Some of the most dramatic and subtle examples of aliasing occur when the signal being sampled also has periodic content.įig.2 Upper left: Animation depicts snapshots of a sinusoid whose frequency is increasing, while it is being sampled at a constant frequency/rate, f s. Audio signals are sampled ( digitized) with an analog-to-digital converter, which produces a constant number of samples per second. Digital cameras provide a certain number of samples ( pixels) per degree or per radian, or samples per mm in the focal plane of the camera. Like the video camera, most sampling schemes are periodic that is, they have a characteristic sampling frequency in time or in space. Temporal aliasing frequencies in video and cinematography are determined by the frame rate of the camera, but the relative intensity of the aliased frequencies is determined by the shutter timing (exposure time) or the use of a temporal aliasing reduction filter during filming. A reversal of direction can be described as a negative frequency. Aliasing has changed its apparent frequency of rotation. In video or cinematography, temporal aliasing results from the limited frame rate, and causes the wagon-wheel effect, whereby a spoked wheel appears to rotate too slowly or even backwards. To prevent this, an anti-aliasing filter is used to remove components above the Nyquist frequency prior to sampling. The high frequencies in the analog signal will appear as lower frequencies (wrong alias) in the recorded digital sample and, hence, cannot be reproduced by the DAC. If a piece of music is sampled at 32,000 samples per second (Hz), any frequency components at or above 16,000 Hz (the Nyquist frequency for this sampling rate) will cause aliasing when the music is reproduced by a digital-to-analog converter (DAC). Music, for instance, may contain high-frequency components that are inaudible to humans. Temporal aliasing is a major concern in the sampling of video and audio signals. Aliasing can be caused either by the sampling stage or the reconstruction stage these may be distinguished by calling sampling aliasing prealiasing and reconstruction aliasing postaliasing. Spatial anti-aliasing techniques avoid such poor pixelizations. If the image data is processed in some way during sampling or reconstruction, the reconstructed image will differ from the original image, and an alias is seen.Īn example of spatial aliasing is the moiré pattern observed in a poorly pixelized image of a brick wall. When a digital image is viewed, a reconstruction is performed by a display or printer device, and by the eyes and the brain. For spatial anti-aliasing, the types of anti-aliasing include fast sample anti-aliasing (FSAA), multisample anti-aliasing, and supersampling.ĭots in the sky due to spatial aliasing caused by halftone resized to a lower resolution Suitable reconstruction filtering should then be used when restoring the sampled signal to the continuous domain or converting a signal from a lower to a higher sampling rate. moiré patterns in digital images) this type of aliasing is called spatial aliasing.Īliasing is generally avoided by applying low-pass filters or anti-aliasing filters (AAF) to the input signal before sampling and when converting a signal from a higher to a lower sampling rate. It can also occur in spatially sampled signals (e.g. It also often refers to the distortion or artifact that results when a signal reconstructed from samples is different from the original continuous signal.Īliasing can occur in signals sampled in time, for instance digital audio, or the stroboscopic effect, and is referred to as temporal aliasing. In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. Halfway through the 24-second loop, the objects appear to suddenly shift and head in the reverse direction, towards the left. The speed of the "camera", moving towards the right, constantly increases at the same rate with the objects sliding to the left. The motion of the 'camera' at a constant shutter speed creates temporal aliasing known as the wagon wheel effect.