Filter Order
The maximum delay, in samples, used in creating each output sample is
called the order
of the filter. In the difference-equation representation, the order is
the larger of 
and 
in Eq.
(5.1). For example,
specifies a particular second-order
filter. If and in
Eq.(5.1) are constrained to be finite (which is, of course,
necessary in practice), then Eq.(5.1) represents the class of all
finite-order causal LTI digital filters.
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Causal Recursive Filters
