Patent GB-1294731-A (GB)
Title: Bivalent signal transmission system
IPC: H04L25/493
Filed: 1970-04-02
Abstract: 1294731 Digital transmission Systems, CIT COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS 2 April 1970 [2 April 1969] 15753/70 Heading H4P In a bipolar transmission system in which transitions from one level to another are separated by a minimum time interval, the signal levels are sampled at a fixed periodic time shorter than the interval and index signals developed corresponding to the levels of the bipolar signal. The sampling pulses are indexed by a counter the contents of which are transferred into a store at each transition. In the Specification the sampling pulses are divided into groups of sixty each corresponding to a unique group of six digits preferably forming one combination of a reflexive binary code, e.g. the Gray code in which each successive group differs from the preceding group by a "1". The properties of the code are exploited not only to indicate the position of transitions but also the polarity. Combinations corresponding to "0" and "1" are not transmitted and combinations 62, 63 represent transition signs +, - respectively. Code 63 may also be used for synchronization. At the transmitter. Fig. 6, the signal M to be coded is applied to terminal E and through bi-stable circuit 11 directly to exclusive OR gate 41 and indirectly to bi-stable circuit 12 providing a comparison of two successive samplings of M at clock frequency (100 kc./sec.). A change in polarity is transmitted through gate 26 to terminal D causing the count in 44 to be transferred into memory 46, and simultaneously a bi-stable circuit 13 is returned to zero. Clock pulses are applied to a terminal H and through a decade divider 43 to terminal H<SP>1</SP>. Counter 44 assumes successive states 2-61 in synchronism with the H clock pulses through gate 24. State 61 is decoded at 45 triggering monostable circuit 14 and with a delay of 2À5 n/sec. trigger 5 monostable circuit 15 both outputs advancing counter 44 to state 63. Monostable 15 triggers monostable 16 which sets counter 44 to zero after a further delay of 2À5 n/sec. so the counter as a whole is returned to "1". If there is no transition when counter 44 reaches a state of 61, bi-stable 13 is "on" hence gate 23 is open thus a pulse from gate 25 corresponding to states 62, 63 of counter 44 can pass through gate 23 and gate 26 to D. State 62, 63 can be transmitted. Natural binary is converted into reflexive binary, e.g. Gray code in 47 and transferred through AND gates 48 into shift register 49 and shifted therefrom by a flow of frequency pulses H1. In the receiver, Fig. 7, shift register 51 converts the reflexive binary to natural binary which is passed through transcoder 53 also AND gates 54 to a memory 55 in response to signals from 76. Memory 55 has six stages, connected through exclusive OR gates 56. A decoder 52 recognizes state 63 which is passed to a terminal X also a counter 58, also receiving lower frequency pulses H<SP>1</SP> from a clock pulse extraction circuit 71, 72, which is recognized by a zero detector 59 the output from which is passed to a delay circuit 76 also AND gate 75, having output terminal Y, which receives state 62 recognition signals from decoder 52. Monostable delay circuit 76 provides a second input to AND gates 54 also returns to zero stages 2-6 of a counter 57 connected to the second inputs of OR gates 56 which are commoned through an AND gate 73 to the output flip-flop 74, which receives X, Y signals hence reproducing signal transitions. Counter 57 assumes the successive states 2-61 which is compared in OR gates 56 with the number stored in memory 55 and when they are equal, AND gate 73 triggers output flip-flop 74 to reproduce signal level transitions. In the state 63 a transition to "1" is applied to flip-flop 74 directly through line X and in state 62 a return to zero signal is applied through AND gates 75 in zero state of counter 58. The state 62, 63 and 0 are never assumed by counter 57. For numerical and telegraph signals the system is stated to be equivalent to a PCM telephone system and that with six code pulses, distortion is not greater than 1.5%.