GB641982A - Improvements in and relating to electric pulse signalling systems - Google Patents

Abstract

641,982. Pulse modulation; multiplex pulse signalling. GENERAL ELECTRIC CO., Ltd., and LEVY, M. M. March 26, 1947, No. 8261. [Class 40 (v)] In a receiver for time-modulated pulses, interference from noise (having a period long compared with the duration of the pulses) is reduced or eliminated by deriving further pulses from both the leading and trailing edges of the signal pulses, and combining these further pulses so that their noise components are in anti-phase. Signal pulses, Fig. 2 (a), are either raised or lowered as a whole by the noise, the results of noise being shown throughout the Figure in dotted lines. Pulses D1, D2 at (b) representing the voltages at C1, C2, are developed, e.g. by double limiting about level C and differentiating the resultant. From pulses D1, D2 are produced pulses (c) and (d) which are width-modulated about a common datum line E. These pulses are subtracted as at (e), the noise components N<SP>1</SP> 1 and N 2 <SP>1</SP> tend to cancel out, and the signal is extracted by integration. Instead of the pulses (c) and (d), pulses width-modulated with respect to a datum F may be produced as at (f) and (g) by applying the pulses D1, D2 of wave (b) to the grid of V1, Fig. 3, and, inverted, to the grid of V1<SP>1</SP>. Both valves are normally biassed to cut-off, but pulses Dl and D2 respectively charge condensers K, K1, to a level P, P1, a level which is maintained since R, K and R1, K1 have long time constants, until discharged by valves V2, V<SP>1</SP>2, which conduct at point F on control pulses applied to their control grids. The output width modulated pulses are added as at (h), Fig. 2, and integrated, or they may first be demodulated and then added. If the pulse waveform is not symmetrical then the phase displacements C1, N1 and C2, N2 will not be equal and therefore the amplitude of waveforms (f) and (g) are made suitably different. When pairs of pulses are push-pull timemodulated, each of the two pulses may be dealt with separately and the combined outputs subtracted, or one circuit may deal with the leading edges of both pulses, and a second circuit with the trailing edges, the outputs being subtracted one from the other. For optimum results, the transmitter should be provided with automatic gain control so that the pulse spacing is always only sufficient for the signal amplitude existing. In an alternative embodiment, the signal pulses are converted to pairs of amplitude-modulated pulses before being combined as above. In a multi-channel system, cross-talk between adjacent channels may be reduced by these means, and further reduced by combining with the pulses of one channel, in suitable sense and phase relation, a small fraction of the signal from the interfering channel. Reference has been directed by the Comptroller to Specification 644,118.