Editing Feedback (section)

==Applications==
===Mathematics and dynamical systems===
[[File:Mandel zoom 00 mandelbrot set.jpg|322px|thumb|Feedback can give rise to incredibly complex behaviors. The [[Mandelbrot set]] (black) within a continuously colored environment is plotted by repeatedly feeding back values through a simple equation and recording the points on the imaginary plane that fail to diverge.|alt=]]
{{Main|Dynamical system|Chaos theory|Edge of chaos|Control theory}}
By using feedback properties, the behavior of a system can be altered to meet the needs of an application; systems can be made stable, responsive or held constant. It is shown that dynamical systems with a feedback experience an adaptation to the [[edge of chaos]].<ref>{{cite journal|last1=Wotherspoon|first1=T.|last2=Hubler|first2=A.|title=Adaptation to the edge of chaos with random-wavelet feedback|journal=J. Phys. Chem. A|date=2009|doi=10.1021/jp804420g|pmid=19072712|volume=113|issue=1|pages=19–22|bibcode=2009JPCA..113...19W}}</ref>

=== Physics ===
Physical systems present feedback through the mutual interactions of its parts. Feedback is also relevant for the regulation of experimental conditions, noise reduction, and signal control.<ref>{{Cite journal |last=Bechhoefer |first=John |date=2005-08-31 |title=Feedback for physicists: A tutorial essay on control |url=https://link.aps.org/doi/10.1103/RevModPhys.77.783 |journal=Reviews of Modern Physics |volume=77 |issue=3 |pages=783–836 |doi=10.1103/RevModPhys.77.783|bibcode=2005RvMP...77..783B }}</ref> The thermodynamics of feedback-controlled systems has intrigued physicist since the [[Maxwell's demon]], with recent advances on the consequences for entropy reduction and performance increase.<ref>{{Cite journal |last1=Sagawa |first1=Takahiro |last2=Ueda |first2=Masahito |date=2008-02-26 |title=Second Law of Thermodynamics with Discrete Quantum Feedback Control |url=https://link.aps.org/doi/10.1103/PhysRevLett.100.080403 |journal=Physical Review Letters |language=en |volume=100 |issue=8 |page=080403 |doi=10.1103/PhysRevLett.100.080403 |pmid=18352605 |issn=0031-9007|arxiv=0710.0956 |bibcode=2008PhRvL.100h0403S }}</ref><ref>{{Cite journal |last1=Cao |first1=F. J. |last2=Feito |first2=M. |date=2009-04-10 |title=Thermodynamics of feedback controlled systems |url=https://link.aps.org/doi/10.1103/PhysRevE.79.041118 |journal=Physical Review E |language=en |volume=79 |issue=4 |page=041118 |doi=10.1103/PhysRevE.79.041118 |pmid=19518184 |issn=1539-3755|arxiv=0805.4824 |bibcode=2009PhRvE..79d1118C }}</ref>

===Biology===
{{Main|Biological interaction}}{{See also|Homeostasis|Allostasis}}
In [[biology|biological]] systems such as [[organism]]s, [[ecosystem]]s, or the [[biosphere]], most parameters must stay under control within a narrow range around a certain optimal level under certain environmental conditions. The deviation of the optimal value of the controlled parameter can result from the changes in internal and external environments. A change of some of the environmental conditions may also require change of that range to change for the system to function. The value of the parameter to maintain is recorded by a reception system and conveyed to a regulation module via an information channel. An example of this is [[insulin oscillation]]s.

Biological systems contain many types of regulatory circuits, both positive and negative. As in other contexts, ''positive'' and ''negative'' do not imply that the feedback causes ''good'' or ''bad'' effects. A negative feedback loop is one that tends to slow down a process, whereas the positive feedback loop tends to accelerate it. The [[mirror neuron]]s are part of a social feedback system, when an observed action is "mirrored" by the brain—like a self-performed action.

