| 摘要 |
PURPOSE:To permit a relatively simple waveform operating circuit to detect a break of light input, by utilizing the periodicity of horizontal synchronizing pulses inserted into a video signal for the detection of the break of light input. CONSTITUTION:A light input break detecting circuit 210 is connected to a photodetector on a mean value AGC system. The light input break detecting circuit 210 has a horizontal synchronizing pulse extructing circuit 203 for extracting horizontal synchronizing pulses in the video signal obtained as the output signal of a receiving and amplifying circuit 106, a cynchronous free running multivibrator 204, a clamping circuits 205, an NAND circuit 206, a monostable multivibrator 207, an integrating circuit 208, and a comparing circuit 209 and according to the presence of a light input at a light input terminal IN, a high or low potential is outputted to the output terminal OUT of the comparing circuit 209. |
