In order for a band-limited (i.e., one with a zero power spectrum for frequencies ) baseband () signal to be reconstructed fully, it must be sampled at a rate . A signal sampled at is said to be Nyquist sampled, and is called the Nyquist frequency. No information is lost if a signal is sampled at the Nyquist frequency, and no additional information is gained by sampling faster than this rate.

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Weisstein, Eric W. "Sampling Theorem." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/SamplingTheorem.html