Effect of under sampling (aliasing effect):-
When a CT band limited signal is sampled at , then the successive cycles of the spectrum of the sampled signal overlap with each other as shown below
Some aliasing is produced in the signal this is due to under sampling.
aliasing is the phenomenon in which a high frequency component in the frequency spectrum of the signal takes as a low frequency component in the spectrum of the sampled signal.
Because of aliasing it is not possible to reconstruct x(t) from g(t) by low pass filtering.
The spectral components in the overlapping regions and hence the signal is distorted.
Since any information signal contains a large no.of frequencies so the decision of sampling frequency is always become a problem.
A signal is first passed through LPF before sampling.
i.e, it is band limited by this LPF which is known as pre-alias filter.
To avoid aliasing
- Pre-alias filter must be used to limit the band width of the signal to Hz.
- Sampling frequency must be .
Pre-alias filter means before sampling is passed through a LPF to make a perfect band limited signal.