Flat display device and its control method

A flat display device which makes drive voltage polarity of pixels of adjacent lines reversed polarity, while drives to reverse drive voltage polarity of a same line on a frame-by-frame basis, wherein the flat display device comprises common voltage generation means for supplying a common voltage signal to a facing electrode of the flat display device and means for generating a common voltage control signal supplied to the common voltage generation means, characterized in that the common voltage signal generation means which generates a common voltage signal which is AC:

• generates, by using an (fh/2) signal generation circuit, a first signal having frequency of (fh/2) (fh:
• horizontal frequency) which reverses lines on a line-by-line basis based on a horizontal synchronization timing signal;
• generates, by using an (fv/2) signal generation circuit, a second signal having frequency of (fv/2) (fv:
• vertical frequency) which reverses frames on a frame-by-frame basis based on a vertical synchronization timing signal;
• generates, by using an (fh×n) signal generation circuit, a third signal of positive polarity and a third signal of negative polarity having frequency of nth of fh based on the horizontal synchronization timing signal and a clock signal;
• generates, by using a multiplication circuit, a fourth signal by performing multiplication of the first and the second signals;
• selects and outputs, from a selection circuit, one of the third signal of positive polarity, the third signal of negative polarity, and the fourth signal that are supplied to the selection circuit;
• determines, by using a selection control circuit, signal selection mode of the selection circuit according to whether one cycle of the second signal contains odd cycles of the first signals or even cycles of the first signals;
• and
• generates the common voltage signal based on the signal obtained from the selection circuit.