Programmable multistage digital filter

Claim 1

A multistage digital filter for producing an output data sequence in response to elements of an input date sequence and to elements of a coefficient data sequence, said multistage digital filter comprising: multiplier means for multiplying elements of a first data sequence by elements of said coefficient data sequence to produce product terms; first memory means for storing accumulated sum, said first memory means having first and second ports for providing access to said accumulated sums; first means for accessing through said first port a first accumulated sum stored in said first memory means, and for adding a product term produced by said multiplier to said first accumulated sum, thereby producing a fist output sum; second means for accessing through said second port a second accumulated sum stored in said first memory means, and for adding a product term produced by said multiplier to said second accumulated sum, thereby producing a second output sum; third means for selectively replacing said first accumulated output sum in said first memory means with said first output sum and to store said second output sum in place of said second accumulated sum, and for selectively producing elements of a second data sequence according to values of said first and second output sums; and fourth means for selecting between a first portion of the elements of said second data sequence and elements of said input data sequence to form said first data sequence, and for alternately generating said output data sequence including a second portion of the elements of said second data sequence.