Normal tissue integrity is preserved by feedback interactions between diverse cell types mediated by adhesion molecules and secreted molecules that act as mediators; failure of key feedback mechanisms in cancer disrupts tissue function.<ref>{{cite journal|last1=Vlahopoulos|first1=SA|last2=Cen|first2=O|last3=Hengen|first3=N|last4=Agan|first4=J|last5=Moschovi|first5=M|last6=Critselis|first6=E|last7=Adamaki|first7=M|last8=Bacopoulou|first8=F|last9=Copland|first9=JA|last10=Boldogh|first10=I|last11=Karin|first11=M|last12=Chrousos|first12=GP|title=Dynamic aberrant NF-κB spurs tumorigenesis: A new model encompassing the microenvironment.|journal=Cytokine & Growth Factor Reviews|date=20 June 2015|pmid=26119834|doi=10.1016/j.cytogfr.2015.06.001|volume=26|issue=4|pages=389–403|pmc=4526340}}</ref>
In an injured or infected tissue, inflammatory mediators elicit feedback responses in cells, which alter gene expression, and change the groups of molecules expressed and secreted, including molecules that induce diverse cells to cooperate and restore tissue structure and function. This type of feedback is important because it enables coordination of immune responses and recovery from infections and injuries. During cancer, key elements of this feedback fail. This disrupts tissue function and immunity.<ref>{{cite journal | last1 = Vlahopoulos | first1 = SA | title = Aberrant control of NF-κB in cancer permits transcriptional and phenotypic plasticity, to curtail dependence on host tissue: molecular mode. | journal = Cancer Biology & Medicine | date = August 2017 | pmid = 28884042 | doi = 10.20892/j.issn.2095-3941.2017.0029 | volume = 14 | issue = 3 | pages = 254–270 | pmc = 5570602}}</ref><ref>{{cite journal|last1=Korneev|first1=KV|last2=Atretkhany|first2=KN|last3=Drutskaya|first3=MS|last4=Grivennikov|first4=SI|last5=Kuprash|first5=DV|last6=Nedospasov|first6=SA|title=TLR-signaling and proinflammatory cytokines as drivers of tumorigenesis.|journal=Cytokine|date=January 2017|volume=89|pages=127–135|doi=10.1016/j.cyto.2016.01.021|pmid=26854213}}</ref>

Mechanisms of feedback were first elucidated in bacteria, where a nutrient elicits changes in some of their metabolic functions.<ref>{{cite journal|last1= Sanwal|first1=BD| title= Allosteric controls of amphilbolic pathways in bacteria.|journal= Bacteriol. Rev.|date=March 1970|volume=34|issue=1|pages=20–39 |pmid=4315011 |pmc=378347|doi=10.1128/MMBR.34.1.20-39.1970}}</ref>
Feedback is also central to the operations of [[gene]]s and [[gene regulatory network]]s. [[Repressor protein|Repressor]] (see [[Lac repressor]]) and [[activator protein|activator]] [[protein]]s are used to create genetic [[operon]]s, which were identified by [[François Jacob]] and [[Jacques Monod]] in 1961 as ''feedback loops''.<ref>{{cite journal|last1= Jacob|first1=F|last2=Monod|first2=J|title= Genetic regulatory mechanisms in the synthesis of proteins.|journal= J Mol Biol|date=June 1961|volume=3|issue=3|pages=318–356 |pmid=13718526|doi=10.1016/S0022-2836(61)80072-7|s2cid=19804795}}</ref> These feedback loops may be positive (as in the case of the coupling between a sugar molecule and the proteins that import sugar into a bacterial cell), or negative (as is often the case in [[metabolic]] consumption).

On a larger scale, feedback can have a stabilizing effect on animal populations even when profoundly affected by external changes, although time lags in feedback response can give rise to [[Lotka–Volterra equation|predator-prey cycles]].<ref>
CS Holling. "Resilience and stability of ecological systems". Annual Review of Ecology and Systematics 4:1-23. 1973</ref>

In [[zymology]], feedback serves as regulation of activity of an enzyme by its direct {{Not a typo|product(s)}} or downstream {{Not a typo|metabolite(s)}} in the metabolic pathway (see [[Allosteric regulation]]).

The [[hypothalamic–pituitary–adrenal axis]] is largely controlled by positive and negative feedback, much of which is still unknown.

