GB1045854A - Improvements in or relating to television camera equipments - Google Patents

Abstract

1,045,854. Television. MARCONI CO. Ltd. July 11, 1963 [Aug. 24, 1962], No. 32631/62. Heading H4F. The output signal from a vidicon television camera tube, which appears at terminal 1, Fig. 1, and is applied through stage 3 to output terminals 4, is D.C. restored at the tips of the dark current DCT, Fig. 3, by a clamp stage 14 which is rendered conducting during the tube blanking intervals by pulses CP applied at terminal 15. In accordance with the invention the true black level of the signal is maintained constant, despite variations in the amplitude DP of the dark current consequent upon variations of the target temperature and/or target electrode voltage by arranging to vary the restoration potential applied to clamp stage 14 in dependence upon the amplitude of the dark current. The camera signal is applied through stage 5 to a clamp stage 7 which is rendered inoperative during the blanking interval by pulses applied from terminal 8 to the base of transistor 6. Stage 7 thus functions as shown in Fig. 2 to clamp the most positive excursion of the actual video signal V which will correspond to true black or may be arranged to do so by including one or more opaque strips on the camera faceplate. The resulting clamped signal then passes to a peak rectifier 13 which develops a potential equal to the dark current and applies it to the emitter of clamp stage 14. The level to which stage 7 restores is determined by adjustable potentiometer 10 connected across Zener diode 11. Stages 7 and 13 are of time constant long compared with a frame period. A second embodiment is described with reference to Fig. 4 (not shown) for use with an input camera signal of opposite polarity to that used in Fig. 1. According to a modification the opaque strips referred to above may be dispensed with so that the circuit stabilizes at the level representing the blackest part of the picture.