Here is a signal, x(t) with X(w) = 0 when |W| > Wm.
With sampling period, T, samples of x(t),x(nT), can be obtained from x(t), where n = 0 +-1, +-2, ....
The sampling frequency is $ frac{2*/pi}{T} $. It is called Ws.
If Ws is greater than 2Wm, x(t) can be recovered from its samples.
Here, 2Wm is called the "Nyquist rate".
To recover, first we need a filter with amplited T when |W| < Wc.
Wc has to exist between Wm and Ws-Wm.
