Patent GB-695198-A (GB)
Title: Improvements in and relating to pulse code signalling systems
Inventor: GARNER BRIAN LEONARD
IPC: H03M1/00
Filed: 1949-12-05
Abstract: 695,198. Multiplex pulse code signalling. GENERAL ELECTRIC CO., Ltd. Dec. 1, 1950 [Dec. 5, 1949], No. 31174/49. Class 40 (v). Apparatus for generating a pulse code signal representative of a voltage differing by " S " from a datum level, comprises means for generating a succession of pulses of progressively decreasing amplitude, means for sub. tracting the peak voltages of successive pulses from a succession of voltage values to produce resultant voltages respectively, the first of said voltage values being " S " and each of the remaining voltages being that obtained in the nearest preceding subtraction in which the resultant voltage is of the same sign as " S " with respect to the datum level, and means for generating pulse signal elements which correspond one to each of the first named pulses and distinguish between those first named pulses which give rise to resultant voltages of the same sign as " S " with respect to the datum level, and those which give rise to resultant voltages of the opposite sign. As shown in Fig. 1 in a five-channel system, the signal 10 is periodically sampled in known manner in intervals t1 to produce pulses 11, 12, &c. whose amplitude S1 &c. represents the amplitude of the waveform 10, the signal 13 of the next channel producing pulses 14 &c. of amplitude S2 &c. in intervals t2. The sampling circuit may comprise a plurality of pentode valves, one for each channel, having a common anode resistance, the appropriate signal channels being fed to the respective control grids and a gating signal as shown in Fig. 2 (c) being supplied from tapping points along a delay line to the respective suppressor grids. A group of five-pulse intervals are allotted to each channel, allowing thirty-two different values of amplitude to be transmitted. The rectangular waveform of curve (a), Fig. 2, is produced in known manner and has a period equal to the period of the pulse intervals of the signal to be transmitted, the interval t1 corresponding to t1 on Fig. 1. The waveform (b) is of the same frequency as (a) but the positive going portions are of smaller duration. The waveform (c) has a recurrence frequency which is equal to the recurrence frequency of adjacent groups of pulse intervals. The waveform at (d) is produced by applying the pulses (c) to shock-excite a parallel tuned circuit having a natural frequency equal to that of waveform (a) the damping being arranged so that the amplitude of each positive peak is one half that of the preceding positive peak and the positive-going portions being retained to produce the waveform (e), the relative amplitudes of the peaks being indicated by numerals 16, 8. &c. The curve (f) shows two successive samples of voltage levers S1, S2 respectively to be converted into a pulse code signal. The waveform (f) is applied to the control grid of a valve 19, and the condenser 20 receives a charge substantially equal to S1, a similar voltage being developed across resistor 21 in the cathode circuit of valve 22. The waveform (e) is applied to the grid of valve 24 and a voltage is produced at point 25 which is the difference between the voltage across condenser 20 and the voltage applied to the grid of valve 24 as amplified by the valve, the first peak of amplitude 16 units of curve (e) producing a signal having the waveform (g) between points 47 and 40, Fig. 2, which is applied to the control grid of a valve 26 having a valve 27 in its cathode circuit, the voltage at point 28 following that at 25. This voltage is applied through a clamping circuit 29 controlled by the pulses (b), which in the absence of the pulses provides a high impedance between point 28 and capacitor 35, but when pulses (b) and - (b) are applied causes capacitor 35 to be charged to the potential of point 28, i.e. the voltage corresponding to 39 in curve (g). The waveform (b) is also applied to the control grid of valve 27 to increase the rate at which the capacitor 35 receives or loses charge. The voltage at point 34 is applied to the control grid of a valve 36 having a valve 37 in its cathode circuit, and a corresponding voltage is produced at point 38, this point being connected through a diode 41 to the cathode of valve 19 so that when the potential at point 38 falls, the cathode of valve 19 will assume the potential of point 38 and thus alter the charge on capacitor 20. Waveform (a) reversed in sign is applied to the grid of valve 37 to hasten the change of charge on capacitor 20. The point 38 is also connected to the cathode of a valve 43 whose grid is connected to three diodes 44, 45, 46 so arranged that application of a positive potential to any one of the anodes of the diodes causes the point 38 to be driven positive to prevent the diode 41 from conducting, waveform (a) being applied to the diode 44 so that conduction of the diode 41 is only permitted between points 40 and 48 on curve (g) when the waveform (a) is negative, and the cathode of valve 19 therefore assumes the voltage level 15. Similarly, after the second peak of curve e has ceased the cathode of valve 19 assumes the voltage level 16 of curve (g). The point 25 is also connected through a diode 49 to the control grid of a valve 50 and when the point 25 falls below earth potential, as at 59, curve (g), the diode conducts causing a positive pulse to appear at the anode of valve 50 which triggers a multivibrator comprising valves 51, 52 and produces at point 53 the leading edge 54 of waveform (h). The time constant is arranged to maintain this condition until a positive pulse applied to the grid of valve 52 through a differentiating circuit 57, 58, from waveform (a) renders this valve conducting to reset the multivibrator, and generate the trailing edge 56 of waveform (h), and the waveform (h) is applied to the diode 46 to ensure that during a positive going pulse of waveform (h) the diode 41 cannot conduct and the voltage at the cathode of valve 19 remains unchanged, so that the voltage level 17 is the same as that at 16 in curve (g). The waveform (h) is also applied through a phase invertor valve 60 to the suppressor grid of a valve 61, whose control grid is supplied with the waveform (a), to gate the valve and allow pulses to appear at the output terminal 63 only when the suppressor grid is positive, and produce the pulse code representations (i) for S1 and S2 respectively, representing relative magnitudes 26 and 17. The pulses (c) are applied to the diode 45 to prevent the diode 41 from conducting whilst the waveform (f) is changing its magnitude. The pulse code signal may take other forms, e.g. finite significance may be given to the condition when no pulse is transmitted and zero significance when a pulse is transmitted, or pulses may be transmitted in one sense having finite significance and in the opposite sense having zero significance. Specification 695,223 is referred to. Reference has been directed by the Comptroller to Specification 630,094.