Patent GB-849891-A (GB)
Title: Improvements in or relating to electric pulse code modulation systems of communication
Inventor: GRAYSON HARRY
IPC: H03K23/76
Filed: 1958-04-15
Abstract: 849,891. Pulse code modulation systems. STANDARD TELEPHONES & CABLES Ltd. April 10, 1959 [April 15, 1958], No. 11905/58. Class 40(5). In a pulse code modulation system, the binary code employed is of the unit disparity, unit distance type i.e. one in which each code group has (2n - 1) digit positions of which either n or (n - 1) are occupied by pulses, the groups being so arranged that a signal amplitude charge of one level produces a charge in only one digit position. The code described with reference to Fig. 2 has the further property that the groups are arranged in such an order that the group representing any even numbered level can be obtained by adding together the groups representing the two adjacent odd numbered levels and limiting the pulses to the same amplitude. As a result the number of coding elements necessary can be almost halved without reducing the coding speed. Not all the possible combinations can be used in this code, the number N 1  used being equal to 2 x 3, (n-1). The 7-digit code of Fig. 2 has either three or four pulses in each group and may be converted to a balanced error-detecting code by the addition of a fourth pulse in the eighth digit position for all the odd numbered levels. The additional pulse takes no part in the actual coding of the signal and may be disregarded at the receiver. If n is not too small, error detecting means may be provided to determine whether each group contains either n or n - 1 pulses, and the additional pulse is not necessary. A coder for producing the 5-digit code represented in the portion of Fig. 2 bounded by the lines A, B is shown in Fig. 3 and comprises ten coding elements 101 representing odd numbered levels 1-17 and the highest level, 18. Each coding element is biased by tappings on potential divider 107 fed by a positive source 103 and a negative source 104. Pulses from source 111 control a sampler 109 and read pulse generator 112, the pulses from sampler 109 being arranged to gate the coding elements 101 in the following manner. When the sampled signal corresponds to an odd numbered level or level 18, only the coding element corresponding to that level is unblocked, all other elements 101 being blocked, but when the sampled signal corresponds to an even numbered level, other than level 18, the two corresponding adjacent odd numbered coding elements are unblocked. A reading pulse from generator 112 after passing through limiting circuit 113 and valve 114 is only passed by the one or two unblocked coding elements. The horizontal output conductor of each coding element is connected to the appropriate vertical digit conductors 1-5 by a rectifier matrix to produce the correct code group simultaneously on the five digit conductors, which conductors are connected to tappings on a delay net 117 to produce the code group in serial form at output terminal 119. In the case of an even numbered level e.g. level 10, coding elements for levels 9, 11 are unblocked and the two output code groups are combined in digit conductors 1-5, the limiting circuit 120, 121 maintaining the correct digit pulse amplitude. Means, Figs. 6, 7 (not shown) may be provided to detect the presence of only two pulses in any code group and to add a third pulse in the sixth digit position. The receiver shown in Fig. 8 is for an 8-digit balanced error-detecting code in which each group should contain four pulses. The decoder 144 is of the type having eight output conductors 145 corresponding to the eight digit positions, positive output pulses being produced on those output conductors corresponding to the digit positions in which the digit pulses of a group occur, and negative pulses being produced on the remaining output conductors. Error detector 147 produces an output pulse whenever the combination of pulses simultaneously fed into it is other than four positive and four negative. Synchronizing. Filter 148 and divider 149 provide synchronizing pulses at the group repetition frequency, these pulses being fed through adjustable delay device 150 to control decoder 144. When synchronism is lost, a group selected by decoder 144 will contain either more or less than four pulses, the output pulse produced by detector 147 operating alarm 151 and stepping delay device 150 by one digit period. This process is repeated until synchronism is regained. Synchronism may alternatively be obtained by transmitting one of the unused code groups or preferably by transmitting the groups 11110000, 00001111 in sequence. These synchronizing groups may also be used for channel synchronizing in a multiplex system.