GB904625A - Improvements in or relating to electric pulse code modulation systems of communication - Google Patents

Abstract

904,625. Pulse code modulation systems; transistor circuits. STANDARD TELEPHONES & CABLES Ltd. Jan. 30, 1959, No. 3419/59. Classes 40 (5) and 40 (6). General.-In a P.C.M. system, the range of amplitude of the signal to be coded is divided into a plurality of equal portions each including a plurality of levels, each portion of the range is selectively applied to a corresponding coding circuit, and the arrangement is such that certain of the digit pulses for each portion of the range are produced directly by the coding circuit associated with that portion, and the remaining digit pulses are derived, under the control of the coding circuit associated with that portion, from at least one of the other coding circuits. The system described produces a 9-digit unit disparity code, Fig. 1, of the type described in Specification 849,891. Each selecting circuit 15, Fig. 2, comprising diodes 32, 33, Fig. 3, biased by resistors 34, 35, receives negative amplitude - modulated pulses from sampler 12 and responds to a different pair of adjacent portions of the range, but all circuits 15 produce an output in the range - 31 to - 25 volts corresponding to a spread of 12 levels (each portion being 6 levels). Each coding sub-unit 16 comprises two identical coding circuits A, B to which the two portions selected by the associated circuit 15 are respectively applied. All circuits 16 produce the least significant digits 8, 9 which undergo one cycle of change in every portion (see Fig. 1). The sub-units are interconnected through crossconnector 22 so as to produce the desired combination of digits in parallel, the outputs 83A, 83B of each sub-unit being connected to the appropriate digit amplifiers 23 through connector 24. Sampling gates 103 are opened simultaneously to obtain a serial output from tapped delay line 25. Circuit details of coding sub-units, Fig. 4.- The input from the circuit 15 is applied at terminal 41, and rectifiers 42A, 42B have a bias of - 28 volts so that each of the two portions is applied to its respective coding circuit A or B. The collector current-input characteristics of transistors 44A, 45A and 44B, 45B are shown in Fig. 5. Each output rectifier 76A, 77A, 78A or 76B, 77B, 78B having a bias of -37.5 volts produces an output on terminal 82, 83A, 84 or 82, 83B, 84 when it is unblocked i.e. when both of the two associated rectifiers such as 60A, 61A have a bias below - 37.5 volts applied thereto. Rectifiers 56A, 61A, 64A, 60A are all controlled in dependence on the collector current of the associated transistor 44A, 45A, but rectifier 65A is blocked or unblocked in dependence on the voltage applied to terminal 67A from terminal 68A or 68B of another coding circuit. The interconnection of the coding circuits is set out in table 1 of the Specification. Digits 8, 9, which appear on terminals 82, 84, are inverted (compare Figs. 1, 5) and are therefore applied to inverting amplifiers 23-8, 23-9, Fig. 2. The auxiliary sub-unit 101 is used to produce digits 2, 3, 7 for certain levels in the upper part of the range.