3 animations about sampling and numerical filtering:
This animation intends to show the spectrum and the harmonics (same color) of a sampled and hold sine wave. The frequency is bursted till frequencies over the Nyquist frequency so that the stroboscopic effect can be seen.
Note that the phases of the harmonics are respected.
This animation intends to show how the complex plan is transformed when approximating the jw operator. 
FIRST ORDER NUMERIC FILTER: