PRODUCT SPECIFICATION
TMC22x5yA
In either of these methods, the “K” signal can be used to cross
fade between the YCOMB and the SIMPLE bandsplit signals.
The resulting comb filter equation can be expressed as:
errors in the demodulated picture. In this example, the
chrominance signal would be demodulated with a 180
degree phase error. Unlike the “simple” decoder technique
any errors in the comb filter decoding produce components
that if re-encoded will never reproduce the original compos-
ite video waveform. It is therefore imperative that the num-
ber and magnitude of comb fails be kept to its absolute
minimum. This is not possible with non-adaptive comb filter
architectures, and all vertical and diagonal transitions in the
picture will cause irreversible picture degradation. For this
reason, all the TMC22x5yA comb filter decoders implement
an adaptive comb filter architecture.
Combed Luma = Simple + (K * Combed High
Frequency Luma)
Combed Chroma = Simple - (K * Combed High
Frequency Luma)
In the case of the chroma comb, the weighted combed high
frequency luma is subtracted from the SIMPLE high pass fil-
ter output to produce the combed chroma signal, and for
luma comb filters the weighted combed high frequency luma
is added to the SIMPLE low pass filter output to provide the
combed luminance signal.
To aid in this decision making process, comprehensive comb
fail signals are generated and fed to a user-programmable
lookup table (XLUT). The output of the lookup table pro-
vides the control for the cross fade between the comb and
simple bandsplit decoder.
Comb Fails
The inputs to the comb filter are monitored to detect discon-
tinuities that would cause the comb filter operation to fail.
Whenever a significant failure is predicted, the comb filter
architecture is modified and an error signal proportional to
the discontinuity is produced. For flat areas of color, it is a
relatively simple to produce an error signal that switches
between the outputs of the comb filter and the simple band
split filter without visibly softening the picture horizontally
or vertically. However, as horizontal frequencies increase
during vertical transitions, so the decision for switching
between the comb and simple bandsplit decoder becomes
more complex.
Comb Fail Detection
The traditional approach of using the low frequency data to
look for vertical luma transitions, and rectifying the high
frequency data to estimate vertical transitions in the chroma
provides adequate comb fail detection. However, chroma
signals that are equal in magnitude but 180 degrees apart in
phase, which can also have a small difference in luma level,
for example green and magenta, can produce undetected
comb fails in the comb filter output.
To overcome problems with simpler comb fail measurement
techniques, the TMC22x5yA generates an array of patented
comb fail and comb filter control signals. To produce these
signals each input to the comb filter is passed through a sim-
ple bandsplit decoder. This provides a luma signal from the
low frequency portion of the comb filter input, and the hue
(phase) and saturation (magnitude) from the high frequency
portion of the comb filter input. These signals are compared
and the differences in luma, hue, and saturation are used to
determine the type of comb filter used to generate the
YCOMB signal and to provide the cross fade control signal
“K”. The “K” signal can be weighted within the XLUT
lookup table, allowing the user to tailor the comb
A line based comb filter can separate the luma and chroma
signals from line repetitive composite video signals, with no
loss of luma or chroma bandwidth. However, if there is a ver-
tical transition, i.e. a change from one scan line to the next,
as shown for a NTSC two line comb in Figure 17, a comb
fail occurs. The comb fail shown in Figure 17, clearly illus-
trates the resulting vertical smearing of the luma and chroma
signals.
In addition to the smearing, the resulting phase of the
chrominance signal with respect to the burst can cause hue
filter response to their system requirements.
65-22x5y-58
Figure 17. Example of a Comb Fail Using a NSTC Two Line Comb Filter
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