From Wikipedia, the free encyclopedia
In
signal processing, the
Nyquist frequency (or
folding frequency), named after
Harry Nyquist, is a characteristic of a
sampler, which converts a continuous function or signal into a discrete sequence. In units of
cycles per second (
Hz), its value is one-half of the
sampling rate (samples per second).
[1][2][A] When the highest frequency (
bandwidth) of a signal is less than the Nyquist frequency of the sampler, the resulting
discrete-time sequence is said to be free of the distortion known as
aliasing, and the corresponding sample rate is said to be above the
Nyquist rate for that particular signal.
[3][4]
In a typical application of sampling, one first chooses the highest frequency to be preserved and recreated, based on the expected content (voice, music, etc.) and desired fidelity. Then one inserts an
anti-aliasing filter ahead of the sampler. Its job is to attenuate the frequencies above that limit. Finally, based on the characteristics of the filter, one chooses a sample rate (and corresponding Nyquist frequency) that will provide an acceptably small amount of
aliasing.
In applications where the sample rate is pre-determined, the filter is chosen based on the Nyquist frequency, rather than vice versa. For example, audio
CDs have a sampling rate of
44100 samples/sec. The Nyquist frequency is therefore 22050 Hz. The anti-aliasing filter must adequately suppress any higher frequencies but negligibly affect the frequencies within the human
hearing range; a filter that preserves 0–20 kHz is more than adequate for this.
