Aliasing is what happens when a signal contains frequencies above the Nyquist limit (half the sample rate) and they are sampled anyway: instead of being captured, they fold back down into the audible range as false, inharmonic tones that were never in the original. Once aliased they cannot be removed — they sit on top of real content. The defence is an anti-aliasing low-pass filter in front of the ADC that strips everything above Nyquist before sampling, and a matching filter after the DAC. Aliasing also appears inside DSP whenever you generate harmonics or resample carelessly, which is why high-quality sample-rate conversion matters.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
Aliasing creates tones that were never in the original sound and cannot be removed once they are recorded. Volume 4 covers aliasing in detail within the basics of digital audio: sampling and sample rates, the Nyquist limit, filtering, bit depth and dynamic range, and why 48 kHz became the video standard. Written by Nikolay Sapunov, CEO at Fora Soft.
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