Abstract: A logging system 2 is described in which an input signal value is compared by a signal processor 12 to a trigger-band 34 and an inner-band 36 centered about a previously logged value 34 and changing at a previously determined rate of change. When the trigger-band is exceeded at point 40, intermediate points are logged representing the maximum and minimum points since the last logged point and the point at which the inner-band was left. The intermediate points enable a more accurate subsequent reconstruction of the actual signal to be made from the logged values.