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Signal
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== Examples of signals == Naturally occurring signals can be converted to electronic signals by various [[sensor]]s. Examples include: * ''[[Motion]]''. The motion of an object can be considered to be a signal and can be monitored by various sensors to provide electrical signals.<ref name= Lu/> For example, [[radar]] can provide an electromagnetic signal for following aircraft motion. A motion signal is one-dimensional (time), and the range is generally three-dimensional. Position is thus a 3-vector signal; position and orientation of a rigid body is a 6-vector signal. Orientation signals can be generated using a [[gyroscope]].<ref name= gyro/> * ''[[Sound]]''. Since a sound is a [[oscillation|vibration]] of a medium (such as air), a sound signal associates a [[pressure]] value to every value of time and possibly three space coordinates indicating the direction of travel. A sound signal is converted to an electrical signal by a [[microphone]], generating a [[voltage]] signal as an analog of the sound signal. Sound signals can be [[sampling (signal processing)|sampled]] at a discrete set of time points; for example, [[compact disc]]s (CDs) contain discrete signals representing sound, recorded at [[44,100 Hz]]; since CDs are recorded in [[stereo]], each sample contains data for a left and right channel, which may be considered to be a 2-vector signal. The CD encoding is converted to an electrical signal by reading the information with a [[laser]], converting the sound signal to an optical signal.<ref name=Chambers/> * ''[[Image]]s''. A picture or image consists of a brightness or color signal, a function of a two-dimensional location. The object's appearance is presented as emitted or reflected [[light]], an electromagnetic signal. It can be converted to voltage or current waveforms using devices such as the [[charge-coupled device]]. A 2D image can have a continuous spatial domain, as in a traditional photograph or painting; or the image can be discretized in space, as in a [[digital image]]. Color images are typically represented as a combination of monochrome images in three [[primary colors]]. * ''[[Video]]s''. A video signal is a sequence of images. A point in a video is identified by its two-dimensional position in the image and by the time at which it occurs, so a video signal has a three-dimensional domain. Analog video has one continuous domain dimension (across a [[scan line]]) and two discrete dimensions (frame and line). * Biological ''[[membrane potential]]s''. The value of the signal is an [[electric potential]] (voltage). The domain is more difficult to establish. Some [[cell (biology)|cell]]s or [[organelle]]s have the same membrane potential throughout; [[neuron]]s generally have different potentials at different points. These signals have very low energies, but are enough to make nervous systems work; they can be measured in aggregate by [[electrophysiology]] techniques. *The output of a [[thermocouple]], which conveys temperature information.<ref name=Priemer/> *The output of a [[pH meter]] which conveys acidity information.<ref name=Priemer/>
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