Aliasing

Aliasing is a well known phenomenon in sampled data analysis. It occurs when the signal being sampled has components at frequencies higher than the Nyquist frequency, which is equal to half the sampling frequency. Aliasing is a consequence of the fact that after sampling, every periodic signal at a frequency greater than the Nyquist frequency looks exactly like some other periodic signal at a frequency less than the Nyquist frequency. For example, suppose we add a 30 cycle per second periodic component to our sampled data sequence u(t). The power spectrum of the augmented signal appears below.

Because the frequency of the new component is above the Nyquist frequency of 25 cycles per second (25 = 1/(2*delt)), the power spectrum shows the contribution of the new component as an alias at 20 cycles per second. To prevent aliasing, frequency components of a signal above the Nyquist frequency must be removed before sampling.

Example Code
Type @sigprc08 at the IDL prompt to run the batch file that creates this display. The source code is located in sigprc08, in the examples/doc/signal directory. See Running the Example Code if IDL does not find the batch file.