Nyquist frequency

Not to be confused with Nyquist rate.

In signal processing, the Nyquist frequency (or folding frequency), named after Harry Nyquist, is a trait of a sampler, which converts a continuous signal into a sequence of numbers. The Nyquist frequency is generally double of the sampling rate. When this is done, the discrete-time sequence is said to be free from distortion (usually called aliasing). For example, audio CD's contain a sampling rate of 44100 samples for every second of audio. When the Nyquist frequency characteristic is applied, the equation being 44100 divide 2, it produces a result of 22050. This number is generally considered to be the amount of frequencies a signal can hold (e.g. a Nyquist frequency of 20000 Hz can contain any frequency from 1 to 20 thousand Hz).

Typical example of Nyquist frequency and rate. To avoid aliasing, the sampling rate must be under double the Nyquist frequency.

The Nyquist frequency is not used for just audio applications and can usually be applied to any signal that has a sample rate.