GB582324A - Improvements in or relating to time-modulated pulse communication systems - Google Patents

Abstract

582,324. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd., CHATTERJEA, P. K., and SCULLY, C. T. Nov. 24,1941, No. 15145. [Class 40 (v)] In a duration-modulated pulse communication system, pulses modulated on each edge in accordance with different portions of a signal wave form are produced by combining two trains of pulses of the same frequency displaced in time so that the leading edges of one train coincide with the trailing edges of the other, the other edges of each train being modulated. The two trains may be produced by modulating a saw-toothed or other " analysing " wave and a similar " image " or reversed wave, or two similar analysing waves may be modulated by the signal wave and a reversed signal wave. As shown diagrammatically in Fig. 3, if a signal wave having instantaneous amplitudes varying between lines 24, 25 about a mean value 17, and a saw-tooth wave 15, are applied to a grid or separate grids of a back-biassed valve, the output comprises pulses which by amplification and limitation may be converted into a pulse-series 19 varying in width as the signal wave form varies between the limits 24, 25, the leading (right hand) edges of the pulses occurring at fixed intervals of time, and the trailing edges being modulated. If this operation is performed on a reversed saw-tooth wave 16, a similar pulse series 20 is produced but the trailing edges are at fixed intervals and the leading edges are modulated in accordance with portions of the signal wave form intermediate of those producing the modulation of 19. A combination of the two pulse series provides the doubly-modulated series 22 each edge being independently modulated between the limits indicated at 21, 23. Instead of reversing the saw-tooth wave, two such identical waves may be modulated respectively by the signal wave form S, and S reversed, as indicated in Fig. 4. Improved representation of the wave form can be obtained if, instead of taking the instantaneous amplitude of the signal wave at the point of intersection with the sloping edge of the saw-tooth as the modulating factor, an average or an R.M.S. value of a portion of the wave form is used. Such an arrangement used in conjunction with a sawtooth wave and its reverse is shown in Fig. 5, the reference on the leads indicating the wave forms, Figs. 6 to 8, which they carry. The averaging is effected with the help of a sawtooth wave H of ten times the frequency of the main saw-tooth wave B generated at 40. The wave H and the signal wave A are applied to a limiter 33 producing a duration-modulated series J in the way previously described which is applied to two channels denoted by suffixes B and A in which after integration during alternate periods in the two channels it is ultimately used to modulate the main sawtooth wave and this wave reversed. The channel B will now be fully described. The wave J together with an equal pulse interval wave E derived from the original saw-tooth wave is applied to a switch 34B which comprises a limiter-amplifier producing a wave K1 (full lines), Fig. 7, in which alternate groups of pulses extending over half periods of the original saw-tooth are lost, the remaining groups being fed to an integrator (condenser) 35B. This integrator is periodically discharged by a pulse wave G, produced at 45B by combining wave E with a delayed copy thereof, so as to produce a wave L1 comprising a building up (integrating) portion t-t1, and a steady portion t1-t2 followed by discharge and repetition of the cycle. Wave L1 and wave D are then combined and limited at 36B to produce wave M1 which comprises pulses of equal duration (half the main period) but of amplitude representative of the mean value of the signal wave during the preceding half period. The wave M1 is then used to modulate the main saw-tooth wave B to produce pulses 01, Fig. 8, with fixed leading edges and modulated trailing edges in accordance with said mean value represented at P1, P2. Similar operations are performed on the main reversed sawtooth wave in channel A using the pulse groups corresponding to those referred to as " lost " in wave K1, the wave form 02, Fig. 8, ultimately produced, having fixed trailing edges and modulated leading edges in accordance with averages P3, P4. The pulse forms O1, 02 are then combined at 38 to produce R. The combined pulse form may be transmitted as short pulses marking the positions of the leading and trailing edges in known manner. The Provisional Specification describes the use of a single isoceles triangular analysing wave to produce the same result as the two analysing waves.