PRODUCT SPECIFICATION
TMC22x5yA
Notch Filter
Bandpass Filter
Amplitude
(dB)
Amplitude
(dB)
Chrominance
Subcarrier
Chrominance
Subcarrier
0
0
-3
-3
Luminance
Chrominance
(& High Frequency
Luminance)
Chrominance
(& High Frequency
Luminance)
-20
-20
FSC
Frequency
FSC
Frequency
Figure 5. Examples of Notch and Bandpass Filters
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 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.
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.
Delay = 1/T
1/2
+
Amplitude
1.0
Another important disadvantage of the “luma 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.
1/2T 1T 3/2T 2T 5/2T 3T 7/2T 4T 9/2T 5T 11/2T 6T
Frequency
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.
Comb Filter Architectures forYC 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
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
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