PURPOSE: To obtain a stable video signal by descrambling the television signal over the entire vertical blanking period by using specific key pulses.

CONSTITUTION: Key pulses outputted by an inverter 13 are integrated by an integrating circuit 14 to generate an integral output waveform. The Schmitt trigger circuit 15 consisting of inverters 16 and 17 and resistances R3 and R4 outputs pulses (h) only for a waveform corresponding to key pulses which are wider than other key pulses on the front end side of the integral output waveform in the vertical blanking period. The pulses (h) are supplied to an RS latch circuit 18, which latches the high level part of the waveform (i) supplied from an inverter 5 and generates a latch output (j), which is given a waveform (K) by a NAND circuit 21 and inverted by an inverter 22 into a signal (l). Then, this signal (l) and output pulses (m) from an inverter 2 are NANDed to obtain pulses for descrambling in the whole vertical blanking period.