Anti-Aliasing

Filtering above the ceiling before sampling stops high frequencies folding back as false tones.

Related

Sample RateHarmonicsTimbre

The problem

Why does downsampling sometimes add a sandy layer to cymbals that was not in the original file?

How it works

If content above Nyquist is not removed before sampling, it folds into the audible band. This is aliasing, and it is a mirror in frequency, not a fade, so the false tones often land far from the original pitch and read as metallic friction or hiss. The anti-aliasing filter must act before conversion, with a narrow transition band; too wide and it cuts useful highs, too gentle and folded content leaks through. Modern converters oversample and filter digitally, easing the analog stage. Downsampling, pitch shifting and wavetable lookup can each reintroduce aliasing, so every stage needs a bandwidth check.

Practice and settings

Record and deliver at 44.1 or 48 kHz and let the converter filter. For large upward shifts, upsample before shifting or use an algorithm with built-in anti-aliasing. When generating wavetables or distortion, limit harmonics at the source instead of repairing later. To check, align spectra before and after the shift and look for non-harmonic content that appears from nowhere.

What to listen for

Hear hi-hat and shaker tails: aliasing adds a sandy layer that does not follow velocity. Hear celesta high notes, where the attack turns dirty. Slide a sawtooth lead upward and listen for a false tone moving the wrong way. Hear vocal sibilance, which can collapse into fine noise.

Common misconceptions

That aliasing only happens on cheap gear; that a low-pass effect fixes it; that louder playback hides it.

Sources

• Ken C. Pohlmann, Principles of Digital Audio — 混叠与过采样的机制核对，未抄原文
• Julius O. Smith III, Digital Audio Resampling — 重采样滤波与过渡带的说明核对，未抄原文

Used for fact-checking only; all prose is original.