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The feedback comb filter uses feedback instead of a
feedforward signal, as shown in Fig. 1.18 (drawn in ``direct form 2''
[426]).
Figure 1.18:
The feedback comb filter.
|
The difference equation describing the feedback comb filter can be
written in ``direct form 1'' [426] as2.5
The feedback comb filter is a special case of an Infinite Impulse
Response (IIR) (``recursive'') digital filter, since there is
feedback from the delayed output to the input [426].
The feedback comb filter can be regarded as a computational physical
model of a series of echoes, exponentially decaying and
uniformly spaced in time. For example, the special case
is a computational model of an ideal plane wave bouncing back and
forth between two parallel walls; in such a model, represents the
total round-trip attenuation (two wall-to-wall traversals, including
two reflections).
For stability, the feedback coefficient must be less than
in magnitude, i.e.,
. Otherwise, if
,
each echo will be louder than the previous echo, producing a
never-ending, growing series of echoes.
Sometimes the output signal is taken from the end of the delay line instead
of the beginning, in which case the difference equation can be written as
This choice of output merely delays the output signal by samples.
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