For other uses, including audio feedback, see Feedback (disambiguation)
Feedback is (generally) information about actions.
In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. Often this is done intentionally, in order to control the dynamic behaviour of the system. Feedback is observed or used in various areas dealing with complex systems, such as engineering, architecture, economics, and biology.
Lines are usually drawn, directed from input through the system and to output. The feedback is shown by another arrowed line, directed from output outside the system to an input, resulting in a loop on the diagram, called feedback loop. This notion is important; for example, the feedback loop is a convenient place for a control device.
The following is an example of a feedback loop used in web-based workflows. Feedback Loops are established by ISPs for permission e-mail marketers to manage subscribers who click the "This is Spam" button in their web mail clients. The Feedback Loop sends a message back to the marketer letting them know to unsubscribe the subscriber.
Main articles: Negative feedback, Positive feedback
Feedback may be negative, which tends to reduce output (but in amplifiers, stabilises and linearises operation), or positive, which tends to increase output. Systems which include feedback are prone to hunting, which is oscillation of output resulting from improperly tuned inputs of first positive then negative feedback. Audio feedback typifies this form of oscillation.
The processing and control of feedback is engineered into many electronic devices and may also be embedded in other technologies.
The most common general-purpose controller is a proportional-integral-derivative (PID) controller. Each term of the PID controller copes with time. The proportional term handles the present state of the system, the integral term handles its past, and the derivative or slope term tries to predict and handle the future.