| 摘要 |
PURPOSE:To surely detect the interruption of optical reception level, by optically transmitting a picture signal clamping a horizontal synchronizing signal to a constant level with PFM modulation and monitoring the frequency signal corresponding to the horizontal synchronizing signal at the reception side. CONSTITUTION:A horizontal clamp level Vc, black level VB, and white level VW of a picture signal are converted respectively into frequencies fC, fB and fW and optically transmitted. The said optical transmission signal is received at an avalanche photodiode (APD) 1 at the reception side and has a prescribed level through the operation of the AGC circuit comprising an amplifier 2, a peak detector 3 and a DC/DC converter 4 is given to a discrimination reproducing circuit 7 and a band pass filter 5. A level detector 6 monitors the level of the frequency signal fC from the filter 5, and when the level is a prescribed value or below, the interruption of optical reception level is alarmed, a gate 8 is interrupted and the reproduced output of the picture signal is stopped. |
