TMC22x5yA
PRODUCT SPECIFICATION
The comb filter architecture performs chrominance or lumi-
nance comb filtering on PAL or NTSC video signals, by
implementing one of sixteen independent chroma and luma
comb filter algorithms. The highest level of the adaptive
comb filter configuration is determined by the STA[3:0]
register bits as shown in Table 3.
several comb filter architectures. These comb filter architec-
tures weight the three lines by varying degrees depending
upon the degree of picture correlation between the inputs to
the comb filter. The simple example in Table 4 shows how
this process works, in which upper denotes error compari-
sons between the two lines stores and lower denotes error
comparisons between the input and the first line store. The
0H, 1H, and 2H terms used in the mathematical description
of the comb filter selection refer to the position with respect
to the internal line stores. The 0H term is the undelayed
input, 1H is the output of line store 1, and 2H is the output of
line store 2.
Table 3. Comb Filter Architecture
STA[3:0]
Comb Filter Description
0
YC or Composite, PAL or NTSC, 3 line
comb
1
2
3
4
5
6
YC or Composite, NTSC, 3 line comb (0H
& 1H)
In this example a 3 line comb is implemented when in the
flat areas of blue or yellow. However, when a difference
between the inputs is detected the 3 line comb filter adapts to
the 2 line comb filter whose inputs have the smallest differ-
ence. This illustrated on line n+4, at which time the comb
filter adapts to inputs from 1H (blue) and 2H (blue) and
ignores the 0H (yellow) inputs. In cases where there is a
difference between all inputs to the comb filter, a 3 line comb
filter is selected and the highest set of comb fail signals are
sent to the XLUT input logic.
YC or Composite, NTSC, 3 line comb (1H
& 2H)
YC or Composite, NTSC, 2 line comb (0H
& 1H)
YC or Composite, NTSC, (2 line) field
comb
YC or Composite, NTSC or PAL, field
comb
This technique would work well if pictures only contained
vertical transitions, which is obviously not the case. There-
fore the weighting of these comb filter taps, (0H, 1H, and
2H), are rarely just the simple ratios shown in Table 4. It is
worth noting that comb filters that use an even number of
lines in the comb filter architecture produce chrominance
and luminance signals that are vertically offset by one pic-
ture line, i.e. in the middle of the even number of lines used
in the comb filter input. While comb filters that use an odd
number of lines, in the comb filter architecture, the chromi-
nance and luminance produced is referenced to the center,
i.e. the middle line, of the comb filter. This approach can
consequentially cause aliasing in decoding composite video
signals containing high frequency diagonal transitions. The
FAST register bit, when set LOW, filters the comb filter
selection to decrease the sensitivity of the adaption algo-
rithm. The second method completely disables the adaption
between different comb filters, by setting the ADAPT[1:0]
register bits accordingly, see Table 5.
YC or Composite, NTSC, (2 line) frame
comb
7
8
YC or Composite, NTSC, frame comb
D1, Y or C C , 3 line comb
B
R
9
D1, Y or C C , 3 line comb (0H & 1H)
B R
10
11
12
13
D1, Y or C C , 3 line comb (1H & 2H)
B R
D1, Y or C C , 3 line comb (0H & 2H)
B
R
D1, Y or C C , (2 line) field comb
B
R
D1, Y or C C , field or 2 line comb
B
R
(0H & 1H)
14
15
D1, Y or C C , (2 line) frame comb
B
R
D1, Y or C C , Frame
B
R
The COMB signal can be produced in two ways. The first
method uses the comb fail detection circuits to select one of
Table 4. Simple Example of an Adaptive Comb Filter Architecture
Error signals
Line Input col- upper upper upper lower lower lower
no.
n+6
n+5
n+4
n+3
n+2
n+1
n
or
luma
sat.
hue
luma
sat.
hue
Comb filter selection
unknown without line n+7
[0H/4] + [1H/2] + [2H/4]
[0] + [1H/2] + [2H/2]
blue
x
0
x
x
x
0
x
x
blue
0
0
0
0
blue
0
0
0
>0
0
0
180
0
yellow
yellow
yellow
black
>0
0
0
180
0
0
[0H/2] + [1H/2] + [0]
0
0
0
0
[0H/4] + [1H/2] + [2H/4]
[0] + [1H/2] + [2H/2]
0
0
0
>0
x
>0
x
>0
x
x
x
x
unknown without line n-1
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REV. 1.0.0 2/4/03
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
