Patent GB-967391-A (GB)
Title: Improvements in or relating to synchronous pulse communication systems
IPC: H04J3/06
Filed: 1961-11-29
Abstract: 967,391. Multiplex pulse code signalling. WESTERN ELECTRIC CO. Inc. Nov. 29, 1961 [Dec. 5, 1960], No. 42784/61. Heading H4L. In a synchronous pulse communication system in which the transmitted pulses are arranged in frames and may be of either polarity i.e. " pseudo-ternary" and the framing pulse is arranged to have the same polarity as a predetermined message pulse in the frame, the receiver is provided with means for selecting the received framing pulses and comparing them in time with locally generated framing pulses and recording any non-coincidence in the occurrence of the two framing pulses and means responsive to a predetermined number of such discrepancies for interrupting the local generation of framing pulses until the reception of the next framing pulse. As shown, in a time division multiplex p.o.m. system, a bipolar wave 10, Fig. 2, is supplied to a transformer T1 at the receiver and is converted into unipolar form by diodes 14, 16, the wave 18 being supplied to terminal S and the wave 20 to terminal R of a bi-stable circuit 22. The wave 10 as shown is a small portion of a frame in which the time slot F is reserved for framing pulses, a framing pulse having the same polarity as the immediately preceding message pulse. Diodes 15, 17 supply unipolar pulse code modulation signals to the decoder 23. The output terminals S<SP>1</SP>, R<SP>1</SP> of the bi-stable device 22 are connected to delay devices 26, 28 respectively each providing a delay equal to one pulse width, i.e. a half time slot, and the delay devices are connected to AND gates 30, 34, so that for message pulses 24, 44 and 48, the gates 30, 34 are not enabled but either gate 30 or 34 is enabled when a framing pulse such as 50 is applied. An output from either gate 30 or 34 will enable an OR gate 52 to prevent an INHIBIT gate 56 from passing a locally generated framing pulse received at its input 58. Thus when framing is correct no pulses are supplied to an error store 60. A clock pulse generator 62 is controlled by the received pulses and produces pulses at the basic pulse repetition frequency which are supplied to an INHIBIT gate 64 and passed to a digit counter 66 when no inhibit pulse is present on the input 68. For every eight pulses received the counter 66 supplies one pulse to the channel counter 70 and the counter 70 after every twenty-four pulses received (i.e. twenty-four channels) supplies a framing pulse to the input 58 of INHIBIT gate 56. Each of the pulses from the generator 62 are supplied via the counter 66 to control the decoder 23 and the demultiplexer 65 is controlled by pulses from the counter 70, decoding and demultiplexing operating only when the decision circuit 63 has not declared the system to be out of frame. As described, the number of successive errors to be tolerated is assumed to be three and these errors are stored at 60. After three successive errors the voltage built up in the circuit 60 will trigger the decision circuit 63 which inhibits the gate 64 and decoding and demultiplexing will cease until the next framing pulse is received and appears at the output of gate 30 or 34. This framing pulse will be supplied via OR gate 52 to enable AND gate 76, which is already energized on its input 78 by the decision circuit 63, and thus cause the voltage stored at 60 to be reduced by an amount proportional to one framing error. The output of decision circuit 63 is removed and the inhibit voltage is removed from the gate 64. Correct reception of subsequent framing signals will cause the error store 60 to be completely depleted.