GB464810A - Improvements in or relating to synchronising means in television transmitters and the like - Google Patents

Abstract

464,810. Television; thermionic relay circuits. GENERAL ELECTRIC CO., Ltd., EDWARDS, G. W., ESPLEY, D. C., and WALTER, D. O. Dec. 9, 1935, No. 34065. [Classes 40 (ill) and 40 (v)] Relates to television transmitters having means for originating synchronizing signals, the frequency of which is a multiple of that of the signals required for framing, the object of the invention being the elimination of the superfluous signals. According to the invention, for this purpose there is employed a valve to whose control grid or grids are applied an impulsive voltage (represented by the curves A, Fig. 1) and an alternating voltage, (represented by the curve B), of suitably chosen frequencies and having amplitudes so related to each other and preferably to a bias voltage applied to one of the control grids, that the effective sum of these voltages rises above zero only at the desired periodicity. In the graphs, T represents the intervals between the impulsive voltages, and T<1> the period of the alternating voltage ; the curve D represents the sum of these voltages, the distance of the line C below O representing the grid bias. For interlaced scanning, where the framing signals are required to have a frequency of 50 per second the alternating voltage B may conveniently be derived from A.C. mains ; in this case means are provided for ensuring that small variations in the phase of the supply will not influence the times of emission of the synchronizing signals. Fig. 2 shows a circuit for carrying out the invention. The impulse voltages applied at I and the alternating lower-frequency voltages applied at A are combined by the arrangement shown, and applied to the control grid 3 of a pentode 1. Grid bias is provided by resistors 9, 10, 11 shunted by condensers 12, 13, 14, a tapping 15 on 11 being connected to the grid 3 through resistors 16, 19 and the secondary 17 of the input transformer 18. A resistor 24, connected to the secondary of the other input transformer 22 is large compared with 10 and 11. Similarly directed rectifiers 26, 27 are connected as shown, and serve to limit the peak of the alternating voltage applied at A, so that the wave-form of this voltage is made flat-topped, slight variations of phase of the supply being thereby rendered innocuous. The grid bias corresponding to C, Fig. 1, is so chosen that the combined voltage applied to the grid 3 rises once only during the period T<1> of the alternating voltage B above zero, i.e. above that value at which an anode current flowing in the valve 1 is just sufficient to cause the emission of a synchronizing signal. For generating the impulsive voltages, a magnetic pickup is preferred. For this purpose, magnetic members mounted on a non-magnetic disc, which is coupled to the scanning disc, cooperate with a fixed electromagnet near which they pass, producing one or more impulses during each revolution. A phase-shifting network may be provided for varying the phase of the alternating voltage B in accordance with any change that may be made in the point at which impulses are generated by the magnetic pick-up. Specifications 425,220 and 457,135 are referred to.