## Direct-Form-I Implementation

The difference equation (Eq.(5.1)) is often used as the recipe
for numerical implementation in software or hardware. As such, it
specifies the *direct-form I* (DF-I) implementation of a digital
filter, one of four direct-form structures to choose from. The DF-I
signal flow graph for the second-order case is shown in
Fig.5.1a. The direct-form II structure, another common choice,
is depicted in Fig.5.1b. The other two direct forms are
obtained by *transposing* direct forms I and II.
Chapter 9 discusses all four direct-form structures.

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Filter Order