Aliasing

Undesirable ‘beating’ effects caused by the presence of frequencies that are too high in an analog input signal that is converted into digits. Passing the input through a suitable low-pass filter, removing all the frequencies above half that of the analog-to-digital converter’s (ADC) clock rate (the Nyquist Frequency), solves the problem. Examples of aliasing include:

1) Temporal aliasing – e.g. wagon wheel spokes apparently reversing, also movement judder seen in the output of standards converters with insufficient temporal filtering.

2) Raster scan aliasing – twinkling effects on sharp boundaries such as horizontal lines. Due to insufficient filtering this vertical aliasing, and its horizontal equivalent, are often seen on the output of lower quality video processing equipment, such as poor DVEs, as detailed images are re-sized.

The appearance of ‘steppiness’ or ‘jaggies’ of poorly filtered images with near horizontal lines in a TV image is also referred to as aliasing.

See also: Anti-aliasing, Interpolation (temporal) Interpolation (spatial), Into digits (Tutorial 1), Nyquist (frequency)