# Translations:Prewhitening - book/2/en

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When the true autocorrelation, shown in Figure 10a, is truncated to the curve shown in Figure 10b, the spectrum is no longer positive for all values of the frequency. Such a situation is untenable. To correct this problem, the truncated autocorrelation can be tapered, as shown in Figure 10c, so that it damps out more rapidly. This increased damping is enough to make the corresponding spectrum positive for all frequencies.