Abstract: A digital buffer store 25 (FIG. 2) stores digitized samples of a video signal. A circuit 206 determines the mean grey level (f(b)) of the stored signals and a circuit 28 forms a histogram indicating the number of pixels having various ones of 8 possible grey leves. A circuit 29 determines the dispersion of the histogram about the mean grey level. In principle, circuit 29 could determine from the means level and the histogram the statistical distribution of pixels amongst the grey levels and from that determine the standard deviation as a measure of dispersion. In practice, the circuit 29 determines an emirical approximation to the dispersion, ##EQU1## where n=number of binary bits=3N.sub.i =number of pixels having a grey level iA=f(b)=mean grey levelN.sub.A =number of pixels having the means grey level.This equation is advantageous as it operates on powers of 2.The circuit of FIG. 2 compares the determined values of f(a) and f(b) with empirically derived desired values and servoes the gain and D.C.