Feedback Comb Filters
The feedback comb filter uses feedback instead of a feedforward signal, as shown in Fig.2.24 (drawn in ``direct form 2'' [449]).
A difference equation describing the feedback comb filter can be written in ``direct form 1'' [449] as3.9



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 becomes


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Feedforward Comb Filter Amplitude Response
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Feedforward Comb Filters