Aliasing 
This movie illustrates the phenomenon of aliasing.
A 600 Hz sinusoid is sampled at 500 samples per second.

Folding 
This movie illustrates the phenomenon of folding.
A 600 Hz sinusoid is sampled at 750 samples per second.

NO Aliasing 
This movie illustrates that the Sampling Theorem is true.
In this case, a 600 Hz sinusoid is sampled at 2000 samples per second.
Since the samples are taken at more than two times the
frequency of the cosine wave, there is no aliasing.
