Patent GB-859156-A (GB)
Title: Improvements in or relating to coding arrangements for electric p.c.m. systems
Inventor: NEU WALTER
IPC: H03M5/00
Filed: 1959-10-09
Abstract: 859,156. Pulse-code modulation systems. STANDARD TELEPHONES & CABLES Ltd. Oct. 9, 1959, No. 34268/59. Class 40(5). In a coding arrangement for a PCM system, samples of a signal wave are represented according to a unit distance unit disparity binary code of (2n-2) digits of the type shown in Fig. 1 which provides for 81 levels in either section A or section B, and a code converter is provided for combining pairs of digits (2r-2), (2r-1), (where 2#r#n) to produce the corresponding digit r of a ternary code. A digit 1 may be provided in the binary code, as shown, to double the possible number of levels, this digit having the greatest significance and also forming digit 1 of the ternary code. The signal may be transmitted by means of the ternary code or by means of a modi- - fled binary code derived from the output of the code converter, this modified code being so arranged as to prevent an abnormally high or low number of digit pulses from being transmitted during periods of slow signal variation. A code converter, Fig. 3 is provided in a single channel system for each pair of binary digits to be combined, the digit pair 2, 3 for example being applied simultaneously to terminal 42, 43 respectively as a positive pulse or no pulse. If the ternary code is to be transmitted, gates 44, 45, 48 may be omitted, a digit 2 pulse alone resulting in a positive pulse at the output 51 of combining network 50, a digit 3 pulse alone resulting in a negative pulse at output 51 by way of inverting amplifier 49, and simultaneously occuring digits 2, 3 pulses or no pulses resulting in zero output at 51. The ternary code so produced is shown in Fig. 1 where blocks shaded upwards to the right represents positive pulses, and those shaded upwards to the left represent negative pulses. If the modified binary code is to be transmitted, a digit 2 pulse alone is passed by gate 44 and appears at terminal 46, a digit 3 pulse alone is passed by gate 45 and appears at terminal 47, if both digit 2, 3 pulses are present a positive pulse appears at each of terminals 46, 47, and if neither digit pulse is present, gate 68 produces an output pulse which is passed by each of gates 44, 45 so that again a positive pulse appears at each of terminals 46, 47. Terminals 46, 47 are connected to a code reconverter to produce the modified binary code as described below with reference to a time-division multiplex system employing only one code converter and reconverter. The multiplexed signal samples of four channels A, B, C, D are fed to the input 11, Fig. 4, of a binary coder 10 whose digit outputs 1-9 are connected to a distributer 64 arranged to distribute the digit pulses in time as shown at 65, 66, 67 of Fig. 5. The groups of digits 1 (if present) appear at terminal 65 and are fed direct to output 70. The groups of even and odd numbered digits appear at terminals 66, 67 respectively as shown in Fig. 5 and are applied to the input terminals 42, 43 of the code converter 68 of Fig. 3 whose output is applied to a reconverter 69 which delays the input from terminal 47 so that the odd numbered digit of each pair appears at the output 70 in the digit position following the corresponding even numbered digit, the resulting interlaced train being shown at 70 in Fig. 5. The digits are not modified by reconverter 69 unless two or more successive pairs of digits occur in which all the digits are the same, in which case the code reconverter 69 operates to produce the digit pairs ++, 00 or vice versa alternately until a different digit pair occurs. Thus the number of pulses transmitted in a given time remains substantially constant and enables the limiting level at the receiver to be maintained constant, this level being generally determined by the average energy of the pulse train received. In the receiver the pulse train is segregated into the three series represented by 65, 66, 67 in Fig. 5 and the even and odd digits, other than digit 1 (if present) are supplied respectively to terminal 42, 43 of a code converter, Fig. 3, the ternary code produced by which is decoded by a conventional ternary decoder, either directly, or after converting the ternary code, which is a reflected code, into a simple ternary code. Specification 849,891 is referred to.