| 摘要 |
PURPOSE:To perform the detection at a high speed, by detecting the presence or absence of a broadcast wave from the AFT voltage formed from the output of a peak AGC type VIF processing circuit and a separated synchronizing signal when a positive modulated signal is received. CONSTITUTION:The output of a VIF processing circuit 5 is selected by a switch circuit S1 and extracted to an output terminal when a positive modulated signal is received. At the same time, the synchronizing signal separated by a synchronism separating circuit 12 is selected by a switch circuit S2 and supplied to a discriminating circuit 11. The output of a VIF processing circuit 7 is supplied to a frequency discriminating circuit 10 to form the AFT voltage. This AFT voltage is supplied to the circuit 11. When the detecting outputs of both AFT voltage and synchronizing signal are set at a level near (0)V, a detecting pulse is generated from the circuit 11 and supplied to a channel selection control circuit 2 to stop a sweeping operation. |
