Aliasing....

A wave diagram showing Aliasing

When a continuous time band limitted signal is sampled at a rate lower than Nyquist rate fs < 2fm then the successive cycles of the spectrum G(w) of the sampled signal g(t) overlap each other ,this is known as aliasing. In fact, aliasing is the phenomenon in which high frequency component in the frequency spectrum of the signal takes identity of a lower frequency component in the spectrum of the sampled signal. Due to aliasing it is not possible to recover original signal and hence the signal is distorted. Therefore a signal is first passed through a low pass filter. This filter does not pass the frequencies above fm Hz. This process is known as band limitting of original signal x(t). This low pass filter is called as pre alias filter because it is used to prevent aliasing effect