The large majority of people cannot explain what each component in a TV set does but most people can tune the channels to receive the TV stations and plug the set into a power socket so they can watch it. The TV is an example of a system and it shows that we do not need to know how specific systems work, we simply need to know what input we have to supply to the system and what the system will generate at the output. A linear system is a black box in which an increase in the input signal brings about a proportional increase in the output signal. A typical example is an amplifier in which the input signal might be a voltage and an output signal might also be voltage. There are other systems which we consider as linear, such as filters, equalisers and plenty of others.
Linearity is related to the ratio of the signal input to the signal output.
If this ratio stays constant, regardless of the value of the input signal, then the system is linear. If it does not, then it is not linear. This is illustrated in the Figure 4.1 . Note that the figure shows two different forms of nonlinearity. A useful quantity associated with linear systems is called gain .