Crenellation
The crenellation or fold of spectrum is a phenomenon which can occur at the time of the digital processing of a signal, when frequencies which cannot normally be represented are introduced by error into the signal, by consequence of sound sampling not respecting the Théorème of sampling of Nyquist-Shannon.
In particular in the field of the video game, one also rather often uses the Anglicism aliasing .
Consequences of crenellation
In the field of the imagery
Crenellation can arrive when one redimensionne an image towards a resolution different from the resolution of digitalization. It follows an effect of pixellisation , or aliasing , on the edges between objects, more or less marked. Many programs have a function of Anticrènelage to mitigate this effect.
In the field of the sound
Sound crenellation consists of the “fold” of a frequency higher than the Fréquence of Nyquist towards a frequency lower than this one. This is particularly awkward since this phenomenon can transform a Harmonique of a sound into a frequency Anharmonique, unpleasant with the ear. Crenellation is often a consequence of the operations of rééchantillonnage (the sound analog of the redimensioning of image, used in the sampler S to vary the height of a sound) or of distortion (used in particular to simulate valve amplifiers).
Solutions
In many cases it is unfortunately not possible mathematically to eliminate crenellation rigorously, but it is however possible to reduce it to negligible levels. A standard technique is to oversample the signal at a sampling rate definitely higher than the wanted frequency, to apply the desired treatments (for example redimensioning of image, rééchantillonnage or distortion of sound), then to apply a low-pass filter to him to eliminate any frequency higher than the frequency from Nyquist. One can then go down again in full safety at the sampling rate wanted, since no frequency higher than the frequency of Nyquist is present. This method requires a very important computing power, it is thus not applicable to all the situations: thus at present, number of video games still prefer to rather devote all the power of the Graphics processor to the posting of additional details than to the reduction of crenellation.
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