In [[psychology]], the body receives a stimulus from the environment or internally that causes the release of [[hormone]]s. Release of hormones then may cause more of those hormones to be released, causing a positive feedback loop. This cycle is also found in certain behaviour. For example, "shame loops" occur in people who blush easily. When they realize that they are blushing, they become even more embarrassed, which leads to further blushing, and so on.<ref>{{cite magazine|last=Scheff |first=Thomas |url=http://www.psychologytoday.com/blog/lets-connect/200909/the-emotionalrelational-world |title=The Emotional/Relational World |magazine=Psychology Today |date=2009-09-02 |access-date=2013-07-10}}</ref>

===Climate science===
{{Main|Climate change feedback}}
[[File:20220726 Feedbacks affecting global warming and climate change - block diagram.svg|thumb|right|upright=1.5| Some effects of global warming can either enhance ([[positive feedback]]s) or inhibit ([[negative feedback]]s) warming.<ref name=NASA_IntegratedSystem>{{cite web |title=The Study of Earth as an Integrated System |url=https://climate.nasa.gov/nasa_science/science/ |website=nasa.gov |publisher=NASA |date=2016 |archive-url=https://web.archive.org/web/20161102022200/https://climate.nasa.gov/nasa_science/science/ |archive-date=2 November 2016 |url-status=live }}</ref><ref name=IPCC_AR6_SGI_FigTS.17>Fig. TS.17, ''[https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary],'' Sixth Assessment Report (AR6), Working Group I, IPCC, 2021, p. 96. [https://web.archive.org/web/20220721021347/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Archived] from the original on 21 July 2022.</ref>]]
The climate system is characterized by strong positive and negative feedback loops between processes that affect the state of the atmosphere, ocean, and land. A simple example is the [[Ice–albedo feedback|ice–albedo positive feedback]] loop whereby melting snow exposes more dark ground (of lower [[albedo]]), which in turn absorbs heat and causes more snow to melt.

===Control theory===
{{Main|Control theory}}
Feedback is extensively used in control theory, using a variety of methods including [[state space (controls)]], [[full state feedback]], and so forth. In the context of control theory, "feedback" is traditionally assumed to specify "negative feedback".<ref name=mees>A. I. Mees ({{circa|1981}}) ''Dynamics of Feedback Systems'', New York: J. Wiley. {{ISBN|0-471-27822-X}}. p. 69: "There is a tradition in control theory that one deals with a ''negative feedback loop'' in which a negative sign is included in the feedback loop{{nbsp}}..."</ref>
{{Further|PID controller}}
The most common general-purpose [[controller (control theory)|controller]] using a control-loop feedback mechanism is a [[PID controller|proportional-integral-derivative]] (PID) controller. Heuristically, the terms of a PID controller can be interpreted as corresponding to time: the proportional term depends on the ''present'' error, the integral term on the accumulation of ''past'' errors, and the derivative term is a prediction of ''future'' error, based on current rate of change.<ref>{{Citation | url = http://www.eolss.net/ebooks/Sample%20Chapters/C18/E6-43-03-03.pdf | title = PID Control | last = Araki | first = M. }}</ref>

===Education===
For feedback in the educational context, see [[corrective feedback]].

===Mechanical engineering===
In ancient times, the [[float valve]] was used to regulate the flow of water in Greek and Roman [[water clock]]s; similar float valves are used to regulate fuel in a [[carburettor]] and also used to regulate tank water level in the [[flush toilet]].

The Dutch inventor [[Cornelius Drebbel]] (1572–1633) built thermostats (c1620) to control the temperature of chicken incubators and chemical furnaces. In 1745, the windmill was improved by blacksmith Edmund Lee, who added a [[windmill fantail|fantail]] to keep the face of the windmill pointing into the wind. In 1787, [[Tom Mead]] regulated the rotation speed of a windmill by using a [[conical pendulum|centrifugal pendulum]] to adjust the distance between the bedstone and the runner stone (i.e., to adjust the load).

