JPS5778261A - Signal detecting circuit - Google Patents

Abstract

PURPOSE:To avoid the malfunction due to a virtual noise, a similar signal and a periodic pulse noise and to surely detect the signals, by surely detecting noise components out of tonal signals of an information transmitting device and picking up the signals of reception signals only having constant frequency. CONSTITUTION:A tonal signal (a) including noise is binary-coded into white and black video information at a binary circuit 1, a binary signal (b) is applied to the 1st, 2nd FFs 2, 3 and the output of the FF2 is applied to counters 6, 7 via delay circuits 4, 5. At the counters 6, 7, the output is compared with a count value before one period with a clock pulse (g) to output a signal only having a prescribed frequency continuously from a comparator 8. Further, the tonal signal (a) is halfwave-rectified and integrated at an integration circuit 10, binary-coded and applied to an AND circuit 12 together with the output of the 2nd FF. The presence of the tonal signal (a) is discriminated and a prescribed frequency signal is detected at the comparator 8, and when the tonal signal (a) exists, the input signal is outputted as an effective tonal signal.