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Unlike a continuous-time signal, a discrete-time signal is not a function of a continuous argument; however, it may have been obtained by sampling from a continuous-time signal, and then each value in the sequence is called a sample. When a discrete-time signal obtained by sampling a sequence corresponds to uniformly spaced times, it has an associated sampling rate; the sampling rate is not apparent in the data sequence, and so needs to be associated as a characteristic unit of the system.
Discrete-time signals may have several origins, but can usually be classified into one of two groups:
- By acquiring values of an analog signal at constant or variable rate. This process is called sampling.
- By observing an inherently discrete-time process, such as the weekly peak value of a particular economic indicator.
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