H.S. Black: Feedback

Feedback, or its application by humans as control theory, despite the protestations of audio designers that eschew its use, is an entirely natural phenomena. Every time you reach out to pick up an object, get up and walk around, or write something on a piece of paper, feedback data from multiple body sensors are being processed by your brain so that your immediate objective can be executed flawlessly and without error. And, here we have not even begun considering the complex bio-chemical metabolic pathways within the human body – or every single other life form on the planet for that matter – that rely on multi-loop, self-regulating (i.e. feedback control) behaviour.  Nature abounds with examples of the miracle of feedback – a bird in flight, the earths weather system (human CO2 emissions notwithstanding), the thermonuclear reaction taking place in the core of a stable main sequence star, and even planet wide climatic control as exemplified by James Lovelock’s Gaia Hypothesis and so forth. It may therefore be more correct  to say that humans discovered feedback but did not invent it.

H.S. Black  published this article about feedback in the Bell Labs in house magazine ‘The Bell Laboratories Record’  in 1934, based on work he had done on the subject starting as early as 1927. In it he explains how feedback can be used to improve the performance of repeater amplifiers. Feedback has been described as the most important development in electrical engineering in the 20th century.

Feedback has been thoroughly and completely explored and codified over the last 80 years resulting in a huge body of published scholarly work across many hundreds of application fields ranging from aerodynamics, power generation, consumer electronics and onto process control in chemical production and self-regulating metabolic processes in the life sciences.  It is no exaggeration to say that our modern world would not function without it, and neither would mother nature.

Next time you board an airplane, rest assured you will only be arriving safely at you destination because there are hundreds of inter-linking feedback loops all operating flawlessly – from the airplane ailerons, tail fins, pressure sensors to the jet engine turbine control systems and on to navigation and guidance and so forth. Audio amplifiers in this context, and within the natural world,  are therefore an exceedingly simple application of  the science of control theory aka feedback.   




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