GB754611A - Improvements in or relating to television pick-up apparatus - Google Patents

Abstract

754,611. Television. FERNSEH GES. Aug. 12, 1954 [Aug. 14, 1953], No. 23433/54. Class 40 (3). Relates to television pick-up apparatus employing a pick-up tube in which the storage target is controlled by a flooding source of electrons whereby the signal generated during flyback has a value in the neighbourhood of picture white. In such apparatus, if the generated signal is passed through a variable-gain A.C.-coupled amplifier and then clamped at a constant level during flyback, variation of the amplifier gain results in a variation in the signal black level. According to one aspect of the invention, Fig. 3, this defect is overcome by controlling the clamping level in accordance with the gain of the amplifier. The generated signal appears at terminal 11 and passes through a variable gain amplifier 13, 14, 15 to an output stage 16 at the grid of which the signal is clamped during the flyback interval by means of a conventional four-diode clamp circuit 17-20 pulsed on at terminals 26 and 27. The first stage 13 of the amplifier is a variable-mu valve controlled by variable bias source 12 and is operated in conditions such that the slope is proportional to the anode current. The voltage drop across the cathode resistor 22 is smoothed by filters 23, 24 and is then employed as the reference voltage at point 21 in the clamp circuit. According to another aspect of the invention, Figs. 1 and 2 (not shown), the white signal appearing during flyback is offset by a pulse of opposite polarity and of amplitude equal to the difference between picture white and black. This pulse, which is made slightly narrower than the white signal, is added to the generated signal at the input of the variable gain amplifier by means of a cathode-follower stage. Preferably its amplitude is controlled as a function of the electron discharge from the flooding source in the pick-up tube.