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Finite impulse response : ウィキペディア英語版
Finite impulse response
In signal processing, a finite impulse response (FIR) filter is a filter whose impulse response (or response to any finite length input) is of ''finite'' duration, because it settles to zero in finite time. This is in contrast to infinite impulse response (IIR) filters, which may have internal feedback and may continue to respond indefinitely (usually decaying).
The impulse response (that is, the output in response to a Kronecker delta input) of an Nth-order discrete-time FIR filter lasts exactly ''N'' + 1 samples (from first nonzero element through last nonzero element) before it then settles to zero.
FIR filters can be discrete-time or continuous-time, and digital or analog.
==Definition==

For a causal discrete-time FIR filter of order ''N'', each value of the output sequence is a weighted sum of the most recent input values:
:\begin
y() &= b_0 x() + b_1 x() + \cdots + b_N x() \\
&= \sum_^ b_i\cdot x(),
\end
where:
*\scriptstyle x() is the input signal,
*\scriptstyle y() is the output signal,
*\scriptstyle N is the filter order; an \scriptstyle Nth-order filter has \scriptstyle (N \,+\, 1) terms on the right-hand side
*\scriptstyle b_i is the value of the impulse response at the ''ith instant for \scriptstyle \ 0 \ \le \ i \ \le \ N \ of an \scriptstyle Nth-order FIR filter. If the filter is a direct form FIR filter then \scriptstyle b_i is also a coefficient of the filter .
This computation is also known as discrete convolution.
The \scriptstyle x() in these terms are commonly referred to as ''s'', based on the structure of a tapped delay line that in many implementations or block diagrams provides the delayed inputs to the multiplication operations. One may speak of a ''5th order/6-tap filter'', for instance.
The impulse response of the filter as defined is nonzero over a finite duration. Including zeros, the impulse response is the infinite sequence:
:
h() = \sum_^b_i\cdot \delta() =
\begin
b_n & \scriptstyle 0 \le n \le N\\
0 & \scriptstyle \text.
\end

If an FIR filter is non-causal, the range of nonzero values in its impulse response can start before ''n'' = 0, with the defining formula appropriately generalized.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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