1. System design

Early active control systems were based on purely analog devices. They were unable to "follow" variations in the noise to be reduced, as their settings could only be modified by external action, for example by acting on potentiometers. As a result, destructive interference conditions could not be ensured over the long term, making these systems difficult to use for practical or industrial applications. The switch to devices based on digital signal processing was to enable the boom in active control. The reasons for this are well known:

• digital systems are not very sensitive to environmental conditions and ageing;

• they eliminate errors resulting from the non-linearities of active components or the tolerances of passive components;

• they benefit from ever-increasing advances...