Patent GB-1375645-A (GB)
IPC: H04N9/093
Filed: 1972-08-25
Abstract: 1375645 Cathode-ray tube circuits RANK ORGANIZATION Ltd 2 Aug 1973 [25 Aug 1972] 39774/72 Heading H4T  In a colour television camera employing a plurality of pick-up tubes and a zoom lens a signal (which may be represented by a resistance value) representative of the magnitude of an operational parameter of the lens is employed to modify the size of the scanned raster on at least one of the tubes to compensate for misregistration due to the effect of variation of lens parameters. Such misregistration is caused mainly by variations in the zoom setting but may also arise from variations in iris and focus settings. Figure 1 shows one embodiment in which a measure of the parameter considered is produced by the position of the slider of a potentiometer P1 the variation in resistance of which relative to (video) earth shunts, in a compensating manner, current sensing resistors R RH,  R RV  and R BH , R BV  in the feed-back paths of the horizontal and vertical scanning circuits of the red and blue pick-up tubes respectively of the camera 11, the green pick-up tube (nots hown) of which acts as a reference. If the misregistration effects in the red and blue channels are not complimentary, as assumed in Fig. 1, a separate potentiometer P1 must be employed for each channel. The various capacitors serve to prevent interaction between the horizontal and vertical circuits and to block D.C. and low frequency components which result from movement of the slider of P1. In a second embodiment which reduces the loading effects on the scan current sensing resistors the shunting networks are arranged to have a high impedance when the lens is zoomed to one end of its range and to change progressively to a lower value as the lens is zoomed towards the other end of its range. In this embodiment the shunting networks comprise variable impedance devices coupled to the zoom setting information via networks including active and/or passive elements and which may have either linear or non-linear transfer characteristics (Fig. 2, not shown). An arrangement of this latter kind in which the variable impedance device comprises a field effect transistor is described with reference to Fig. 3 (not shown). The variable impedance device may, alternatively, consist of other forms of transistor, analogue multipliers, voltage dependent resistors, combination of resistors and Zener diodes or ladder networks with switched transistors. Also, if the zoom setting is a digital form, D/A converters may be employed. In a further embodiment a zoom indicative voltage V z  (Fig. 4) is employed to adjust the magnitude of current soures CS 1 , CS 2  synchronized with the horizontal and vertical scans via generators WG 1 , WG 2  and fed to the scan amplifiers AR 1 , AR 2 . This arrangement may be modified as shown in Figure 5. In another embodiment the voltage V z  (Fig. 6) controls a sawtooth generator ST 1  synchronized by horizontal drive pulses, the output of the sawtooth generator, which may be either positive or negative forming the control quantity for (in this case) the red horizontal channel. In a modification of the latter embodiment the output of the sawtooth generator is constant and the voltage V z  is employed to control the gain of an amplifier through which the sawtooth is supplied (via a buffer amplifier, to the network C8, R9 (Fig. 7, not shown). Where differential registration signals are available at the camera the voltage V z  may be summed with such signals (Fig. 8, not shown).