The use of the [[centrifugal governor]] by [[James Watt]] in 1788 to regulate the speed of his [[steam engine]] was one factor leading to the [[Industrial Revolution]]. Steam engines also use float valves and [[relief valve|pressure release valves]] as mechanical regulation devices. A [[mathematical analysis]] of Watt's governor was done by [[James Clerk Maxwell]] in 1868.<ref name=maxwell/>

The ''[[SS Great Eastern|Great Eastern]]'' was one of the largest steamships of its time and employed a steam powered rudder with feedback mechanism designed in 1866 by [[John McFarlane Gray]]. [[Joseph Farcot]] coined the word ''[[Servomechanism|servo]]'' in 1873 to describe steam-powered steering systems. Hydraulic servos were later used to position guns. [[Elmer Ambrose Sperry]] of the [[Sperry Corporation]] designed the first [[autopilot]] in 1912. [[Nicolas Minorsky]] published a theoretical analysis of automatic ship steering in 1922 and described the [[PID controller]].<ref name="Minorsky">{{cite journal |author=Minorsky, Nicolas |year=1922 |title=Directional stability of automatically steered bodies |journal=Journal of the American Society for Naval Engineers |volume=34 |issue= 2|pages=280–309 |doi= 10.1111/j.1559-3584.1922.tb04958.x}}</ref>

Internal combustion engines of the late 20th century employed mechanical feedback mechanisms such as the [[Ignition timing#Vacuum timing advance|vacuum timing advance]] but mechanical feedback was replaced by electronic [[engine control unit|engine management systems]] once small, robust and powerful single-chip [[microcontroller]]s became affordable.

===Electronic engineering===
[[File:Ideal feedback model.svg|thumb|The simplest form of a feedback amplifier can be represented by the ''ideal block diagram'' made up of [https://www.google.com/search?tbo=p&tbm=bks&q=%22A+unilateral+block+or+network+is+one+in+which+power+may+be+transmitted+in+one+direction+only.%22&num=10&gws_rd=ssl unilateral elements].<ref name="Chen">
{{cite book|title=Circuit Analysis and Feedback Amplifier Theory|author=Wai-Kai Chen|publisher=CRC Press|date=2005|isbn=9781420037272|location=Boca Raton, FL, USA |id=423825181|pages=13.1–13.14|chapter=Chapter 13: General feedback theory|quote=[In a practical amplifier] the forward path may not be strictly unilateral, the feedback path is usually bilateral, and the input and output coupling networks are often complicated.|chapter-url=https://books.google.com/books?id=ZlJM1OLDQx0C&pg=SA13-PA1}}
</ref>|280px|right]]

The use of feedback is widespread in the design of [[electronics|electronic]] components such as [[amplifier]]s, [[oscillator]]s, and stateful [[logic circuit]] elements such as [[flip-flop (electronics)|flip-flop]]s and [[counter (digital)|counter]]s. Electronic feedback systems are also very commonly used to control mechanical, thermal and other physical processes.

If the signal is inverted on its way round the control loop, the system is said to have ''[[negative feedback amplifier|negative feedback]]'';<ref name=KalS>
{{cite book |title=Basic Electronics: Devices, Circuits and IT Fundamentals |author=Santiram Kal |url=https://books.google.com/books?id=_Bw_-ZyGL6YC&q=%22it+is+called+negative+feedback%22+%22if+the+feedback+signal+reduces+the+input+signal%22&pg=PA191 |quote=If the feedback signal reduces the input signal, ''i.e.'' it is out of phase with the input [signal], it is called negative feedback. |isbn=9788120319523 |year=2009 |publisher=PHI Learning Pvt. Ltd |page=191}}
</ref> otherwise, the feedback is said to be ''positive''. Negative feedback is often deliberately introduced to increase the [[BIBO stability|stability]] and accuracy of a system by correcting or reducing the influence of unwanted changes.  This scheme can fail if the input changes faster than the system can respond to it.  When this happens, the lag in arrival of the correcting signal can result in over-correction, causing the output to [[oscillation|oscillate]] or "hunt".<ref>With mechanical devices, hunting can be severe enough to destroy the device.</ref> While often an unwanted consequence of system behaviour, this effect is used deliberately in electronic oscillators.

