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A signal is an abstract element of information, or more exactly usually a flow of information (in either one or several dimensions). The signal can be either analog or digital. The former is a continuous flow of information in some framework (the signal value does not need to be continuous), while digital signals are constant within some constant temporal (or spatial) intervals.

A typical signal is sound, for example, speech sound whereby the signal carries the information of the spoken words, the identity of the speaker and for example, emotional cues. Another typical signal is a radio transmission which, in turn, can carry the speech sound-signal. Both sound and radio signals are analog signals.

Digital signals are most usually either representations of analog signals within a digital framework (i.e., a computer), or computer generated signals.

The frequency spectrum of a signal can be evaluated with the Fourier transform, which is, in itself a signal.

In Linux, UNIX and other POSIX-compliant operating systems, and in several real-time operating systems as well, a signal is and asynchronous event transmitted between one process and another.

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