Order of a digital filter The order of a digital filter is the number of previous inputs (stored in the processor's memory) used to calculate the current output. Examples (1) and (2) above are zero-order filters, as the current output yn depends only on the current input xn and not on any previous inputs. And the co-author with W.-S Lu of Two-Dimensional Digital Filters (Marcel Dekker, 1992). Antoniou served as Associate Editor and Chief Editor for the IEEE.