Editing Signal (section)

{{short description|Varying physical quantity that conveys information}}
{{other uses}}
{{distinguish|Cignal|Cygnal}}
{{Use American English|date=June 2019}}

[[File:William Powell Frith The signal 1858.jpg|thumb|right|In ''The Signal'' by [[William Powell Frith]], a woman sends a signal by waving a white handkerchief.]]

A '''signal''' is both the process and the result of [[Signal transmission|transmission]] of [[data]] over some [[transmission media|media]] accomplished by embedding some variation. Signals are important in multiple subject fields including [[signal processing]], [[information theory]] and [[biology]]. 

In signal processing, a signal is a function that conveys [[information]] about a phenomenon.<ref name=Priemer/> Any quantity that can vary over space or time can be used as a signal to share messages between observers.<ref name=PC/> The ''[[IEEE Transactions on Signal Processing]]'' includes [[audio signal|audio]], [[video]], speech, [[image]], [[sonar]], and [[radar]] as examples of signals.<ref name="IEEE" /> A signal may also be defined as {{em|any}} observable change in a quantity over space or time (a [[time series]]), even if it does not carry information.{{efn|Some authors do not emphasize the role of information in the definition of a signal.<ref name=Sinha/>}}

In nature, signals can be actions done by an organism to alert other organisms, ranging from the release of plant chemicals to warn nearby plants of a predator, to sounds or motions made by animals to alert other animals of food. Signaling occurs in all organisms even at cellular levels, with [[cell signaling]]. [[Signaling theory]], in [[evolutionary biology]], proposes that a substantial driver for [[evolution]] is the ability of animals to communicate with each other by developing ways of signaling. In human engineering, signals are typically provided by a [[sensor]], and often the original form of a signal is converted to another form of energy using a [[transducer]]. For example, a [[microphone]] converts an acoustic signal to a voltage waveform, and a [[Loudspeaker|speaker]] does the reverse.<ref name="Priemer" />

Another important property of a signal is its [[Entropy (information theory)|entropy]] or [[information content]]. [[Information theory]] serves as the formal study of signals and their content. The information of a signal is often accompanied by [[Noise (electronics)|noise]], which primarily refers to unwanted modifications of signals, but is often extended to include unwanted signals conflicting with desired signals ([[crosstalk]]). The reduction of noise is covered in part under the heading of [[signal integrity]]. The separation of desired signals from background noise is the field of [[signal recovery]],<ref name="Wilmshurst" /> one branch of which is [[estimation theory]], a probabilistic approach to suppressing random disturbances.

Engineering disciplines such as electrical engineering have advanced the design, study, and implementation of systems involving [[Data transmission|transmission]], [[data storage device|storage]], and manipulation of information. In the latter half of the 20th century, electrical engineering itself separated into several disciplines: [[electronic engineering]] and [[computer engineering]] developed to specialize in the design and analysis of systems that manipulate physical signals, while [[Industrial design|design engineering]] developed to address the functional design of signals in [[User interface|user–machine interfaces]].