Altera FIR Compiler - Manual (Page 45)

Chapter 4: Functional Description 4–3
FIR Compiler
© May 2011 Altera Corporation FIR Compiler User Guide
Symmetrical Architecture Selection
Many FIR filters have symmetrical coefficient values. The FIR Compiler examines the
coefficients and automatically determines the filter’s symmetry: even, odd, or none.
After detecting symmetry, the wizard selects an optimum algorithm to minimize the
amount of computation needed. The FIR compiler determines coefficient symmetry
after the coefficients are rounded. If symmetry is present, two data points are added
prior to the multiplication step, saving a multiplication operation (taking advantage
of filter symmetry reduces the number of multipliers by about half).
1 The wizard gives you the option to force non-symmetrical structures. If the
symmetry-optimized architecture is not available, this option is disabled.
Odd and even filter structures are shown in Figure 4–1 and Figure 4–2 .
Symmetrical Serial
Symmetrical serial filters take an additional clock cycle to perform the FIR
computation (so the filter can compute the carry). Additional logic cells are required
for the symmetrical adder resources.
Because non-symmetrical serial FIR filters do not require this resource, non-
symmetrical filters may be smaller and/or faster.
You can use the Resource Estimator in the Architecture Specification area of the
Parameterize FIR Compiler
page to determine the best solution available. Refer to
Figure 3–9 on page 3–15 ).
Figure 4–1. Seven-Tap Symmetrical FIR Filter
Data In
Data Out
Z -1
C
0
Z -1
C
1
C
2
C
3
Z -1
Z -1
Z -1
Z -1
Z -1