Anti aliasing filter reduces errors due to aliasing. If a signal is sampled at 8 kS/S, the max frequency of the input should be 4 kHz. Otherwise, aliasing errors will result. Typically a 3.4kHz will have an image of 4.6 khz, and one uses a sharp cut off filter with gain of about 1 at 3.4kHz and gain of about 0.01 at 4.6 kHz to effectively guard against aliasing. Thus one does not quite choose max frequency as simply fs/2 where fs is sampling frequency. One has to have a guard band of about 10% of this fmax, and chooses max signal frequency as 0.9*fs/2

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