Nyquist frequency signal, whereas "Nyquist frequency" is a property of a discrete-time system. When the function domain is time, sample rates are usually expressed in samples per second, and the unit of Nyquist frequency is cycles per second (hertz). When the function domain is distance, as in an image sampling system, the sample rate might be dots per inch and the corresponding Nyquist frequency would be in cycles/inch. Referring again to Figure 1, undersampling of the sinusoid at 0.6 f is what allows there to be a lower-frequency "alias", which is a different function that produces the same set of samples. That
Unit of length in vacuum during a time interval of seconds." It is approximately equal to . Other units are derived from the metre by adding prefixes from the table below: For example, a kilometre is . In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or of a metre. Other non-SI units are derived from decimal multiples of the metre. The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly by international treaty in 1959. Common Imperial units and U.S. customary units of length include: In addition, the following