| 摘要 |
PURPOSE:To faithfully detect a background level even though the background has two steps by constituting a picture signal extracting circuit with a peak holding circuit, a differentiating circuit, a timer circuit, a switch circuit and a differential amplifying circuit. CONSTITUTION:A peak holding circuit 10 is prepared to hold the peak value of the output of an image sensor, together with a differentiating circuit 11 which outputs a pulse when the output of the image sensor has a sudden change, and a timer circuit 12 which is started by the output pulse of the circuit 11 and produces a signal in case a pulse is outputted again from the circuit 11 within a fixed time. A switch circuit 13 functions to obtain the coincidence between the capacitor voltage of the circuit 10 and the output of the image sensor. Then a differential amplifying circuit 14 amplifies the difference between the output of the circuit 10 and the sensor output. Thus it is possible to faithfully detect a background level despite the stepped change of the background level. |
