PURPOSE: To generate a pattern with fidelity by changing the signal amplitude of a selected brightness within a specific range, compressing the signal amplitude of other part in matching with a range with an exponential curve and suppressing a level change in an output video signal.

CONSTITUTION: A clamp waveform shape section 1 eliminates a synchronous signal portion from an input video signal and inputs the result to a variable output amplifier section 2 in which the strength of contrast is adjusted. Its output is added to a position designation voltage from a control voltage generating section 3, the sum is fed to a function amplifier 5 and when the amplitude is a value corresponding to a nonlinear portion, the amplitude is limited by an exponential curve and only the range of the lightness of the linear portion passes. In the portion to be emphasized, since the output of the generating section 3 controlled by position control is fed to the video signal from the amplifier section 2, the potential is offset and for which portion of the signal the linear region is to be set is decided. The output from the amplifier section 5 is inputted to a reference output amplifier 6, the reference pulse signal is compared with a reference voltage and controlled automatically to be a prescribed level.