[[Harry Nyquist]] at [[Bell Labs]] derived the [[Nyquist stability criterion]] for determining the stability of feedback systems. An easier method, but less general, is to use [[Bode plot]]s developed by [[Hendrik Wade Bode|Hendrik Bode]] to determine the [[Gain margin|gain margin and phase margin]]. Design to ensure stability often involves [[frequency compensation]] to control the location of the [[pole (complex analysis)|pole]]s of the amplifier.

Electronic feedback loops are used to control the output of [[electronics|electronic]] devices, such as [[amplifiers]]. A feedback loop is created when all or some portion of the output is fed back to the input. A device is said to be operating ''open loop'' if no output feedback is being employed and ''closed loop'' if feedback is being used.<ref>P. Horowitz & W. Hill, ''The Art of Electronics'', Cambridge University Press (1980), Chapter 3, relating to operational amplifiers.</ref>

When two or more amplifiers are cross-coupled using positive feedback, complex behaviors can be created. These ''[[multivibrator]]s'' are widely used and include:
* astable circuits, which act as oscillators
* monostable circuits, which can be pushed into a state, and will return to the stable state after some time
* bistable circuits, which have two stable states that the circuit can be switched between

====Negative feedback====
Negative feedback occurs when the fed-back output signal has a relative phase of 180° with respect to the input signal (upside down). This situation is sometimes referred to as being ''out of phase'', but that term also is used to indicate other phase separations, as in "90° out of phase". Negative feedback can be used to correct output errors or to desensitize a system to unwanted fluctuations.<ref name=Bhattacharya>
For an analysis of desensitization in the system pictured, see {{cite book |author=S.K Bhattacharya |title=Linear Control Systems |pages=134–135 |quote=The parameters of a system ... may vary... The primary advantage of using feedback in control systems is to reduce the system's sensitivity to parameter variations. |chapter=§5.3.1 Effect of feedback on parameter variations |isbn=9788131759523 |publisher=Pearson Education India |year=2011 |chapter-url=https://books.google.com/books?id=e5Z1A_6jxAUC&q=%22primary+advantage+of+using+feedback+in+control+system+is+to+reduce+the+system%27s+sensitivity+to+parameter+variations%22&pg=PA135}}
</ref> In feedback amplifiers, this correction is generally for waveform [[distortion]] reduction<ref>{{Cite web |title=Negative Feedback & Distortion |url=https://learnabout-electronics.org/Amplifiers/amplifiers34.php#:~:text=Using%20negative%20feedback%20to%20control,and/or%20cut%20off%20regions. |access-date=2024-06-07 |website=learnabout-electronics.org}}</ref> or to establish a specified [[Gain (electronics)|gain]] level. A general expression for the gain of a negative feedback amplifier is the [[asymptotic gain model]].

====Positive feedback====
Positive feedback occurs when the fed-back signal is in phase with the input signal. Under certain gain conditions, positive feedback reinforces the input signal to the point where the output of the device [[oscillates]] between its maximum and minimum possible states. Positive feedback may also introduce [[hysteresis]] into a circuit. This can cause the circuit to ignore small signals and respond only to large ones. It is sometimes used to eliminate noise from a digital signal. Under some circumstances, positive feedback may cause a device to latch, i.e., to reach a condition in which the output is locked to its maximum or minimum state. This fact is very widely used in digital electronics to make [[Flip-flop (electronics)|bistable]] circuits for volatile storage of information.

The loud squeals that sometimes occurs in [[audio system]]s, [[public address system|PA systems]], and [[rock music]] are known as [[audio feedback]]. If a microphone is in front of a loudspeaker that it is connected to, sound that the microphone picks up comes out of the speaker, and is picked up by the microphone and re-amplified.  If the [[loop gain]] is sufficient, howling or squealing at the maximum power of the amplifier is possible.

