PURPOSE: To control the amplitude of a luminance signal in response to the amplitude of a red, a blue, a green color signal or the like in a color television receiver or the like.

CONSTITUTION: The absolute amplitude of color difference signals (R-Y), (G-Y) with respect to a pedestal level is detected by an adder/subtractor circuits 13,14 for color difference signals. The absolute amplitudes of the detected color difference signals are added by an adder circuit 16 and the sum is given to a divider circuit 17, in which data are compressed. The compressed data represents the quantity of the color signal. On the other hand, a luminance signal (Y) is formed into signal data based on a pedestal level. The color signal to be compressed and the luminance signal based on the pedestal level are inputted to a ROM 18. The ROM 18 stores several tens of kinds of degree of expansion (conversion characteristic) of the luminance signal by using the amplitude of the color signal as a parameter. The ROM 18 is used to convert the input luminance signal into a luminance signal with an amplitude in response to the amplitude of the color signal inputted in parallel.