Low pass nonrecusine digital filter

Claim 1

A digital filter for filtering a series of first binary code words occuring at a first sampling frequency fs, and for producing a filtered version of the first words occuring at another sampling frequency f''s, comprising: at least one first digital filter subunit having an input terminal, an output terminal and a first cut-off frequency which is smaller than fs; means to apply said series of first words to the input terminal of the first filter subunit, at least one interpolation digital filter subunit having an input terminal, an output terminal and a second cut-off frequency which is smaller than f''s; each sUbunit including a source of filter coefficients, means for providing a weighted sum of a predetermined series of words with a corresponding series of the filter coefficients, and a clock pulse generator; the pulse generator in the first subunit operating at a frequency which exceeds the first cut-off frequency to provide at the output terminal of the first subunit a series of second code words occuring at a sampling frequency exceeding said first cut-off frequency and constituting a version of said first words; means to apply said second code words to the input terminal of said interpolation filter subunit; and the pulse generator in the interpolation filter subunit operating at the desired sampling frequency f''s.