2. The signal

2.1 General

An automatic system is affected by certain time-varying quantities: the inputs. There are two types of inputs (figure 8 ):

• useful inputs, or commands, that you want the system to obey;

• parasitic inputs or disturbances, to which one would like its operation to be insensitive.

Disturbances are by nature unpredictable; they are random functions of time, as are commands, since an automatic system is called upon to work under a priori non-determined conditions of use. As a result, an adequate description of its operation must, in principle, make use of methods for studying random functions....