Frequencies above half the sampling rate fold into lower frequencies.
If a signal contains components above f_s/2, they fold into lower frequencies, creating false signals (alias) that cannot be removed after sampling. Examples: wagon wheels appearing to rotate backwards, moiré patterns. Prevent with analog anti-alias filters and adequate f_s.
A 9 kHz tone sampled at 12 kHz appears as 3 kHz.
Aliasing corrupts digital measurements by folding high frequencies down, so it must be prevented by the sampling rate and a filter.
Frequencies above half the sampling rate fold into lower frequencies.
Aliasing corrupts digital measurements by folding high frequencies down, so it must be prevented by the sampling rate and a filter.
A sampled signal cannot tell apart frequencies that differ by multiples of the sampling frequency, so high components appear at lower apparent frequencies.
Sample above twice the highest frequency of interest, and use an analogue anti-alias low-pass filter before the converter.
Typical applications include explaining why a wagon wheel in a film appears to turn backwards, preventing false low-frequency peaks in a vibration spectrum and choosing an anti-alias filter for a data acquisition system.
Study "Sampling" first (topic 23 of 101 in Signals, Control & Comms). It covers the groundwork this topic builds on.
Folding frequency. Alias frequency ‖f-k f_s‖. Stroboscopic effect.
A 9 kHz tone sampled at 12 kHz appears as 3 kHz.
The next topic is "Quantisation", which builds directly on this one.
It is classed as Beginner in Signals, Control & Comms (topic 24 of 101). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.
Want to practise? Open the interactive course for quick checks, quizzes, simulations and progress tracking.