Digital Audio System

Low-pass filter (left):

• prevents components with frequency $> f_s/2$ from entering the ADC.

COMPUTER:

• sound storage/processing;
• processing may introduce components with frequencies $> f_s/2$.

Low-pass filter (right):

• defines audible bandwidth up to $f_s/2$;
• does not prevent introduction of components with frequency $> f_s/2$ (``damage'' already done)!

What happens when frequencies exceed $f_s/2$?