-In this paper a simple expression is derived for the formula of variable quantizing step size. This formula can be used to improve the following ability of the discrete adaptive delta modulation codec. Then it is shown that the previously published formulas for video signal can be reduced to this expression. Next, clarifying the relation between the first-order difference\xiof input signal and the mechanism of appearance of the slope overload noise by using this expression, the quantizing noise is classified into granular noise, the first class slope overload noise and the second class slope overload noise. Then these noises are simply expressed as a function of\xi. Finally, using this result, the signal-to-noise ratios for sinusoidal input and Gaussian input are obtained and in consequence the utility of our simple analytical method is confirmed.