An anti-aliasing filter suppresses signal components outside the intended acquisition band before sampling. For voltage recording it is an analogue filter ahead of the analog-to-digital converter, with a transition band allowed below half the sampling rate. Once two frequencies have become indistinguishable through aliasing, a digital filter generally cannot recover the lost distinction. The Nyquist–Shannon sampling theorem concerns an already bandlimited function; a physical filter approximates that prerequisite.
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An anti-aliasing filter is a signal processing filter used to prevent aliasing when sampling a signal. Aliasing occurs when a continuous signal is sampled at a rate that is insufficient to accurately capture the changes in the signal, leading to distortion or misrepresentation of the original signal's features in the sampled data.