====Oscillator====
[[File:OpAmpHystereticOscillator.svg|thumb|A popular [[Relaxation oscillator#Comparator–based electronic relaxation oscillator|op-amp relaxation oscillator]]]]
An [[electronic oscillator]] is an [[electronic circuit]] that produces a periodic, [[oscillation|oscillating]] electronic signal, often a [[sine wave]] or a [[Square wave (waveform)|square wave]].<ref name="Snelgrove">{{cite encyclopedia
  | last = Snelgrove
  | first = Martin
  | title = Oscillator
  | encyclopedia = McGraw-Hill Encyclopedia of Science and Technology, 10th Ed., Science Access online service
  | publisher = McGraw-Hill
  | year = 2011
  | url = http://accessscience.com/abstract.aspx?id=477900&referURL=http%3a%2f%2faccessscience.com%2fcontent.aspx%3fid%3d477900
  | access-date = 1 March 2012
  | archive-url = https://web.archive.org/web/20130719125711/http://accessscience.com/abstract.aspx?id=477900&referURL=http%3A%2F%2Faccessscience.com%2Fcontent.aspx%3Fid%3D477900
  | archive-date = 19 July 2013
  | url-status = dead
  }}</ref><ref name="Chattopadhyay">{{cite book   
  | last = Chattopadhyay
  | first = D. 
  | title = Electronics (fundamentals And Applications)
  | publisher = New Age International
  | year = 2006
  | pages = 224–225
  | url = https://books.google.com/books?id=n0rf9_2ckeYC&q=%22negative+resistance%22&pg=PA224
  | isbn = 978-81-224-1780-7}}</ref> Oscillators convert [[direct current]] (DC) from a power supply to an [[alternating current]] signal.  They are widely used in many electronic devices.  Common examples of signals generated by oscillators include signals broadcast by [[Radio transmitter|radio]] and [[television transmitter]]s, clock signals that regulate computers and [[quartz clock]]s, and the sounds produced by electronic beepers and [[video game]]s.<ref name="Snelgrove" />

Oscillators are often characterized by the [[frequency]] of their output signal:
* A [[low frequency oscillation|low-frequency oscillator]] (LFO) is an electronic oscillator that generates a frequency below ≈20&nbsp;Hz.  This term is typically used in the field of audio [[synthesizers]], to distinguish it from an audio frequency oscillator.
* An audio oscillator<!--Please don't link – circular reference--> produces frequencies in the [[audio frequency|audio]] range, about 16&nbsp;Hz to 20&nbsp;kHz.<ref name="Chattopadhyay" />
* An RF oscillator produces signals in the [[radio frequency]] (RF) range of about 100&nbsp;kHz to 100&nbsp;GHz.<ref name="Chattopadhyay" />

Oscillators designed to produce a high-power AC output from a DC supply are usually called [[Inverter (electrical)|inverters]].

There are two main types of electronic oscillator: the linear or harmonic oscillator and the nonlinear or [[relaxation oscillator]].<ref name="Chattopadhyay" /><ref name="Garg">{{cite book   
  | last = Garg
  | first = Rakesh Kumar  
  | author2=Ashish Dixit |author3=Pavan Yadav
  | title = Basic Electronics
  | publisher = Firewall Media
  | year = 2008
  | pages = 280
  | url = https://books.google.com/books?id=9SOdnsHA2IYC&pg=PA280
  | isbn = 978-8131803028}}</ref>

====Latches and flip-flops====
[[File:JohnsonCounter2.png|thumb|A 4-bit [[ring counter]] using [[Flip-flop (electronics)#D flip-flop|D-type flip flops]]]]

A latch or a [[Flip-flop (electronics)|flip-flop]] is a [[electronic circuit|circuit]] that has two stable states and can be used to store state information. They typically constructed using feedback that crosses over between two arms of the circuit, to provide the circuit with a state. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in [[sequential logic]]. Latches and flip-flops are fundamental building blocks of [[digital electronics]] systems used in computers, communications, and many other types of systems.

Latches and flip-flops are used as data storage elements. Such data storage can be used for storage of ''[[state (computer science)|state]]'', and such a circuit is described as [[sequential logic]]. When used in a [[finite-state machine]], the output and next state depend not only on its current input, but also on its current state (and hence, previous inputs). It can also be used for counting of pulses, and for synchronizing variably-timed input signals to some reference timing signal.

