### General Causal FIR Filters

The most general case--a TDL having a tap after *every* delay
element--is the general causal *Finite Impulse Response (FIR)*
filter, shown in Fig.2.22. It is restricted to be *causal*
because the output may not depend on *``future''* inputs
, , etc. The FIR filter is also called a
*transversal filter*. FIR filters are described in greater
detail in [449].

The *difference equation* for the th-order FIR filter in Fig.2.22
is, by inspection,

*transfer function*is

The STK class for implementing arbitrary direct-form FIR filters is
called Fir. (There is also a class for IIR filters named Iir.)
In Matlab and Octave, the built-in function `filter` is
normally used.

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Feedforward Comb Filters

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TDL for Parallel Processing