Overholder man ikke ovenstående filtrering, fås det som kaldes [[harmonisk aliasering]], som i virkeligheden skyldes forsømmelse af den rette filtrering inden diskretisering. Virkningen af en sådan forsømmelse, vil resultere i forkerte power spektra, når det oprindelige input signal indeholder frekvenser, som er forskelligt fra de entydigt adskilbare frekvenser.<ref>[http://www.asp.ucar.edu/colloquium/1992/notes/part1/node62.html asp.ucar.edu: 8. Spectral Analysis: 8.9 Aliasing] Citat: "...The Fourier transform determined from the discrete series thus will include contributions not only from the analyzed frequency $\nu$ but also from all other frequencies that differ from that frequency by an integer multiple of the sampling frequency. This mixing of contributions from different frequency components is called "aliasing." There is no way to separate the various components that contribute to $\tilde g(\nu)$ in (8.55), once they are mixed during sampling...For this reason, we can only analyze Fourier coefficients over a unique range of frequencies varying by $1/\Delta T$, because beyond this range the analysis will simply repeat...To reduce aliasing, low-pass filters can reduce the high-frequency components of a signal before sampling. If this is not done, higher-frequency contributions from the variance spectrum will appear as false contributions to lower parts of the estimated spectrum, and they cannot be eliminated after sampling because information distinguishing the separate contributions from frequencies above and below the Nyquist frequency is lost...", [https://web.archive.org/web/20140312172131/http://www.asp.ucar.edu/colloquium/1992/notes/part1/node62.html backup]</ref>
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