Aliasing

aliasedaliastemporal aliasingaliasesaliasing artifactsaliasing distortionanti-aliasingare physically equivalent to vectors inside itdigital aliasingfolding frequency
In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled.wikipedia
215 Related Articles

Spatial anti-aliasing

anti-aliasinganti-aliasedantialiasing
Spatial anti-aliasing techniques avoid such poor pixelizations.
In digital signal processing, spatial anti-aliasing is a technique for minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution.

Anti-aliasing filter

anti-aliasinganti-aliasing (AA) filteroptical low-pass filter
Aliasing is generally avoided by applying low pass filters or anti-aliasing filters to the input signal before sampling.
Since the theorem states that, unambiguous reconstruction of the signal from its samples is possible when the power of frequencies above the Nyquist frequency is zero, a real anti-aliasing filter trades off between bandwidth and aliasing.

Moiré pattern

moirémoiremoire pattern
Aliasing can also occur in spatially sampled signals, for instance moiré patterns in digital images.
Moiré patterns are often an artifact of images produced by various digital imaging and computer graphics techniques, for example when scanning a halftone picture or ray tracing a checkered plane (the latter being a special case of aliasing, due to undersampling a fine regular pattern).

Reconstruction filter

filteringmultivariate interpolation kernelsreconstruction
Suitable reconstruction filters should then be used when restoring the sampled signal to the continuous domain.
The sampling theorem describes why the input of an ADC requires a low-pass analog electronic filter, called the anti-aliasing filter: the sampled input signal must be bandlimited to prevent aliasing (here meaning waves of higher frequency being recorded as a lower frequency).

Digital audio

digital musicaudiodigital
Aliasing can occur in signals sampled in time, for instance digital audio, and is referred to as temporal aliasing.
Analog signals that have not already been bandlimited must be passed through an anti-aliasing filter before conversion, to prevent the aliasing distortion that is caused by audio signals with frequencies higher than the Nyquist frequency (half the sampling rate).

Sampling (signal processing)

sampling ratesamplingsample rate
In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. If a piece of music is sampled at 32000 samples per second (Hz), any frequency components above 16000 Hz (the Nyquist frequency for this sampling rate) will cause aliasing when the music is reproduced by a digital to analog converter (DAC). See Sampling (signal processing), Nyquist rate (relative to sampling), and Filter bank.
That fidelity is reduced when s(t) contains frequency components whose periodicity is smaller than two samples; or equivalently the ratio of cycles to samples exceeds ½ (see Aliasing).

Nyquist frequency

NyquistN/2 different frequenciesNyquist component
If a piece of music is sampled at 32000 samples per second (Hz), any frequency components above 16000 Hz (the Nyquist frequency for this sampling rate) will cause aliasing when the music is reproduced by a digital to analog converter (DAC).
It is sometimes known as the folding frequency of a sampling system.

Wagon-wheel effect

wagon wheel effect
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.
In these recorded media, the effect is a result of temporal aliasing.

Analog-to-digital converter

ADCanalog to digital converteranalog-to-digital conversion
Audio signals are sampled (digitized) with an analog-to-digital converter, which produces a constant number of samples per second.
The SNR of an ADC is influenced by many factors, including the resolution, linearity and accuracy (how well the quantization levels match the true analog signal), aliasing and jitter.

Nyquist rate

Nyquist limitNyquist sampling rateNyquist
See Sampling (signal processing), Nyquist rate (relative to sampling), and Filter bank.
1) as a lower bound for the sample rate for alias-free signal sampling (not to be confused with the Nyquist frequency, which is half the sampling rate of a discrete-time system) and

Distortion

distortedharmonic distortiondistort
It also refers to the distortion or artifact that results when the signal reconstructed from samples is different from the original continuous signal.
Other forms of audio distortion that may be referred to are non-flat frequency response, compression, modulation, aliasing, quantization noise, wow and flutter from analog media such as vinyl records and magnetic tape.

Undersampling

bandpass samplingundersampled
Undersampling, which creates low-frequency aliases, can produce the same result, with less effort, as frequency-shifting the signal to lower frequencies before sampling at the lower rate.
When one undersamples a bandpass signal, the samples are indistinguishable from the samples of a low-frequency alias of the high-frequency signal.

Nyquist–Shannon sampling theorem

sampling theoremsampling theoryNyquist
is met for the highest frequency component of the original signal, then it is met for all the frequency components, a condition called the Nyquist criterion.
When the bandlimit is too high (or there is no bandlimit), the reconstruction exhibits imperfections known as aliasing.

Signal reconstruction

reconstructreconstructionPixel filtering
When a digital image is viewed, a reconstruction is performed by a display or printer device, and by the eyes and the brain.
Aliasing

Whittaker–Shannon interpolation formula

interpolation/sampling theoryreconstructingsinc interpolation
But the fidelity of a theoretical reconstruction (via the Whittaker–Shannon interpolation formula) is a customary measure of the effectiveness of sampling.
(See Aliasing.)

Jaggies

jaggednessjagginessjaggyness
Jaggies
"Jaggies" is the informal name for artifacts in raster images, most frequently from aliasing, which in turn is often caused by non-linear mixing effects producing high-frequency components or missing or poor anti-aliasing filtering prior to sampling.

Wave field synthesis

wave fieldWFS
Spatial aliasing, particular of angular frequency, can occur when reproducing a light field or sound field with discrete elements, as in 3D displays or wave field synthesis of sound.
There are undesirable spatial distortions caused by position-dependent narrow-band break-downs in the frequency response within the rendition range – in a word, aliasing.

Stroboscopic effect

stroboscopicstrobe effectstroboscopically
Stroboscopic effect
The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous motion is represented by a series of short or instantaneous samples.

Sawtooth wave

sawtoothsawsaw wave
Six sawtooth waves are played in succession, with the first two sawtooths having a fundamental frequency of 440 Hz (A4), the second two having fundamental frequency of 880 Hz (A5), and the final two at 1760 Hz (A6).
If the waveform is digitally created directly in the time domain using a non-bandlimited form, such as y = x - floor(x), infinite harmonics are sampled and the resulting tone contains aliasing distortion.

Wavelength

wavelengthsperiodsubwavelength
Waves must be sampled at more than two points per wavelength, or the wave arrival direction becomes ambiguous.
This produces aliasing because the same vibration can be considered to have a variety of different wavelengths, as shown in the figure.

Signal processing

signal analysissignalsignal processor
In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled.

Artifact (error)

artifactartifactsartefacts
It also refers to the distortion or artifact that results when the signal reconstructed from samples is different from the original continuous signal.

Digital image

digital imagesimagesimage
Aliasing can also occur in spatially sampled signals, for instance moiré patterns in digital images.

Low-pass filter

low-passlow pass filterlow pass
Aliasing is generally avoided by applying low pass filters or anti-aliasing filters to the input signal before sampling.