Aliasing is a type of digital distortion that occurs when the sampling frequency (sample rate) is too low to correctly capture an audio signal.

The Theorem and the Cause

According to the Nyquist-Shannon Theorem, the sampling frequency must be at least twice the highest frequency you want to record.

For example, to record sounds up to 20 kHz (the limit of human hearing), the sampling rate must be at least 40 kHz.

That's why the CD standard was established at 44.1 kHz.

Aliasing happens when this rule is broken. If a sound with a frequency higher than half the sampling frequency enters the converter, the system cannot "read" the wave correctly. It creates a new "phantom" frequency that did not exist in the original sound.

Visual Analogy

Think of the wheels of a car in movies. As the car accelerates, the wheels appear to rotate slower, stop, or even spin backward. This happens because the frame rate (the "sampling" of the film) is too low to capture the actual movement of the wheels. Aliasing in audio is the sonic equivalent of this phenomenon. The Resulting Sound: This "phantom" frequency generated by aliasing is an inharmonic distortion, meaning it has no musical relationship with the original sound, resulting in an unpleasant and dissonant digital artifact. To avoid aliasing, A/D converters use an anti-aliasing filter, which is a low-pass filter that removes any frequency above half the sampling frequency before the audio is digitized.