In "The Signal" painting by William Powell Frith, a woman waves a handkerchief as a signal to a person able to see this
action, in order to convey a message to this person.
In communication systems, signal processing, and electrical engineering, a signal is a function that
"conveys information about the behavior or attributes of some phenomenon".[1] A signal may also be
defined as an "observable change in a quantifiable entity".[2] In the physical world, any quantity exhibiting
variation in time or variation in space (such as an image) is potentially a signal that might provide
information on the status of a physical system, or convey a message between observers, among other
possibilities.[3] The IEEE Transactions on Signal Processing states that the term "signal"
includes audio, video, speech, image, communication, geophysical, sonar, radar, medical and musical
signals.[4] In a later development, a signal is redefined as an "observable change in a quantifiable
entity";[5] here, anything which is only a function of space, such as an image, is excluded from the category
of signals. Also, it is stated that a signal may or may not contain any information.
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. Signaling theory, in
evolutionary biology, proposes that a substantial driver for evolution is the ability for 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 speakerdoes the reverse.[1]
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. The term noise usually means an undesirable random disturbance,
but is often extended to include unwanted signals conflicting with the desired signal (such as crosstalk).
The prevention of noise is covered in part under the heading of signal integrity. The separation of desired
signals from a background is the field of signal recovery,[6] one branch of which is estimation theory, a
probabilistic approach to suppressing random disturbances.
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. In the latter
half of the 20th century, electrical engineering itself separated into several disciplines, specialising in the
design and analysis of systems that manipulate physical signals; electronic engineering and computer
engineering as examples; while design engineering developed to deal with functional design of user–
machine interfaces.
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, signals are analog and digital representations of analog physical quantities.
In terms of their spatial distributions, signals may be categorized as point source signals (PSSs) and
distributed source signals (DSSs).[5]
In a communication system, a transmitter encodes a message to create 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. The telephone transmitter converts the sounds into an electrical signal.
The signal is transmitted to the receiving telephone by wires; at the receiver it is reconverted into sounds.
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. Continuous-time
signals are often referred to as continuous signals.
A second important distinction is between discrete-valued and continuous-valued. Particularly in digital
signal processing, a digital signal may be defined as a sequence of discrete values, typically associated
with an underlying continuous-valued physical process. In digital electronics, digital signals are the
continuous-time waveform signals in a digital system, representing a bit-stream.
Another important property of a signal is its entropy or information content.
Analog and digital signals
Analog signal
Main article: Analog signal
An analog signal is any continuous signal for which the time varying feature (variable) of the signal is a
representation of some other time varying quantity, i.e., analogous to another time varying signal. For
example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with
the pressure of the sound waves. It differs from a digital signal, in which the continuous quantity is a
representation of a sequence of discrete values which can only take on one of a finite number of
values.[7][8] The term analog signal usually refers to electrical signals;
however, mechanical, pneumatic, hydraulic, human speech, and other systems may also convey or be
considered analog signals.
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. In an electrical
signal, the voltage, current, or frequency of the signal may be varied to represent the information.
Any information may be conveyed by an analog signal; often such a signal is a measured response to
changes in physical phenomena, such as sound, light, temperature, position, or pressure. The physical
variable is converted to an analog signal by a transducer. For example, in sound recording, fluctuations in
air pressure (that is to say, sound) strike the diaphragm of a microphone which induces corresponding
fluctuations in the current produced by a coil in an electromagnetic microphone, or the voltage produced by
a condenser microphone. The voltage or the current is said to be an "analog" of the sound.
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