Trends in feedback systems (editorial)

There are a number of new and powerful techniques rapidly being developed for the analysis and synthesis of feedback control systems. Four of these techniques are illustrated by some of the papers in this issue. One of the most fruitful of these is in the field of statistics, and in particular its application to nonlinear systems. R. C. Booton presents a paper which derives the cross-power density spectrum for a component with saturation or clipping. From this cross-power operator one can determine the quantity of output signal which is correlated with the input, the amount of harmonic distortion power present in the output, and an equivalent "gain" term satisfactory for use in testing for the build-up of periodic oscillations.