Patent GB-1103567-A (GB)
Title: Improvements in or relating to pulse transmission systems
IPC: H04J3/06
Filed: 1966-09-30
Abstract: 1,103,567. Pulse transmission system. WESTERN ELECTRIC CO. Inc. 30 Sept., 1966 [9 Nov., 1965], No. 43727/66. Heading H4L. A pulse transmission system comprises a transmission terminal for multiplexing on a common time divided transmission facility 20, Fig. 1, pulse trains from a plurality of asynchronous pulse transmitters 10-12, the terminal including means for inserting signals into the pulse trains to increase their pulse repetition rate, a generator of a first predetermined pulse code word M providing a system synchronization facility, generators of a second code word C representative of those pulse trains into which said signals have been inserted, and means for transmitting the first and second code words in sequence in predetermined portions of the transmission space of said common facility. In a first system, four pulse transmitters operating at nominally equal frequencies of 55 Megabits per second are sequentially time multiplexed on to a 220 Megabits per second system. 144 information bit positions I 1  . . . I 144 , Fig. 3A, are provided, a framing bit pulse F being positioned between the first and last information bit, and a coding bit position S being provided between information bit positions I 72 and I 73 . The pulses in the coding bit positions S run in a 54 bit cycle, Fig. 2, the first 18 pulses constituting the code word M (so designed that it will still be recognizable with up to two errors) the second group of 18 pulses constituting the code word C and the last group of 18 pulses constituting a G or an R code word. For the present system, the G code word is used comprising 18 zeros. The four transmitters 10-12 (only three shown) each feed a respective synchronizing circuit 14-16 comprising a store which is read out to a respective contact of a multiplexing rotary switch 21 at 55 megabits a second and in correct respective sequence. Thus for the third transmitter, which will be considered, its pulses are read out by read-out pulses # 3 , Fig. 3D, to appear in information bit positions I 3 , I 7  ,I 11 , &c. The read-out pulses # 3  and framing pulses F are produced by a clock and framing circuit 18, the framing pulses being applied to switch contact f. The M, C and G or R code word pulses are produced by a synchronizing signal generator 19 and fed to switch contact S. The generator 19 also includes means which periodically interrogates the contents of each synchronizing circuit store, and when the content falls below a predetermined value, indicating a faulty slow input frequency, causes a pulse to be inserted into the store input, (or equivalently stops the read out for one pulse) to increase the frequency and produce the desired synchronisms of the multiplexed signals. The synchronizing signal generator determines which if any transmitter has had its output so added to and accordingly produces a group of three consecutive one pulses, which, when the transmitter is the first are positioned in the 2nd, 3rd, 4th positions of the C code word-in the 5th, 6th, 7th positions for the second transmitter-in the 8th, 9th, 10th positions for the third transmitter and in the 11th, 12th, 13th positions for the fourth transmitter. All other C code word positions are zeros. Thus after demultiplexing the information signals at the receiver terminal, using a rotary switch 25 synchronized by recognizing the M code words, a synchronizing signal receiver 40 is able to determine which transmitter output has been added to by decoding the C code word, and remove said inserted pulse. The inserted pulse is inserted in the frame bounded by the 15th and 16th bit positions of the C code word, and the stores are interrogated during the preceding 15th bit position of the M code word. In a second system 144 pulse trains at a nominal frequency of 1À544. Megabits per second are time multiplexed on to the 220 Megabit per second line. Since however, using the C code arrangement of the first system, this would require 3 x 144 = 432 bit positions, resulting in prohibitively long intervals between the synchronizing M code words, a different coding system must be used. Accordingly every 36th G code word (18 zeros) is replaced by an R code word, and the R code words serve as framing means for progressive variations in the transmitters representative by the C code word. Thus the C code word after an R code word can represent the 1st to the 4th transmitter, the next C code word can represent the 5th to the 8th transmitter, and so on. Using this second system, a mixture of high and low speed pulse trains may be multiplexed.