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it occurs when you sample a continous time signal at a sub nyquest rate . It will lead to lowfrequency overtones taking over the identity of the hf components .
sepwww.stanford.edu/sep/ prof/bei/krch/paper_html/node12.html
aliasing occurs when sample rate is less than nyquist rate.the signal spectra overlaps with each other.so we can not extract the signal from its sampled version.so to overcome aliasing
1> we use a antialiasing filter before sampling.
2>sampling should always greater than nyquist rate.
sampling theorm states that in order to be able to reconstruct the original signal using it's discrete samples, the sampling frequency should be twice or more than the highest frequency content of the continous signal...
you should read the proof of this theorm in Signals and systems by oppenheim to understand this better,it's easy if you know your fourier transform properties...
alliasing happens when the condition is not satisfied,the periodic spectrum will overlap and the continious signal reconstructed will be distorted...
aliasing occurs when sampling rate is less than nyquist rate. Normally it should be grater than the nyquist rate, which avoids the suffering of sine waves due to aliasing. So we keep it greater than nyquist rate
Do You want an example? Ok: try to sample a sin(2x) [who have period equal to PI ==> bandwindth is equal to 1/period = 1/PI] at the frequency 1/PI (who is equal to the bandwidth of Your signal, so don't respect the Shannon-Nyquist Formula).
When You go to interpolate Your samples, You will reconstruct the function SIN(x) and not SIN(2X)!!! Have You understand the word ALIASING now?! You obtain in recostruction an ALIAS of Your original signal.
I'm Paolone from Italy: in italian, this phenomena is known as "EQUIVOCAZIONE".
There is an example:
You may notice that in some old film, the wheel of a car often runs at the wrong direction. That is because the sample frequency (about 25sample/s) is too slow for the motion of the wheel, so the sampled signal is aliased.
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