A Digital Waveguide Filter (DWF) can be defined as any digital filter built by stringing together digital waveguides.C.8 In the case of a cascade combination of unit-length waveguide sections, they are essentially the same as microwave filters  and unit element filters  (Appendix F). This section shows how DWFs constructed as a cascade chain of waveguide sections, and reflectively terminated, can be transformed via elementary manipulations to well known ladder and lattice digital filters, such as those used in speech modeling [299,366].
DWFs may be derived conceptually by sampling the unidirectional traveling waves in a system of ideal, lossless waveguides. Sampling is across time and space. Thus, variables in a DWF structure are equal exactly (at the sampling times and positions, to within numerical precision) to variables propagating in the physical medium in an interconnection of uniform 1D waveguides.