File:FFT aliasing 600.gif

English: Animation showing how aliasing (frequency ambiguity) is a result of sampling a continuous waveform.

Upper left: Animation depicts a sequence of sinusoids, each with a higher frequency than the previous ones. These "true" signals are also being sampled (blue dots) at a constant frequency/rate, ${\displaystyle f_{s}.}$

Upper right: The continuous Fourier transform of the sinusoid (not the samples). The single non-zero component, depicting the actual frequency, means there is no ambiguity.

Lower right: The discrete Fourier transform of just the available samples. The presence of two components means the samples can fit at least two different sinusoids, one of which is the true frequency (upper-right).

Lower left: Using the same samples (now in orange), the default reconstruction algorithm (i.e. in the absence of collateral information) produces the lower-frequency sinusoid.