INTERPOLATOR/DECIMATOR FILTER STRUCTURE AND A NOTCH FILTER THEREFOR

A filter structure including a notch filter is designed to have a transmission zero at a frequency slightly displaced from one half of the lower sampling rate of the interpolator/decimator, and thus providing increase in attenuation at the half-rate frequency. The notch filter is comprised of two like all-pass-network filters and has feed forward and feedback connections, the latter connection including a coefficient multiplier. The feedback connection is made between a tapped output of the second of the network filters and an input node. The tapped filter is characterized by throughput and a tapped output transform functions X(Z) and Y(Z) given by the following expressions: EQU X(Z)=[Z.sup.-1 -K]/[1-KZ.sup.-1 ; and, EQU Y(Z)=.alpha.Z.sup.-1 /[1-KZ.sup.-1 ; where Z.sup.-1 is the unit delay operator, K the multiplier coefficient and, .alpha., a structure dependant constant.