- Finite Impulse Response (FIR) filters are defined by scaled and time-delayed versions of the filter input signal only, as given by the following difference equation:
*n*< 0. - An FIR filter can be represented by a block diagram as shown in Fig. 1 below.
- The
*z*^{-1}terms represent unit delays. Note that while this representation for a delay element is common and widely accepted in the signal processing community, the specification of delay in terms of powers of*z*is a*z*-domain characterization (to be described below) while the block diagram itself is a time-domain representation. - With respect to the filter block diagram, FIR filters make use of feed-forward terms only.
- The impulse response of an FIR filter is only as long as the maximum delayed input term in its difference equation.
- The summation of feedforward input terms can result in destructive signal interference, or cancellations, at certain frequency values.
- The FIR filter is said to have an order equivalent to the number of unit delays in its difference equation.

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