Patent GB-652396-A (GB)
Title: Improvements in or relating to electric pulse signalling systems
IPC: H04J3/04
Filed: 1948-02-04
Abstract: 652,396. Cathode-ray tubes. GENERAL ELECTRIC CO., Ltd., FARREN, L. I., HEAD, N. E., and LEVY, M. M. Feb. 4, 1948, No. 3275. [Class 39 (i)] [Also in Group XL (c)] Feed-back from an electron-collecting electrode to the signal input is provided in a cathoderay tube demodulator for time - modulated pulses, to steepen the leading and trailing edges of the output duration-modulated pulses. The signal pulses may deflect the beam and/or modulate its intensity, and a multiple-target tube may provide separating means for the interlaced channel trains of a multi-channel system. In one embodiment the electron beam 41, Fig. 1, is rotated by deflecting coils 12, 13, so as normally to trace out a path on apertured plate 14 just below apartures 15. Signal pulses applied to plate 18 of the radial deflection plates 18, 19, deflect the beam to pass through aperture 15 and strike the target electrode 16, from which the output pulses are fed to the low-pass filter 30. When the beam leaves the plate 14, the fall in current in resistance 22 produces a potential change which is amplified in circuit 24 and applied to deflecting-plate 19 in such a sense as to increase the deflection of the beam away from the lower edge of the aperture 15. Thus the path traced out by the beam in the neighbourhood of aperture PQRS, Fig. 3, in the absence of the feed-back circuit, would be represented by the full-line 20, 26 when the signal pulse 26 is received. When the feed-back circuit is in use according to the invention, the beam path is represented by the dotted line 20, 27, 28, 29, 20, the pulse output from the target 16 now having a steepened leading edge occurring at a time determined by that of the signal pulse and a steep trailing edge which is fixed in time, so that it is duration-modulated. The apertured plate 14 may be replaced by a number of targets the sense of the feed-back being reversed by providing one stage more or less in the amplifier 24 or connecting the feedback to electrode 18. Alternatively the feedback may be taken from target 16. When a single channel train is being demodulated a single target 16 co-operates with all the apertures or one aperture only may be provided. In a multi-channel system with sixteen channels, for example, one tube may deal with the odd and another with the even-numbered channels, and a separate target 16 feeding a separate low-pass filter 30 is provided for each aperture. Screening plates 39, 40 and radial screens, between adjacent channel apertures, may be provided. When the apertures 15 are replaced by plates, one of the deflecting electrodes, say 19, is in the form of a number of electrodes, each target being coupled by a separate feed-back circuit 24 to one of the electrodes 19. Alternatively a common feedback circuit and deflecting electrode may be used if separating valves are provided between each of the targets and the feed-back circuit. Arrangements are described in which the regularly recurrent edge of the durationmodulated output pulses is the leading edge, Fig. 4 (not shown). The beam may, in other arrangements, sweep linearly across suitablyarranged apertures or targets. Instead of or in addition to using the signal pulses to deflect the beam they may be used to vary the beam intensity. In this embodiment they are applied to the tube grid to switch on the beam and the feed-back is applied to the grid, Fig. 5 (not shown). The feed-back voltage may be provided by a local pulse generator triggered on or off by voltages generated when the beam crosses the edge of an aperture or plate.