| 摘要 |
PURPOSE:To simplify the circuit constitution and to save number of components by mixing a synchronizing signal from an oscillator and a character signal when no video signal exists. CONSTITUTION:When a video signal is fed to a video signal amplifier 9, a changeover switch 8 is thrown to the position of contact (a), horizontal/vertical synchronizing signals HS, VS are compared with signals HS, VS from an oscillator 17 and the oscillator 17 is controlled in response to the phase difference. Thus, the character signal and the video signal are synchronized. When no signal exists, the absence is detected by a no-signal detector 12 and the switch 8 is thrown to the position of contact (b). In this case, a synchronizing separator 14 is brought into free-run state, the signals HS, VS from the oscillator 17 are mixed, a composite synchronizing signal S is obtained, it is fed to a character signal superimposing circuit 7, mixed with the character signal, fed to a monitor and displayed on it. |
