JPS6486782A - Image display device - Google Patents

Abstract

PURPOSE:To clamp a text signal TEXT at a fixed DC level and to prevent a horizontal mark or change of luminance at a weak electric field and the rolling of the horizontal mark in a vertical cycle at the time of interlacing from being generated by prohibiting the supply of a television signal TV to the processing circuit of the signal TV at the time of a TEXT mode. CONSTITUTION:In the TEXT mode, a horizontal blanking period TB and a horizontal blanking pulse HBLK go to 'H' levels, and a TV/TEXT switching pulse goes to the 'H' level, thereby, transistors Q1' and Q2 are turned ON, and potential at a point (a) drops gradually. When the potential drops to Vcc-VF (VF represents a voltage between the base and the emitter of a transistor Q3), the transistors Q3 and Q4 are turned ON, and the potential at a point (b) goes to the ground potential, and a transistor Q5 is turned OFF, therefore, the processing circuit of the television signal TV goes to a no input state. By setting such state, since the output voltage of a matrix 16 becomes the DC voltage, it follows that the text signal TEXT is clamped at the DC voltage, then, a stable clamping operation can be performed.