Digital signal selective circuit

Abstract From the Canadian patent

A digital signal selective or detecting circuit which is responsive to a periodic input signal having a frequency within a specified range, such as an audio frequency used as a cueing signal in a VTR only when the, signal in such frequency range exists for a predetermined time; includes a clock pulse generator providing reference clock pulses, a first counting circuit counting the reference clock pulses whenever the periodic signal is present, a circuit proviaing a counting pulse whenever an operating characteristic of the periodic signal such as, the transition between its positive and negative portions occurs at a time when the count. in the first counting circuit is between first and …