
PRODUCT SPECIFICATION
TMC22x5yA
41
centered at the subcarrier frequency produces the chroma 
signal. This simple technique works well in pictures contain-
ing large flat areas of color, however this is rarely the case. 
If, as is generally true, the picture contains high frequency 
luma and chroma transitions, for example herring bone suit 
jackets, branches of trees, text, etc., cross color and cross 
luma artifacts are evident. 
The presence of cross color or cross luma is generally 
acceptable when viewing the decoded picture on a monitor 
from several feet, as would be the case in most homes on 
commercial television sets. However, these artifacts become 
increasingly difficult to process, or ignore, when the image is 
to be compressed or manipulated. In these cases more 
sophisticated methods of separating the luma and chroma 
signals, such as frame, field, or line based comb filter decod-
ers, are required.
Another important disadvantage of the “l(fā)uma notch filter and 
bandpass chroma” technique is that once a notch filter has 
been used on the luminance channel this portion of the lumi-
nance frequency spectrum is lost. This effect becomes 
increasingly objectionable if the decoder component outputs 
are subsequently re-encoded into a composite video signal.
Comb Filter Architectures for YC Separation
A comb filter uses the relationship between the number of 
subcarrier cycles per line period, to cancel the chrominance 
signal over multiple line periods. This is shown for an NTSC 
two line comb filter in Figure 6. In NTSC there a 227.5 sub-
carrier cycles per line period, therefore the subcarrier can be 
canceled by simply adding two consecutive field scan lines. 
In PAL(B/I/ etc.) there are 283.7516 subcarrier cycles per 
line period, ignoring the 0.0016 cycle advance caused by the 
25Hz offset, the PAL subcarrier can be canceled by adding 
the first and third line of three consecutive field scan lines. 
Due to the 270 degree advance, it is not possible to use infor-
mation from consecutive field lines without adding a PAL 
modifier. A PAL modifier produces a 90 degree phase shift in 
the chrominance signal by multiplying the chrominance 
signal by a signal at two times the subcarrier frequency that 
is phased locked to the subcarrier burst reference in the com-
posite video waveform. In addition the PAL modifier inverts 
the V component of the chrominance signal. This document 
refers to line based comb decoders when discussing decoders 
that use inputs from sequential scan lines, i.e. lines from the 
same field, field based comb decoders when describing 
decoders that use inputs from sequential fields, and finally 
frame based comb decoders when examining decoders that 
use inputs from sequential frames.
Figure 6.
Composite Line-Based Comb Decoders
The phase relationship of the quadrature modulated chromi-
nance signal can also be represented as in Figure 7. The three 
line comb based decoder is clearly biased towards 1H which 
illustrates the inherent one line delay through a 3 line comb, 
while a two line comb based decoder is biased towards 0H. 
In the following discussions a flat color represents video of 
constant luma and chroma magnitude and phase.
In NTSC, adding two adjacent lines of flat color will cancel 
the chroma and leave the luma whereas subtracting two lines 
of flat color will cancel the luma and leave the chroma. In a 3 
line comb filter the flat color on 0H and 2H is added to pro-
vide the flat color average before adding or subtracting from 
1H.
In PAL, adding the flat color from 0H and 2H will cancel the 
chroma and leave the luma while subtracting the flat color 
from 0H and 2H will cancel the luma and leave the chroma. 
However, chroma generated in this manner has no simple 
Delay = 1/T
+
1/2
1/2T
1T
3/2T
2T
5/2T
3T 7/2T
4T
9/2T
5T 11/2T
6T
Frequency
Amplitude
1.0
Figure 5. Examples of Notch and Bandpass Filters
Chrominance
Subcarrier
Notch Filter
Bandpass Filter
Chrominance
Subcarrier
Frequency
F
SC
F
SC
Frequency
Amplitude
(dB)
Amplitude
(dB)
Luminance
-20
-3
0
-20
-3
0
Chrominance
(& High Frequency
Luminance)
Chrominance
(& High Frequency
Luminance)