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log in contents move to sidebar hide top 1 definitions 2 classification toggle classification subsection 2 1 analog and digital signals 2 1 1 analog signal 2 1 2 digital signal 2 2 energy and power 2 3 deterministic and random 2 4 even and odd 2 5 periodic 2 5 1 time discretization 2 6 amplitude quantization 3 examples 4 signal processing 5 signals and systems 6 gallery 7 see also 8 notes 9 references 10 further reading toggle the table of contents signal 58 languages afrikaans العربية aymar aru azərbaycanca български भोजपुरी বাংলা bosanski català کوردی čeština чӑвашла dansk deutsch ελληνικά esperanto español eesti euskara فارسی français galego עברית हिन्दी hrvatski bahasa indonesia italiano 日本語 қазақша 한국어 кыргызча lietuvių bahasa melayu nederlands norsk bokmål polski português runa simi română русский scots srpskohrvatski српскохрватски සිංහල simple english slovenčina slovenščina soomaaliga shqip српски srpski svenska தமிழ் ไทย türkçe українська tiếng việt 吴语 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia varying physical quantity that conveys information for other uses see signal disambiguation not to be confused with cignal or cygnal 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 transmission of data over some 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 1 any quantity that can vary over space or time can be used as a signal to share messages between observers 2 the ieee transactions on signal processing includes audio video speech image sonar and radar as examples of signals 3 a signal may also be defined as any observable change in a quantity over space or time a time series even if it does not carry information a 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 speaker does the reverse 1 another important property of a signal is its 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 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 5 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 transmission 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 design engineering developed to address the functional design of signals in user machine interfaces definitions edit definitions specific to sub fields are common in electronics and telecommunications signal refers to any time varying voltage current or electromagnetic wave that carries information in signal processing signals are analog and digital representations of analog physical quantities in information theory a signal is a codified message that is the sequence of states in a communication channel that encodes a message in a communication system a transmitter encodes a message to create a signal which is carried to a receiver by the communication 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 classification edit signals can be categorized in various ways the most common verification needed 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 signals may also be categorized by their spatial distributions as either point source signals psss or distributed source signals dsss 2 in signals and systems signals can be classified according to many criteria mainly according to the different feature of values classified into analog signals and digital signals according to the determinacy of signals classified into deterministic signals and random signals according to the strength of signals classified into energy signals and power signals analog and digital signals edit 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 extreme 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 digital signals are quantized while analog signals are continuous analog signal edit main article analog signal an analog signal is any continuous signal for which the time varying feature 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 sound pressure 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 6 7 the term analog signal usually refers to electrical signals however analog signals may use other mediums such as mechanical pneumatic or hydraulic 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 electrical fluctuations the voltage or the current is said to be an analog of the sound digital signal edit main article digital signal a binary signal also known as a logic signal is a digital signal with two distinguishable levels 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 8 9 10 a logic signal is a digital signal with only two possible values 11 12 and describes an arbitrary bit stream other types of digital signals can represent three valued logic or higher valued logics alternatively a digital signal may be considered to be the sequence of codes represented by such a physical quantity 13 the physical quantity may be a variable electric current or voltage the intensity phase or polarization of an optical or other electromagnetic field acoustic pressure the magnetization of a magnetic storage media etc digital signals are present in all digital electronics notably computing equipment and data transmission 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 digital signals often arise via sampling of analog signals for example a continually fluctuating voltage on a line that can be digitized by an analog to digital converter circuit wherein the circuit will read the voltage level on the line say every 50 microseconds and represent each reading with a fixed number of bits the resulting stream of numbers is stored as digital data on a discrete time and quantized amplitude signal computers and other digital devices are restricted to discrete time energy and power edit according to the strengths of signals practical signals can be classified into two categories energy signals and power signals 14 energy signals those signals energy are equal to a finite positive value but their average powers are 0 0 e s 2 t d t displaystyle 0 e int _ infty infty s 2 t dt infty power signals those signals average power are equal to a finite positive value but their energy are infinite p lim t 1 t t 2 t 2 s 2 t d t displaystyle p lim _ t rightarrow infty frac 1 t int _ t 2 t 2 s 2 t dt deterministic and random edit deterministic signals are those whose values at any time are predictable and can be calculated by a mathematical equation random signals are signals that take on random values at any given time instant and must be modeled stochastically 15 even and odd edit even and odd signals f x x 2 displaystyle f x x 2 is an example of an even signal f x x 3 displaystyle f x x 3 is an example of an odd signal an even signal satisfies the condition x t x t displaystyle x t x t or equivalently if the following equation holds for all t displaystyle t and t displaystyle t in the domain of x displaystyle x x t x t 0 displaystyle x t x t 0 an odd signal satisfies the condition x t x t displaystyle x t x t or equivalently if the following equation holds for all t displaystyle t and t displaystyle t in the domain of x displaystyle x x t x t 0 displaystyle x t x t 0 periodic edit a signal is said to be periodic if it satisfies the condition x t x t t t t 0 t m a x displaystyle x t x t t quad forall t in t_ 0 t_ max or x n x n n n n 0 n m a x displaystyle x n x n n quad forall n in n_ 0 n_ max where t displaystyle t fundamental time period 1 t f displaystyle 1 t f fundamental frequency the same can be applied to n displaystyle n a periodic signal will repeat for every period time discretization edit discrete time signal created from a continuous signal by sampling signals can be classified as continuous or discrete time in the mathematical abstraction the domain of a continuous time signal is the set of real numbers or some interval thereof whereas the domain of a discrete time dt signal is the set of integers or other subsets of real numbers what these integers represent depends on the nature of the signal most often it is time a continuous time signal is any function which is defined at every time t in an interval most commonly an infinite interval 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 amplitude quantization edit if a signal is to be represented as a sequence of digital data it is impossible to maintain exact precision each number in the sequence must have a finite number of digits as a result the values of such a signal must be quantized into a finite set for practical representation quantization is the process of converting a continuous analog audio signal to a digital signal with discrete numerical values of integers examples edit naturally occurring signals can be converted to electronic signals by various sensors 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 16 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 the position and orientation of a rigid body is a 6 vector signal orientation signals can be generated using a gyroscope 17 sound since a sound is a 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 sampled at a discrete set of time points for example compact discs 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 18 images 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 represent...
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