PURPOSE: To detect changed line by performing comparison between frames with a small amount of memory.

CONSTITUTION: Pixel data coming from a color LUT in the form of 8bits×4 are latched by a latch 51 and 32bit data and 0 are added with an adder 52. The rotate-shift by one bit of the obtained 64bit date is performed by a rotate shift register 53 and the shifted data are added to next 32bit date. After this addition is repeated for a fixed time, values of the rotate shift register 53 are compared as a 'Signature' with the 'Signature' of the same place as a front frame. A comparator 54 outputs Result=1 in the case Signatures are different and outputs Result=0 in the case Signatures are equal. At this time, the count value of the number of lines of a timing control part 55 is simultaneously outputted. The Signature date of a present frame are stored in a memory 46 for the comparison with next frame.