Patent GB-1466781-A (GB)
Title: Camera tube circuit
IPC: H04N5/228
Filed: 1974-02-27
Abstract: 1466781 Cathode-ray tube circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 27 Feb 1974 [13 March 1973] 8888/74 Heading H4T In a television system employing a pick-up tube having an anti-comet tail (ACT) electron gun comprising a cathode, a control electrode, anode electrodes and an electron lens elecrode so energized that in the line fly-back time preceding a line sweep the beam scans the target with an increased diameter, intensity and potential (to neutralize locally over-exposed areas), a circuit arrangement is provided which automatically controls the potential of the lens electrode against variations in the potential of the cathode and the control electrode such that optimum charge neutralization is produced when the potentials of either of the latter two electrodes varies, e.g. due to tube ageing. Referring to Figs. 1 and 2, during the line scan period, THS the cathode c is at ground potential (Uc = 0 volt), the lens electrode g3 is at the same potential as the anode electrode g2, g4 = (+Us1) and the negative potential of the control electrode g1 is such that the correctly focused beam e1 is produced and operates in normal manner to produce an image signal via the signal plate of the target electrode tg which to brought to cathode potential. During the line fly-back portion THBS (of the blanking interval THB which also includes the actual blanking time THBB) the (negative) voltage Ug1 on the control electrode gl is reduced so that a beam e2 the intensity of which is several hundred times that of the beam e1 is produced, the electrode g3, which has a small positive potential during the portion THBS, serving by producing a node in the beam (between the electrodes g2, g4) to cause an increase in the beam diameter at the target tg. The fly-back of the latter beam then neutralizes overexposed areas and to ensure that only such areas are neutralized the cathode potential Uc is increased positively so that the target tg is neutralized to this increase potential and desired information in the potential range between ground and the (increased) cathode potential is not influenced. In the above operation, optimum neutralizing of the over-exposed areas is dependent of the position of the beam nodes in the beam e2 and therefore on the voltage between the lens electrode g3 and the anode electrodes g2 and g4 and additionally, on the voltage Uc on the cathode and the voltage between the cathode and the control electrode g1. The circuit arrangement of the present invention provides automatically for optimum neutralization by controlling the voltage Ug3 on the lens electrode 93 in such manner as to compensate for the effect of variations in the voltages Uc and Ug1 (due, e.g. to tube ageing). In operation, a signal G produced by the current flow to the electrodes g2, g4 due to voltage source + Us 1 is supplied to amplifier and clamping circuit 9, (supplied with a clamping pulse F from pulse generator 8) to form the signal J which is supplied to a sample and hold circuit 10. In the latter circuit sampling pulses D cause samples of signal J, to be derived during part of the fly-back portion THBS over two successive field scans V1, V2 (signal K) and supplied to an amplifier and clamp circuit 11 controlled by pulses A. The output L of circuit 11 comprising the difference between the two samples is then supplied via gate 12 opened by pulses C, as waveform M to a storage and limiter circuit 13 the output, waveform Nof which, is employed in a modulation signal generator 14 supplied with the waveforms A and B to produce a modulation signal, waveform P. The latter amplitude modulates the pulse train E in a modulator 15 to produce the required voltage Q=Ug3 for the lens electrode g3. Fig. 3 shows the relationship between the voltage Ug3 and the current Ig2, 4 to the electrodes g2, g4 (solid line curve) the circuit operating to reduce the level of Ug3 to the valve U30, the "knee" of the characteristic, which is the optimum operating point. Circuit details of the blocks 9 to 15 are described with reference to Fig. 4 (not shown). When the pick-up tube is one of the (three) tubes of a colour television camera the block 9 may be switched cyclically to all of the tubes and the output of block 12 switched cyclically to separate respective groups of (three) blocks 13 to 15 associated with the respective tubes.