop 64 so that the gate 22 is no longer inhibited and each signal from gate 54 sets a flip-flop 23 to its " 1 " state which inhibits the transmission of clock pulses via gate 21 to the counter 11, the flipflop 23 being reset after a short delay via gate 24. The flip-flop 23 is set and reset in this way until the correct clock phase is found, each cycle of the flip-flop 23 retarding the phase of the counter 11 by one bit period. The " 1 " output of the flip-flop 48 is connected to the reset terminals of the stages of a counter 81 via an OR gate 82 and differentiator and clipper 83 and also after inversion at 50, to the inhibit terminal of gate 84. Thus when flip-flop 48 is in its" " 1 " state (indicating frame loss) the counter 81 is reset and the F 1  and F 0  clock signals are applied via OR gate 85 to the counter 81. The counter 81 attempts to count to a predetermined number (e.g. sixteen) in response to the clock pulses but when an out-of-frame condition exists it is continually reset to zero by the " search reset " signal from the gate 54. When framing is re-established the counter 81 counts to the chosen number sixteen this is detected at 86 and flip-flop 48 and counter 43 are reset and normal operation is resumed.