aliasingilmiötä

Aliasingilmiö, often referred to as aliasing, is a phenomenon that occurs when a continuous signal is sampled at a rate that is too low to accurately represent its original frequency components. This undersampling leads to the creation of false, lower frequencies that were not present in the original signal. In essence, the sampling process is not frequent enough to capture the rapid oscillations of the high-frequency components, causing them to be misinterpreted as slower ones.

The most common manifestation of aliasing is seen in digital signal processing, particularly with audio and image data. In audio, aliasing can result in unwanted distortion or the perception of frequencies that are not actually in the recording. In images, it is often observed as jagged edges or moiré patterns in areas with fine, repetitive textures. For example, when photographing a striped shirt with a digital camera, if the stripe pattern has a frequency close to half the sampling frequency of the camera's sensor pixels, aliasing can create spurious wavy patterns.

To prevent aliasing, the Nyquist-Shannon sampling theorem states that the sampling rate must be at least twice the highest frequency present in the signal. This minimum sampling rate is known as the Nyquist rate. When this condition is met, the original signal can be perfectly reconstructed from its samples. If the sampling rate is lower than the Nyquist rate, aliasing is unavoidable, and anti-aliasing filters are often employed before sampling to remove or attenuate frequencies above the Nyquist limit, thereby mitigating the aliasing effect.