A memory unit includes a buffer memory unit having first and second write buffer memories and first and second read buffer memories, and a frame memory section. The buffer memory unit is connected to the frame memory and a gray signal converter. The first and the second write buffer memories alternately store a sequence of data segments of current gray data from a signal source and alternately output the current data segments to the frame memory for storing. The first and the second read buffer memories read out a sequence of data segments of previous gray data from the frame memory and alternately output the previous data segments to the gray signal converter. The frame memory stores a plurality of previous and current data segments, and the total amount of the data segments stored in the frame memory is equal to or larger than those for one frame.