In comparing a signal

with its amplitude-distorted form

, where

is a monotonically increasing function of its argument, one is led to consider the correlation function begin{equation} R(s) triangleq int_{-infty}^{infty} dtg (f(t))f(t-s). end{equation} A rigorous proof is given of the inequality

. Generalizations are presented for the cases of finite domains and of signals defined in two-dimensional space.