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