GB994121A - Image-orthicon circuit arrangements - Google Patents

Abstract

994,121. Television; cathode-ray tube circuits. FERNSEH G.m.b.H. Sept. 30, 1963 [Sept. 28, 1962], No. 38357/63. Headings H4F and H4T. During the flyback interval of the scanning process of an image - orthicon pick - up tube voltage pulses are applied to an electrode of the pick-up tube such that the beam current is increased to a multiple of its value during the scanning periods, the output signal being clipped during the flyback intervals to yield a noise-free reference level. As shown the camera 1 includes an image-orthicon pickup tube 11, the output signal from which passes to a clipping circuit 12 and then to a preamplifier 13. After further amplification in an intermediate amplifier 21, the signal passes to a clamp circuit 22, which restores the directcurrent component, and then to a blanking amplifier 23 and main amplifier 24. Linefrequency voltage impulses, Fig. 2a, applied to the camera at 32, synchronize a scan generator 14 which controls the horizontal deflection of the electron beam in tube 11 and also produce, during the flyback intervals of the scanning process, a positive-going change in the potential of the control electrode of the electron gun which develops the scanning beam in tube 11. In addition, the impulses synchronize the operation of clamp-drive circuit 33 which developes pulses at appropriate amplitude and polarity to operate the clamp circuit 22. The amplitude of the impulses is chosen so that during the flyback intervals the beam current rises to a multiple, e.g. five times, of its value during the scanning period and the resultant output signal is shown in Fig. 2b. The output signal produced during the flyback interval carries superimposed noise and to obtain an undisturbed reference level the signal is clipped by stage 12 at a potential below the black level as shown by the horizontal broken line in waveform 2b. The clipper stage 12 (Fig. 3, not shown) includes a diode appropriately biased by means of a potential divider. In order that the reference level is maintained at a desired value below the black level, preferably so that the difference in potential between the clipping level and the black level represents a constant percentage of the peak-to-peak amplitude of the picture signal, variable resistors forming part of the potential divider in the clipper stage are coupled for ganged operation so that the voltage applied to an intermediate dynode of the electron multiplier, which effects the gain of the multiplier, and the bias potential for the diode in the clipper stage are controlled simultaneously to bring the output signal voltage to the same amplitude when different pick-up tubes are used. The beam current is also adjusted so that the output current of the pick-up tube which represents black is constant. A further impulse having the same polarity as the picture signals may be developed so that it occurs during the flyback intervals and has an amplitude such that when it is combined with the clipped picture signal the reference level is returned to the picture black level.