Multistage digital signal multiplying and adding apparatus

Multistage digital signal multiplying and adding apparatus suitable for a multistage filter comprises a plurality of addition stages (1,2,N) each having adding devices (11,12,13) for forming a succession of partial products together with selectors (34) operable in a first condition to connect sum and carry outputs of the adding devices within a stage to further adding devices within the stage or a second condition to supply outputs to a further stage. The output of one stage is connected to a subsequent stage during a first cycle of product formation of the subsequent stage so that the output is accumulated with the first partial product of the subsequent stage.