## Signal Flow Graph

One possible *signal flow graph* (or *system diagram*)
for Eq.(5.1) is given in
Fig.5.1a for the case of and .
Hopefully, it is easy to see how this diagram represents the
difference equation (a box labeled ``'' denotes a one-sample
delay in time). The diagram remains true if it is converted to the
*frequency domain* by replacing all time-domain signals by their
respective *z* transforms (or Fourier transforms); that is, we may replace
by and by . transforms and their usage
will be discussed in Chapter 6.

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Causal Recursive Filters

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Difference Equation