The difference equation for this filter is given by:
For stability, the coefficient
.
The transfer function of the feedback comb filter is
from which the amplitude response is found as
The magnitude response of a feedback comb filter, calculated with the Matlab script combs.m, is shown in Fig. 23 for , , and = -0.1, -0.5, and -0.9.
Figure 23:
Magnitude response of a feedback comb filter with , , and = -0.1, -0.5, and -0.9.
The feedback comb filter can be regarded as a computational physical model of a series of echoes, exponentially decaying and uniformly spaced in time. When this happens in a real space, the phenomena is referred to as ”flutter echo.”