JPS5799837A - Error signal detecting circuit - Google Patents

Abstract

PURPOSE:To raise the error signal detecting capacity, by counting a pulse of an input signal per unit time, and operating a signal demodulating circuit when the input signal has entered the prescribed range continuously by the prescribed number of times. CONSTITUTION:An optical input is converted to an electric signal by a photoelectric converting part 10 and is inputted to a signal counter 13. By a reference counter 11 and a decoder 12, a reference signal is generated from a reference clock having a period T, and a clear signal and a check timing signal are generated when a counting value is ''0'', and when the counting value is N, respectively. The counter 13 sets an FF14 when an input pulse has been counted by (m) piece, resets the FF14 and also sets an FF15 when the clear signal is ''1'', and resets the FF15 when it has been counted by (n)(>m) pieces. When a period of the input signal is denoted as (t), an FF17 is set at the time of a range of mt<NT<nt. When an output of the FF17 is logic ''1'' during the period exceeding a time constant of a time constant circuit 18 continuously, an FF20 is set, a signal demodulating circuit 21 is operated, and if said output has slipped out of the prescribed range even once, the FF20 is reset, and the circuit 21 is not operated.