VCT 38xxA
ADVANCE INFORMATION
2.2.5. Digitally Controlled Clock Oscillator
The comb filter uses the middle line as reference,
therefore, the comb filter delay is one line. If the comb
filter is switched off, the delay lines are used to pass
the luma/ chroma signals from the A/D converters to
the luma/ chroma outputs. Thus, the comb filter delay
is always one line.
The clock generation is also a part of the analog front-
end. The crystal oscillator is controlled digitally by the
control processor. The clock frequency can be
adjusted within ±150 ppm.
Various parameters of the comb filter are adjustable,
hence giving to the user the ability to adjust his own
desired picture quality.
2.2.6. Analog Video Output
The input signal of the Luma ADC is available at the
analog video output pin. The signal at this pin must be
buffered by a source follower. The output voltage is
2 V, thus the signal can be used to drive a 75-Ω line.
The magnitude is adjusted with an AGC in 8 steps
together with the main AGC.
Two parameters (KY, KC) set the global gain of luma
and chroma comb separately; these values directly
weigh the adaption algorithm output. In this way, it is
possible to obtain a luma/chroma separation ranging
from standard notch/bandpass to full comb decoding.
The parameter KB allows to choose between the two
proposed comb booster modes. This so-called feature
widely improves vertical high-to-low frequency transi-
tions areas, the typical example being a multiburst to
DC change. For KB=0, this improvement is kept mod-
erate, whereas, in case of KB=1, it is maximum, but
the risk to increase the “hanging dots” amount for
some given color transitions is higher.
2.3. Adaptive Comb Filter
The adaptive comb filter is used for high-quality lumi-
nance/chrominance separation for PAL or NTSC sig-
nals. The comb filter improves the luminance resolu-
tion (bandwidth) and reduces interferences like
cross-luminance and cross-color artifacts. The adap-
tive algorithm can eliminate most of the mentioned
errors without introducing new artifacts or noise.
Using the default setting, the comb filter has separate
luma and chroma decision algorithms; however, it is
possible to switch the chroma comb factor to the cur-
rent luma adaption output by setting CC to 1.
A block diagram of the comb filter is shown in Fig. 2–2.
The filter uses two line delays to process the informa-
tion of three adjacent video lines. To have a fixed
phase relationship of the color subcarrier in the three
channels, the system clock (20.25 MHz) is fractionally
locked to the color subcarrier. This allows the process-
ing of all color standards and substandards using a
single crystal frequency.
Another interesting feature is the programmable limita-
tion of the luma comb amount; proper limitation, asso-
ciated to adequate luma peaking, gives rise to an
enhanced 2-D resolution homogeneity. This limitation
is set by the parameter CLIM, ranging from 0 (no limi-
tation) to 31 (max. limitation).
The CVBS signal in the three channels is filtered at the
subcarrier frequency by a set of bandpass/notch fil-
ters. The output of the three channels is used by the
adaption logic to select the weighting that is used to
reconstruct the luminance/chrominance signal from
the 4 bandpass/notch filter signals. By using soft mix-
ing of the 4 signals switching artifacts of the adaption
algorithm are completely suppressed.
The DAA parameter (1:off, 0:on) is used to disable/
enable a very efficient built-in “rain effect” suppressor;
many comb filters show this side effect which gives
some vertical correlation to a 2-D uniform random
area, due to the vertical filtering. This unnatural-look-
ing phenomenon is mostly visible on tuner images,
since they are always corrupted by some noise; and
this looks like rain.
Bandpass
Filter
CVBS Input
Luma Output
Bandpass/
1H Delay Line
1H Delay Line
Notch
Filter
Chroma Output
Bandpass
Filter
Chroma Input
Fig. 2–2: Block diagram of the adaptive comb filter (PAL mode)
12
Micronas
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