Effect of under sampling (aliasing effect):-
When a Continuous Time 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 are 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 always becomes a problem.
A signal is first passed through LPF before sampling.
i.e, it is band limited by this LPF which is known as a pre-alias filter.
To avoid aliasing
- Pre-alias filter must be used to limit the bandwidth of the signal to Hz.
- Sampling frequency must be .
Pre-alias filter means before sampling is passed through an LPF to make a perfect band-limited signal.