Introduction

Introduction

Look up ‘feedback’ on Google, and you will find page after page listing feedback services for web sites or other services. Meaning, in this case, user feedback – comments (good or otherwise) concerning a service. This is one kind of feedback, but there are many other kinds, which I want to explore with you. From electronics to psychology, from systems of natural growth to the human body, from chemistry to economics and from the whole planet to the cosmos, feedback forms a hugely important part of the way the world works.

 

What is Feedback?

We have all heard what happens when a public address system is not set up correctly; someone speaks into the microphone and a howl or whine comes out of the speakers – and not from the person speaking. This audio feedback is one kind of feedback. It’s a type of feedback called Positive Feedback. The ‘positive’ in this case does not mean that it is any sense good or desirable – as it implies in user feedback. In fact, it is a plain nuisance, and the bugbear of sound engineers. It happens when the amplification (or gain) is turned up too high, and when the speakers are not far enough from the microphone – and maybe pointing towards it as well.

Why does it happen? Well, the sound picked up by the microphone is amplified and then fed to the speakers. Sound from the speakers is reaching the microphone, and is then being amplified again. This creates a Loop, which is a characteristic of any feedback system.

Note that the loop we have here is a Closed Loop. The output is connected back to the input forming a kind of closed circuit. This feedback is called positive because the feedback increases the input. This type of feedback is generally a bad thing and is inherently destabilizing. It is often nasty, and can even be explosive, as we shall see later.

If the speakers are too close to the mike (microphone), then the sound reaching the mike is amplified over and over again until it produces the familiar howl.

A properly set up system will try and avoid the feedback by putting the main speakers a long way from the performer, and then having smaller monitor speakers pointing back at the performer, and away from where the microphones.

Why is it a howl? Well, not all frequencies (tones) are treated the same. Amplifiers these days are well designed and will amplify audible frequencies pretty much the same, but the same is not true for microphones and speakers. It is very difficult to make these treat all frequencies the same, and low and high ones will be louder or softer than middle frequencies. And the shape and character of the surroundings will also affect how the sound reaches the mike from the speakers. What this all amounts to is that one frequency – a fairly high one – is amplified round the loop more than others. Because the loop exaggerates this difference, then that frequency will dominate the others, and so a high pitch howl results.

If the amplifier is turned down – reducing the gain – the feedback howl is stopped. This is because the sound going round the loop is not being increased so much, and is dying away rather than growing to the howl. What is critical here is that the Loop Gain has to be greater than 1 for the feedback to produce the howl. This means that a sound going right round the loop gets back bigger than it started; and then gets bigger and bigger each time it goes round the loop. Of course in electronics, the time it takes to go round the loop is very small, although the time taken by the sound waves is noticeable, though still pretty small.

What stops the sound from growing? As you can see, with a loop gain greater than 1, it grows each time round the loop. But an amplifier is limited in how much it can amplify; it cannot produce more power than is supplied to it, and the electronic components will have a maximum power capability. So the howl grows until it is limited by these effects.

Audio feedback is usually undesirable, but some rock and pop artists (notably Jimi Hendrix) have used it for effect, though with some degree of control over the amount of feedback so they can make the howl rise and fall.

So that’s feedback, or at least one example of one type of feedback. But there are many examples of feedback in all manner of different systems.

Let’s start off by looking at the types of feedback, and some of the places that they occur.

        Types of feedback

A general description of feedback is that involves a flow from a systems output back to its input, so the always has to be some kind of loop involved.

Positive feedback always increases the input, and so is inherently destabilizing, because things can run away, with the input and output increasing somewhat out of control.

Negative feedback on the other hand decreases the input by feedback from the output, so it is inherently stabilizing, and generally a good thing.

Generally speaking, positive feedback tends to occur by accident – and can be the cause of accidents, whereas negative feedback is used as a design method to improve things. But this is by no means always true. One deliberate use of positive feedback is in electronic oscillators which I shall return to later, and natural systems also use negative feedback – in the human endocrine system for example, or in maintaining the earth’s oxygen level.

Feedback appears in many disparate disciplines of study – cybernetics, electronics, control systems, robotics, biology, chemistry and economics to name a few. We shall look at all these in some detail later, but first let’s start with electronics.