Abstract: A stereo audio decoder decoding a multiplex signal into a stereo audio signal. A first filter module filters the multiplex signal to generate a summation signal. A sub-carrier module modulates the multiplex signal according to a sub-carrier frequency to generate a sub-carrier mixed signal having a first high frequency and a first low frequency component. The first low frequency component has a sub-carrier phase offset between the stereo audio decoder and the multiplex signal. The second filter module filters out the first high frequency component of the sub-carrier mixed signal to generate a sub-carrier pure signal having only the first low frequency component. A corrector generates a difference signal according to a correction signal and the multiplex signal. A channel separator obtains a left channel signal and a right channel signal of the stereo audio signal by decoding the summation signal and the difference signal.