| 摘要 |
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 providing 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 second predetermined counts, and a second counting circuit counting each counting pulse and providing an output when the second counting circuit attains a predetermined count. Preferably a Schmitt trigger is employed for shaping the periodic input signal, and a JK bistable multivibrator is associated with the Schmitt trigger and the clock pulse generator for synchronizing the output of the Schmitt trigger with the reference clock pulses.
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