Sampling & Aliasing with Overtones

Aliasing for Signals with Overtones

For signals with overtones, undersampling cal lead to inharmonic aliases long before the fundamental itself exceeds the Nyquist frequency. For a harmonic signal with a fundamental frequency \(f_0\), the alias frequencies for all \(N\) harmonics can be calculated:

\(f_m = \sum\limits_{n=1}^{N} \Big| n f_0 - f_s \Big\lfloor \frac{n f_0}{f_s} \Big\rfloor \Big|\)

Interactive Example

For the following example, a sawtooth with 20 partials is used without band limitation. At a pitch of about \(2000 Hz\), the aliases become audible. For certain fundamental frequencies, all aliases will be located at actual multiples of the fundamental, resulting in a correct synthesis despite aliasing. In most cases, the mirrored partials are inharmonic and distort the signal.

Pitch (Hz):

Output Gain:

Time Domain:

Frequency Domain:

Anti-Aliasing Filters

In analog-to-digital conversion, simple anti-aliasing filters can be used to band-limit the input and discard signal components above the Nyquist frequency. In case of digital synthesis, however, this principle can not be applied. When generating a square wave signal with an infinite number of harmonics, aliasing happens instantaneously and can not be removed, afterwards.

Band Limited Generators

In order to avoid the aliasing, band-limited signal generators are provided in most audio programming languages and environments.

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