e c g approaches 2 u00d7 e c u2212 e f which is identical to the limiting condition for eq a1 thirdly because we are interested in providing gain based on relative rather than absolute differences in the slow varying envelope signal the gain factor should be normalised with respect to the average level of slow varying envelope signal as per eq a3 the effect of the numerator in eq a3 compresses the linear gain factor as defined in eq a2 into a range of 0 to 2 the gain factor is now proportional to the modified 2 nd order derivative and inversely proportional to the average level of the slow varying envelope channel signal g 2 u00d7 e c u2212 2 u00d7 e p u2212 e f e c e p e f a3 finally the gain factor according to eq a3 can fall below zero when e c lt e p e f 2 thus eq a4 is imposed on g n so that the gain is always greater than or equal to zero an analysis of the limiting cases for the gain factor can be used to describe its behaviour as a function of the slow varying envelope signal for the limiting case when e p is much smaller than e c i e during a period of rapid rise in the.