Gazohyojisochinokontorasutochoseikairo

摘要

PURPOSE:To obtain a suitable contrast by following the mean level of a video signal, making the upper limit reference potential and the lower limit reference potential of an A-D converter variable, and making a voltage width between them variable. CONSTITUTION:A reference potential generating circuit 11 generates a low-level reference potential VL and a high-level reference potential VH in accordance with the white level voltage and a black level voltage of the video signal. The width between the VL and the VH is adjusted by making the reference potential of an OP amplifier in the circuit 11. The reference potentials VL and VH are inputted to the reference terminals of comparators 121-12n directly or after being divided by resistors R1-Rm. The comparators 121-12n operates while a bias is given from a bias circuit 14 and inputs their output signals to a decoder 13. The decoder 13 decodes the input signals into, for example, a digital signal of 4 bits and sends it as the output or an A-D converter 3 to a shift register 9.