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A signal as referred to in communication systems, signal processing, and electrical engineering is a function that “conveys information about the behavior or attributes of some phenomenon”. In nature, signals can take the form of any action by one organism able to be perceived by other organisms, ranging from the release of chemicals by plants to alert nearby plants of the same type of a predator, to sounds or motions made by animals to alert other animals of the presence of danger or of food. Signaling occurs in organisms all the way down to the cellular level, with cell signaling. The formal study of the information content of signals is the field of information theory.
The information in a signal is usually accompanied by noise. Engineering disciplines such as electrical engineering have led the way in the design, study, and implementation of systems involving transmission, storage, and manipulation of information. Definitions specific to sub-fields are common. For example, in information theory, a signal is a codified message, that is, the sequence of states in a communication channel that encodes a message. In the context of signal processing, arbitrary binary data streams are not considered as signals, but only analog and digital signals that are representations of analog physical quantities. In a communication system, a transmitter encodes a message to a signal, which is carried to a receiver by the communications channel. For example, the words “Mary had a little lamb” might be the message spoken into a telephone.
In telephone networks, signaling, for example common-channel signaling, refers to phone number and other digital control information rather than the actual voice signal. Signals can be categorized in various ways. The most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are often referred to as time series in other fields. A second important distinction is between discrete-valued and continuous-valued. Particularly in digital signal processing a digital signal is sometimes defined as a sequence of discrete values, that may or may not be derived from an underlying continuous-valued physical process.
A digital signal has two or more distinguishable waveforms, in this example, high voltage and low voltages, each of which can be mapped onto a digit. Characteristically, noise can be removed from digital signals provided it is not too large. Two main types of signals encountered in practice are analog and digital. The figure shows a digital signal that results from approximating an analog signal by its values at particular time instants. An analog signal uses some property of the medium to convey the signal’s information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. The physical variable is converted to an analog signal by a transducer.
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A digital signal is a signal that is constructed from a discrete set of waveforms of a physical quantity so as to represent a sequence of discrete values. Alternatively, a digital signal may be considered to be the sequence of codes represented by such a physical quantity. With digital signals, system noise, provided it is not too great, will not affect system operation whereas noise always degrades the operation of analog signals to some degree. One of the fundamental distinctions between different types of signals is between continuous and discrete time. If for a signal, the quantities are defined only on a discrete set of times, we call it a discrete-time signal. A simple source for a discrete time signal is the sampling of a continuous signal, approximating the signal by a sequence of its values at particular time instants. If a signal is to be represented as a sequence of numbers, it is impossible to maintain exact precision – each number in the sequence must have a finite number of digits.
Signals in nature can be converted to electronic signals by various sensors. The motion of an object can be considered to be a signal, and can be monitored by various sensors to provide electrical signals. For example, radar can provide an electromagnetic signal for following aircraft motion. 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 an emitted or reflected electromagnetic wave, one form of electronic signal.