JPS57162581A - Signal detecting circuit - Google Patents

Abstract

PURPOSE:To hold an average state regardless of an AFC for a signal detecting circuit, by delivering in a stable way the signal according to whether the input electric wave is higher than a certain level by making use of the fact that the low level voltage of a horizontal synchronizing signal varies by the electric field intensity. CONSTITUTION:For instance, the input horizontal synchronizing signal is supplied via a parallel circuit of a capacity C4 and a resistance R8 to a gate circuit 2 including a level control means and having the prescribed threshold value VTH. The output of the circuit 2 is fetched into the 1st signal processing circuit 3, and the circuit 3 delivers a high level signal with application of a prescribed standard signal (a) with a strong electric field and delivers a low level signal with application of a signal (b) of the low level voltage VL and with the weak electric field. The 2nd signal processing circuit 4 delivers a signal showing the existence of a prescribed field intensity with application of a prescribed horizontal synchronizing signal. In case the electric field intensity is unstable and the input of the circuit 2 goes up and down the value VTH, the unstable field intensity is held stable via feedback circuit 6. In such way, the presence or absence of the broadcast electric wave can be automatically detected regardless of the lead-in time of an AFC circuit.