| 摘要 |
PURPOSE:To obtain correct focusing even at a low brightness and in the close-up state by deciding the lens system so that the total time of a video signal per one field in excess of a high threshold value from the edge of one horizontal scanning line is minimized. CONSTITUTION:A level 5 at the input of a pulse A of a video signal 10 (at right edge of a horizontal scanning line) is held by a sample-and-hold circuit 11. A pulse B is inputted to a comparator circuit 12 at the left edge of the succeeding horizontal scanning line and the video signal level 4 at the left edge of the horizontal scanning line is compared with the said level 5 and a slice level 6 higher by 10% than any of both the said levels is set by a slice level generating circuit 13. The slice level 6 and the video signal 10 are compared by a comparator circuit 14 and the total sum sigmat is calculated by an integration device 16 from signals (t) from a reference signal generating circuit 15 for a period only when the input video signal is larger. |