Flip-flops can be either simple (transparent or opaque) or [[clock signal|clock]]ed (synchronous or edge-triggered).  Although the term flip-flop has historically referred generically to both simple and clocked circuits, in modern usage it is common to reserve the term ''flip-flop'' exclusively for discussing clocked circuits; the simple ones are commonly called ''latches''.<ref name="pedroni">
{{cite book| author = Volnei A. Pedroni| title = Digital electronics and design with VHDL| url = https://books.google.com/books?id=-ZAccwyQeXMC| year = 2008| publisher = Morgan Kaufmann| isbn = 978-0-12-374270-4| page = 329 }}</ref><ref name="ee42">[http://rfic.eecs.berkeley.edu/ee100/pdf/lect24.pdf Latches and Flip Flops] {{Webarchive|url=https://web.archive.org/web/20161005192018/http://rfic.eecs.berkeley.edu/ee100/pdf/lect24.pdf |date=5 October 2016 }} (EE 42/100 Lecture 24 from Berkeley)  ''"...Sometimes the terms flip-flop and latch are used interchangeably..."''</ref>

Using this terminology, a latch is level-sensitive, whereas a flip-flop is edge-sensitive. That is, when a latch is enabled it becomes transparent, while a flip flop's output only changes on a single type (positive going or negative going) of clock edge.

===Software===
Feedback loops provide generic mechanisms for controlling the running, maintenance, and evolution of software and computing systems.<ref name="Giese-at-al-engineering-saso-systems">
{{Cite news
|author1=H. Giese |author2=Y. Brun |author3=J. D. M. Serugendo |author4=C. Gacek |author5=H. Kienle |author6=H. Müller |author7=M. Pezzè |author8=M. Shaw |title=Engineering self-adaptive and self-managing systems
|year= 2009
|publisher=Springer-Verlag
}}</ref> Feedback-loops are important models in the engineering of adaptive software, as they define the behaviour of the interactions among the control elements over the adaptation process, to guarantee system properties at run-time. Feedback loops and foundations of control theory have been successfully applied to computing systems.<ref name=Hellerstein-feedbackloop-book>
{{Cite book
|author1=J. L. Hellerstein |author2=Y. Diao |author3=S. Parekh |author4=D. M. Tilbury|author4-link= Dawn Tilbury |title=Feedback Control of Computing Systems
|year= 2004
|publisher=John Wiley & Sons
}}
</ref> In particular, they have been applied to the development of products such as [[IBM Db2]] and [[IBM Tivoli]]. From a software perspective, the [[autonomic computing|autonomic]] (MAPE, monitor analyze plan execute) loop proposed by researchers of IBM is another valuable contribution to the application of feedback loops to the control of dynamic properties and the design and evolution of autonomic software systems.<ref name="muller-autonomic-computing">
{{Cite news
|author1=J. O. Kephart |author2=D. M. Chess |title=The vision of autonomic computing
|year= 2003
}}</ref><ref name="autonomic-computing">
{{Cite news
|author1=H. A. Müller |author2=H. M. Kienle |author3=U. Stege |name-list-style=amp |title=Autonomic computing: Now you see it, now you don't—design and evolution of autonomic software systems
|year= 2009
}}</ref>

==== Software Development ====
{{Main|Software review}}

====User interface design====
{{Main|User interface design}}

Feedback is also a useful design principle for designing [[user interface]]s.

===Video feedback===
[[Video feedback]] is the [[video]] equivalent of [[acoustic feedback]].  It involves a loop between a [[video camera]] input and a video output, e.g., a [[Television|television screen]] or [[video monitor|monitor]].  Aiming the camera at the display produces a complex video image based on the feedback.<ref name=hofstadter>{{cite book|last=Hofstadter|first=Douglas|title=I Am a Strange loop|url=https://archive.org/details/iamstrangeloop00hofs|url-access=registration|year=2007|publisher=Basic Books|location=New York|isbn=978-0-465-03079-8|page=[https://archive.org/details/iamstrangeloop00hofs/page/67 67]}}</ref>

===Human resource management===
{{main|performance appraisal}}
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