A video signal processing circuit. includes: a bell table, a first switch, a carrier generating unit, a second switch and a calculation processing unit. The bell table inputs a color difference signal to output a modulation signal having an amplitude defined by the SECAM Standard of the color difference signal. The first switch selects one of the color difference signal and the modulation signal to output a first signal. The carrier generating unit inputs the color difference signal to generate a carrier signal having a frequency defined by the SECAM Standard of the color difference signal. The second switch selects one of the carrier signal and a carrier signal having frequency defined by the PAL Standard and/or the NTSC Standard to output a second signal. The calculation processing unit calculates the first and second signals to output a color difference modulation signal.