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The effect of exchange interaction A between grains on signal (S) and signal-to-noise (S/N) ratio of single-layer perpendicular recording media was investigated by computer simulation. Oscillation of S and S/N, which could be due to the model of the media, was observed with variation of head field. The maximum values of S (to be called Smax) and S/N (to be called S/Nmax) in oscillation increase with increasing A in a small A range, saturate and decrease in a large A range. On the other hand, the minimum S and minimum S/N (to be called Smin and S/Nmin) in oscillation decrease monotonically with increasing A. The value of A corresponding to the maximum of S/Nmax in a A-S/Nmax curve increases with increasing anisotropy field of a medium and decreases with increasing recording density. The Smax or S/Nmax in oscillation may correspond to S and S/N of media with small grains or amorphous substances and Smin or S/Nmin may correspond to S and S/N of media with large grains, respectively. Therefore, S and S/N of the media composed of small grains or amorphous substances with high Hk could be increased with increasing A.