u2147 j u03c6 u2062 n 1 u2062 u220f m 1 n 1 u2062 u2062 u2147 j u03c6 z m equation u2062 u2062 4 where x e j u03c6 denotes the z transform of a discrete time sequence x n the z m represent the roots of the z transform and g is the gain factor each factor in eq 4 corresponds in the z plane to a vector starting at z m and ending at e j u03c6 hence where e j u03c6 gets very close to one of these zeros one of the factors in eq 4 gets very small in amplitude and undergoes an important argument modification which corresponds to spiky change in the group delay function so a simple observation on group delay spectrums does not provide the desired information the plots are usually too noisy due to the zeros close to unit circle in fig5 b a group delay function for a speech frame is presented together with zeros of z transform of the same signal closely located to the unit circle each zero creates a spike in the group delay function hiding resonance peaks to appear as in fig4 in the solution according to the invention the problem is first redefined in a more general framework of u2018 differential phase spectrum u2019 the differential phase spectrum is defined as the negative derivative of the phase spectrum calculated from.