Anti-aliasing filter

An anti-aliasing filter is a low-pass filter placed before the A/D converter. It removes content above the Nyquist frequency so there's nothing left to cause aliasing.

A real filter can't cut off instantly, so it needs room to roll off. That's why sample rates sit a little above twice the limit of hearing (see Shannon-Nyquist). At 44.1 kHz the filter must go from passing 20 kHz to blocking 22.05 kHz, which takes a very steep filter. Higher sample rates allow gentler filters.