Editing Signal (section)

=== Analog and digital signals ===
[[File:Digital-signal-noise.svg|thumb|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 signal|analog]]'' and ''[[digital signal|digital]]''. The figure shows a digital signal that results from approximating an analog signal by its values at particular time instants.  Digital signals are ''[[Quantization (signal processing)|quantized]]'', while analog signals are continuous.

====Analog signal====
{{main|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 (signal processing)|digital signal]], in which the continuous quantity is a representation of a sequence of [[discrete value]]s which can only take on one of a finite number of values.<ref>{{Cite web|url=https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/signals/digital/digital.htm|title=Digital signals|website=www.st-andrews.ac.uk|access-date=2017-12-17|url-status=live|archive-url=https://web.archive.org/web/20170302055200/http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/signals/digital/digital.htm|archive-date=2017-03-02}}</ref><ref>{{Cite web|url=https://learn.sparkfun.com/tutorials/analog-vs-digital/digital-signals|title=Analog vs. Digital - learn.sparkfun.com|website=learn.sparkfun.com|language=en|access-date=2017-12-17|url-status=live|archive-url=https://web.archive.org/web/20170705100418/https://learn.sparkfun.com/tutorials/analog-vs-digital/digital-signals|archive-date=2017-07-05}}</ref>

The term ''analog signal'' usually refers to [[electrical signal]]s; however, analog signals may use other mediums such as [[Classical mechanics|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]], [[Electric current|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====
{{main|Digital signal}}
[[File:Original message.jpg|thumb|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 [[waveform]]s of a physical quantity so as to represent a sequence of [[discrete space|discrete]] values.<ref>{{cite book |url=https://books.google.com/books?id=1eO7kLWUmYIC&q=digital%20signal%20logic&pg=PA4 |title=Digital Design with CPLD Applications and VHDL |author=Robert K. Dueck |archive-url=https://web.archive.org/web/20171217234501/https://books.google.com/books?id=1eO7kLWUmYIC&lpg=PA49&ots=EBhxEwk7bY&dq=digital%20signal%20logic&pg=PA4 |archive-date=2017-12-17 |quote=A digital representation can have only specific discrete values|isbn=1401840302 |year=2005 |publisher=Thomson/Delmar Learning }}</ref><ref>{{Cite book |title = Digital Signal Processing |url=https://books.google.com/books?id=H_5SAAAAMAAJ |publisher=Pearson Prentice Hall |date=2007-01-01 |isbn=9780131873742 |first1=John G. |last1=Proakis |first2=Dimitris G. |last2=Manolakis|author2-link= Dimitris Manolakis |url-status=live |archive-url=https://web.archive.org/web/20160520061336/https://books.google.com/books?id=H_5SAAAAMAAJ |archive-date=2016-05-20 |df=ymd}}</ref><ref name=waveform>{{cite book |url=https://books.google.com/books?id=WJ9qkNpScFAC&q=%22a+digital+signal+is%22+communication&pg=PA3  |title=Analogue and Digital Communication Techniques |archive-url=https://web.archive.org/web/20171217234501/https://books.google.com/books?id=WJ9qkNpScFAC&pg=PA3&dq=%22a+digital+signal+is%22+communication&hl=sv&sa=X&ved=0CFoQ6AEwB2oVChMI-MD09ZjjxwIVBAssCh1wnQu4 |archive-date=2017-12-17 |quote=A digital signal is a complex waveform and can be defined as a discrete waveform having a finite set of levels|isbn=9780080527147 |last1=Smillie |first1=Grahame |date=1999-04-02 |publisher=Elsevier }}</ref> A ''logic signal'' is a digital signal with only two possible values,<ref>{{cite web |url=http://www.chegg.com/homework-help/definitions/digital-signal-4 |title=Digital Signal |access-date=2016-08-13 |archive-date=2019-04-02 |archive-url=https://web.archive.org/web/20190402155546/https://www.chegg.com/homework-help/definitions/digital-signal-4 |url-status=live }}</ref><ref>{{cite book |title=The Art of Electronics |author1=Paul Horowitz |author2=Winfield Hill |isbn=9780521809269 |publisher=Cambridge University Press |date=2015}}</ref> 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.<ref name=Khanna>{{cite book |author=Vinod Kumar Khanna |url=https://books.google.com/books?id=Vf2qXAbn58oC |title=Digital Signal Processing |date=2009 |isbn=9788121930956 |page=3 | publisher=S. Chand |quote=A digital signal is a special form of discrete-time signal which is discrete in both time and amplitude, obtained by permitting each value (sample) of a discrete-time signal to acquire a finite set of values (quantization), assigning it a numerical symbol according to a code ... A digital signal is a sequence or list of numbers drawn from a finite set.}}</ref> The physical quantity may be a variable electric current or voltage, the intensity, phase or [[polarization (waves)|polarization]] of an [[optical]] or other [[electromagnetism|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 (signal processing)|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&nbsp;[[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.  [[Computer]]s and other [[Digital data|digital]] devices are restricted to discrete time.