| 摘要 |
PURPOSE:To prevent the occurrence of offensive noise in a weak electric field by detecting the level of the AGC voltage of a video signal processing circuit and disconnecting a sound signal with the aid of a detected output in the weak electric field. CONSTITUTION:The output terminal of a comparator 4 is connected to the base of a transistor 8 whose collector and emitter are connected to a sound signal line 7 and an earth, respectively. In terms of a VIF and an AGC terminal 2, their voltages rise in the weak electric field. Said voltages are impedance- converted by a transistor 1, and applied to one input terminal of a comparator 4 through a semi-fixed resistance 3. The output of the compartor 4 comes to a high level, supplied to the base of the muting transistor 8 to turn on the transistor 8. Accordingly the sound signal is fallen to the earth, whereby the sound signal from a speaker can be eliminated. |
