Subfilter

Subfilter is a term used in signal processing and related disciplines to denote an individual filter that is part of a larger filtering system. Because the literature does not prescribe a single definition, the exact meaning of subfilter can vary by context. In general, a subfilter implements a portion of the overall frequency response or impulse response and is combined with other subfilters to realize the complete filter.

In polyphase filter banks and multistage filters, an operator is decomposed into several subfilters that can be processed in parallel or with reduced hardware complexity. The outputs of the subfilters are recombined to form the final filtered signal. In two-dimensional filtering, a separable filter may be implemented as two one-dimensional subfilters that operate along different axes, with their results convolved to obtain the final result.

Subfilters can be FIR or IIR and may vary in length, bandwidth, and delay. The design choice affects locality in time, phase linearity, and computational cost; engineers may trade off between precision and implementation resources. In hardware, subfilters are common in digital signal processors and field-programmable gate arrays, where modularity enables reuse and pipelining.

Because "subfilter" is not a standardized term, readers may encounter it in different ways; it is often synonymous with "component filter," "stage filter," or "branch" within a filter bank. Understanding the specific architecture is necessary to determine the role and characteristics of a given subfilter.