Here are some movies that illustrate the concepts of aliasing and folding when a sinusoid is sampled below the Nyquist rate.

By visualizing the spectrogram of a synthesized chirp and listening to the sound, we experience the fact that a D-to-C converter cannot create output signals with frequencies higher than one half of the sampling frequency.

The Continuous-Discrete Sampling Demo (con2dis) is a program that shows the continuous and discrete spectra (and signals) during sampling.
Features:

• Users can change the input frequency and sampling rate.
• Frequency axis can be labeled in hertz or radians/sec.
• Reconstruction through D/A is also shown.

Here are some movies that illustrate the reconstruction process

These movies give an alternate view of the sampling process by using the strobing nature of a camcorder (30 frames per second) to show aliasing of a pattern on a rotating disk.

These movies were generated in MATLAB to show the strobe/sampling effect on a rotating disk. With MATLAB the rotation rate can be calibrated exactly, so that forward and backward movement of the spokes on the disk (due to aliasing) can be tracked.