The difference equation for this filter is given by:
For stability, the coefficient
.
The feedback comb filter can be regarded as a computational physical model of a series of echoes, exponentially decaying and uniformly spaced in time.
The magnitude response of a feedback comb filter is shown in Fig. 22 for , , and = -0.1, -0.5, and -0.9.
Figure 22:
Magnitude response of a feedback comb filter with , , and = -0.1, -0.5, and -0.9.
The following MSP patches implement a feedback comb filter (left: note the 1 ms delay minimum using the tapout˜ object; right: control in samples).
The comb˜ object in MSP implements a combined feedback/feedforward comb filter with inputs for various parameters (delay time and coefficients). This object has the same limitations with respect to delay length (1 ms minimum) as mentioned above.