This paper describes a digital signal processing method for reducing interference when receiving an analog FM stereo signal. The proposed method uses the left and right audio signals of a stereo receiver as input signals. Unlike conventional FM receiver strategies that reduce the stereo separation broadband or in frequency bands to keep the noise at a bearable level, here the noise is reduced to a quality comparable to mono while at the same time preserving the stereo separation and frequency response. This is achieved by applying signal processing rules derived from the observation of matrixed source signals recorded in intensity, time-of-arrival, and equivalence stereophony. The main part of the noise reduction is based on lowering the magnitude spectrum of the disturbed difference signal to the level of the sum signal, eliminating the excessive width of the stereo base caused by noise. The signal processing method is compatible with the FM stereo transmission standard and applicable worldwide.
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