| 摘要 |
PURPOSE:To prevent focusing on a background by stabilizing the angle of field by gating a video signal with horizontal and vertical synchronizing gate signals, by using only a high frequency signal of the screen center part as a focusing signal and by applying pulse-width control over gate signals. CONSTITUTION:One-shot multivibrator 11 has a pulse length of approximately 15 microseconds and the output signal of one-shot multivibrator 17 corresponds to a part about a half as large as the horizontal center screen of a video signal applied to terminal 3. Further, one-shot multivibrator 12 has a pulse length of about 5 milliseconds and its output signal corresponds to a part about a half as large as the vertical center screen. Output signals of one-shot multivibrators 17 and 18 are supplied to analog gate circuit 19 and the component of the center screen position of the high frequency component of the output signal of gate circuit 19 is extracted and led to detector 5. As the helicoid rotates, control voltage of multivibrators 11 and 12 fluctuate to vary in pulse width respectively and at the same time, as control voltage of multivibrators 17 and 18 fluctuate, their pulse widths are varied by presetting the voltage division ratio of potentiometer 21, the voltage of electric power source 20 and the value of voltage inverting circuit 13. |
