LF43168
DEVICES INCORPORATED
Dual 8-Tap FIR Filter
F
IGURE
3.
S
YMMETRIC
C
OEFFICIENT
S
ET
E
XAMPLES
8 7 6 5
8 7 6 5 4 3 2 1
7 6 5 4 3 2 1
4 3 2 1
Even-Tap, Even-Symmetric
Coefficient Set
Odd-Tap, Even-Symmetric
Coefficient Set
Even-Tap, Odd-Symmetric
Coefficient Set
F
IGURE
4.
E
VEN
-S
YMMETRIC
C
OEFFICIENT
F
ILTER
C
ONFIGURATIONS
(N
O
D
ECIMATION
)
DATA IN
DATA IN
A
B+A
B
A
B+A
B
A
B+A
B
A
B+A
B
A
B+A
B
A
B+A
B
A
B+A
B
A
B+A
B
COEF 0
COEF 1
COEF 2
COEF 3
COEF 0
COEF 1
COEF 2
COEF 3
2
EVEN-TAP FILTER
ODD-TAP FILTER
output to 16 bits, bits 5-8 of Control
Register 1 should be set to “0011”. This
will cause a “1” to be added to bit
position 2
–7
.
Symmetric Coefficients
The LF43168 filter architecture is
optimized for symmetric filter coeffi-
cient sets. Figure 3 shows examples of
the different types of symmetric
coefficient sets. In even-symmetric
sets, each coefficient value appears
twice (except in odd-tap sets where
the middle value appears only once).
In odd-symmetric sets, each coefficient
appears twice, but one value is
positive and one is negative. If the
two data input values that will be
multiplied by the same coefficient are
added or subtracted before being sent
to the filter multiplier, the number of
multipliers needed for an N-tap filter
is cut in half. Therefore, an 8-tap filter
can be implemented with four multi-
pliers if a symmetric coefficient set is
used.
FILTER CONFIGURATIONS
Figures 4-6 show the data paths from
filter input to filter multipliers for all
symmetric coefficient filters. Figure 7
shows the interleaved coefficient filter
configuration. Each diagram shows
one of the two FIR filters when the
device is configured for dual filter
mode. The diagrams can be expanded
to include both filters when the device
is configured for single filter mode.
Even-Symmetric Coefficient Filters
Figure 4 shows the two possible
configurations when the device is
programmed for even-symmetric
coefficients and no decimation.
Note that coefficient 3 on the odd-
tap filter must be divided by two to
get the correct result (The coefficient
must be input to the device already
divided by two).
Video Imaging Products
7
03/28/2000–LDS.43168-H
LOGIC [ LOGIC DEVICES INCORPORATED ]
