communications A theorem stating that when an analogue

waveform is digitised, only the frequencies in the waveform

below half the sampling frequency will be recorded. In

order to reconstruct (interpolate) a signal from a sequence of

samples, sufficient samples must be recorded to capture the

peaks and troughs of the original waveform. If a waveform is

sampled at less than twice its frequency the reconstructed

waveform will effectively contribute only noise. This

phenomenon is called "aliasing" (the high frequencies are

"under an alias").