| 摘要 |
PURPOSE:To prevent the loss of a photographic range of a camera to improve its accuracy by keeping a detection level constant when a maximum voltage of the video signal has a certain level or higher in case that the position of an X-ray diaphragm is determined in accordance with the video signal. CONSTITUTION:A system P which detects a peak voltage of the video signal limits the peak voltage to a certain value VC or smaller by a peak level clamping circuit. This clamp level can be set freely by a variable resistance VR1. Then, a threshold level is set to muVp if the peak voltage passing through a buffer 1' and a variable resistance CR is VC or lower, and it is set to muVC if the peak voltage exceeds VC. The aperture width of the X-ray diaphragm in the video signal is detected by a comparator. Since the threshold is set to a cartain voltage in this manner, the aperture diaphragm does not break the photographic range to make it accurate even if a part of a picture is very bright. |
