The phenomenon of aliasing occurs if the sampling frequency is less than the Nyquist rate. If sampling rate is less than or greater than the Nyquist rate, it is called under sampling or over sampling. Aliasing phenomenon occurs only for under sampling.
Consider a continuous signal in the time and frequency domain shown in Fig.1.33.
Fig. 1.33 Continuous signal in the time and frequency domain
If the sampling frequency is too low the frequency spectrum overlaps, and becomes corrupted as shown in Fig. 1.34.
Fig. 1.34 Aliasing ...