
TMC22x5yA
PRODUCT SPECIFICATION
48
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.
Table 3. Comb Filter Architecture
The 
COMB
 signal can be produced in two ways. The first 
method uses the comb fail detection circuits to select one of 
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.
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. 
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.
STA[3:0]
0
Comb Filter Description
YC or Composite, PAL or NTSC, 3 line 
comb
YC or Composite, NTSC, 3 line comb (0H 
& 1H)
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
YC or Composite, NTSC, (2 line) frame 
comb
YC or Composite, NTSC, frame comb
D1, Y or C
B
C
R
, 3 line comb
D1, Y or C
B
C
R
, 3 line comb (0H & 1H)
D1, Y or C
B
C
R
, 3 line comb (1H & 2H)
D1, Y or C
B
C
R
, 3 line comb (0H & 2H)
D1, Y or C
B
C
R
, (2 line) field comb
D1, Y or C
B
C
R
, field or 2 line comb 
(0H & 1H)
D1, Y or C
B
C
R
, (2 line) frame comb
D1, Y or C
B
C
R
, Frame
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Table 4. Simple Example of an Adaptive Comb Filter Architecture
Line 
no.
n+6
n+5
n+4
n+3
n+2
n+1
n
Input col-
or
blue
blue
blue
yellow
yellow
yellow
black
Error signals
upper
hue
x
0
0
180
0
0
x
Comb filter selection
unknown without line n+7
[0H/4] + [1H/2] + [2H/4]
[0] + [1H/2] + [2H/2]
[0H/2] + [1H/2] + [0]
[0H/4] + [1H/2] + [2H/4]
[0] + [1H/2] + [2H/2]
unknown without line n-1
upper
luma
x
0
0
>0
0
0
x
upper
sat.
x
0
0
0
0
0
x
lower
luma
x
0
>0
0
0
>0
x
lower
sat.
x
0
0
0
0
>0
x
lower
hue
x
0
180
0
0
>0
x