Aliasing

The erroneous interpretation of high-frequency signals as lower-frequency signals. Such misinterpretations are an expected result of making discrete measurements with sampling devices such as analog-to-digital (A/D) converters.

In motion sequences temporal aliasing is the effect that occurs when the frame rate is not high enough for the sampling rate. For example, the spokes of a wheel may appear to turn backwards if the frame rate of the video input isnít fast enough to capture the motion of the wheel.

Example

A continuous 10Hz sine wave.

The signal is then sampled at 50Hz.

The signal is reconstructed as a 10Hz sine wave.

Reducing the sample rate to 25Hz still satisfies nyquist.

The resultant reconstructed frequency is correct at 10Hz but the accuracy of the reconstruction is not so good.

Reducing the sample rate to 11Hz has a drammatic effect.

The reconstructed signal is a sine wave with a frequency of 1Hz.

Aliasing must be avoided in digital signal analysis to prevent errors, FFT analyzers always contain low pass filters in their input stages to eliminate frequency components higher than one-half the sampling frequency. These filters are automatically updated to give an approriate cutoff frequency as the sampling frequency is changed, and this occurs when the frequency range of the analyzer is changed.

Another example is to consider a clock.

If you were to only look at the clock every 50 minutes then the minute hand would appear to rotate anticlockwise.

The hour hand would still rotate in the correct direction as you have satisfied nyquist.

The second hand would jitter around depending on how accurate you were with your observations.


See also: Alias, Anti-Aliasing Filter, Downsampling, Nyquist Frequency.

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Subjects: Noise & Vibration Physics