Patent GB-662823-A (GB)
Title: Improvements relating to a method and corresponding devices for the transmission of signals by coded pulses
IPC: H03M1/00
Filed: 1949-07-25
Abstract: 662,823. Pulse code modulation circuits. LIBOIS, L. J., and GLOESS, P. F. M. July 25, 1949 [July 27, 1948], No. 19558/49. Classes 40 (v) and 40 (vi). Time or duration modulated pulses are converted to code pulse groups by causing each pulse to initiate N signal waves of period T/2<SP>0</SP>, T/2<SP>1</SP>, T/2<SP>2</SP>... T/2<SP>n-1</SP>, where N is the number of possible code pulses in a group and T is the maximum modulation time interval, the waves being analysed at a fixed time and code pulses being generated if the individual waves present a predetermined characteristic, e.g. polarity, when the analysis takes place, the code pulses being staggered in time to form a group. The signal waves may be sinusoidal in character or may consist of pulse trains. In one embodiment, the input pulses 4, Fig. 3, have their leading edge modulated in time, the trailing edge 9 being fixed, and the circuit produces 3-element code groups. The pulse 4, after a delay of T/8 in line 20, is applied to cut-off three valves such as 26 having resonant cathode circuits 29, 27 tuned to frequencies 1/T, 2/T, 4/T respectively, the resulting waves 12, 13, 14 being applied to the suppressor grids of valves 5, 6, 7, respectively. At the conclusion of the pulse the valves 26 become conducting to terminate the waves, by damping the resonant circuits 29, 27. At an appropriate time an analysing pulse 16 is applied simultaneously to the control grids of valves 5, 6, 7 and produces an output code pulse from those valves whose suppressor grids are positive. These pulses are applied to tappings on a delay line 10 from the output of which a pulse group 11 is derived. The analysing pulse 16 is timed to coincide exactly with positive or negative maxima of the waves 12, 13, 14, by full-wave rectification of wave 14 in the double triode 39 to the anodes of which it is applied in push-pull. The grids are supplied with the undelayed pulse 4 which blocks both triodes until the time of occurrence of the trailing edge 9 and, thereafter, the next positive peak of the wave 14 applied to one of the anodes produces an output pulse 16, across the resistance 44, at the required time. In a second embodiment, the input pulses 45, Fig. 5, time modulated between the limits 46, 47, which have a time displacement T, are used to produce trains of pulses 52, 53, 54, each train consisting of pulses and gaps of equal duration and their recurrence periods being T, T/2, T/4 respectively. The input pulse 45 produces the pulse 52 by applying the former to condenser 70 through the input matching impedance 71 of a delay line 69 the other end of which is open and reflects back a pulse which terminates pulse 52 after a time T/2. The pair of pulses 74 produced at the point 73 is fed, after reversal in transformer 79, to a valve 75 the output from which, by means of delay line 77 and its associated condenser produces the two pulses 53 and the four pulses 84. A further similar stage 76, 78 produces the four pulses 64 and the eight pulses 61. As in the first embodiment, the pulse trains 52, 53, 54 produce or fail to produce code pulses, from the valves 55, 56, 57, respectively, which pulses are separated in time to form the code group 66 by the delay line 65. The analysing pulses 58, 108, 107 which are applied to the suppressor grids of valves 55, 56, 57, respectively are differently timed by the delay line 95 to compensate for circuit delays in the production of the pulse trains 52, 53, 54 from the input pulse 45, the line 95 being fed with a pulse 64 through valve 93 at the appropriate time. The output pulses from valves 55, 56, 57, are taken from their suppressor grids which provide a negative going pulse, if the control grid is positive when the suppressor grid receives a negative pulse. The pulse 64 from the output of transformer 92 is timed by deriving an input pulse 90 from the screen grid 89 of valve 85 when a fixed channel timing pulse 48, derived from the time modulator for pulse 45, and applied to the control grid, coincides with one of the eight pulses 61 applied to the suppressor grid. A feed-back condenser 91 connected between the screen and suppressor grids gives the circuit two stability positions, ensuring a more definite operation of the timing arrangement.