On the properties of frequency transformations for variable cutoff linear phase digital filters

In a recent paper by Oppenheim, Mecklenbrauker, and Mersereau [1] a class of variable cutoff linear phase digital filters has been proposed. The implementation of this class of filters is achieved by replacing a subnetwork in a prototype network such that it performs a frequency mapping of the prototype filter response. By varying a small number of coefficients in the subnetwork the frequency transformation can be varied. In this letter we examine the characteristics of this transformation in greater detail and discuss the range of parameters over which it can be useful.