When in Eq. (6.7), we may perform a step of long division of to produce an FIR part in parallel with a strictly proper IIR part, e.g.,
When , we define . This type of decomposition is computed by the residuez function in the Matlab Signal Processing Tool Box (a function for computing a complete partial fraction expansion, as illustrated in §6.8.8 below). In Octave, which doesn't have residuez as of this writing, the same result can be obtained using its residue function, as derived in §H.5.
An alternate FIR part is obtained by performing long division on the reversed polynomial coefficients to obtain
We may compare these two PFE alternatives as follows. Let denote , , and . (I.e., we use a subscript to indicate polynomial order, and `' is omitted for notational simplicity.) Then for we have two cases:
In the first form, the coefficients are ``left justified'' in the reconstructed numerator, while in the second form they are ``right justified''. The second form is generally more efficient for modeling purposes, since the numerator of the IIR part ( ) can be used to match additional terms in the impulse response after the FIR part has ``died out''.
In summary, an arbitrary digital filter transfer function with distinct poles can always be expressed as a parallel combination of complex one-pole filters, together with a parallel FIR part when . When there is an FIR part, the strictly proper IIR part may be delayed such that its impulse response begins where that of the FIR part leaves off.
In artificial reverberation applications, the FIR part may correspond to the early reflections, while the IIR part provides the late reverb, which is typically dense, smooth, and exponentially decaying . The predelay (``pre-delay'') control in some commercial reverberators is the amount of pure delay at the beginning of the reverberator's impulse response. Thus, neglecting the early reflections, the order of the FIR part can be viewed as the amount of predelay for the IIR part.