PRELIMINARY DATA SHEET
VDP 31xxB
2.2. Adaptive Comb Filter
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 stan-
dard notch/bandpass to full comb decoding.
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 resolution
(bandwidth) and reduces interferences like cross-lumi-
nance and cross-color artifacts. The adaptive algorithm
can eliminate most of the mentioned errors without
introducing new artifacts or noise.
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 moderate,
whereas, in case of KB=1, it is maximum, but the risk to
increasethe“hangingdots”amountforsomegivencolor
transitions is higher.
A block diagram of the comb filter is shown in Fig. 2–3.
The filter uses two line delays to process the information
of three adjacent video lines. To have a fixed phase rela-
tionship of the color subcarrier in the three channels, the
system clock (20.25 MHz) is fractionally locked to the
color subcarrier. This allows the processing of all color
standards and substandards using a single crystal fre-
quency.
Using the default setting, the comb filter has separate
luma and chroma decision algorithms; it is however pos-
sible to switch the chroma comb factor to the current
luma adaption output by setting CC to 1.
The CVBS signal in the three channels is filtered at the
subcarrier frequency by a set of bandpass/notch filters.
The output of the three channels is used by the adaption
logic to select the weighting that is used to reconstruct
theluminance/chrominancesignalfromthe4bandpass/
notch filter signals. By using soft mixing of the 4 signals
switching artifacts of the adaption algorithm are com-
pletely suppressed.
Another interesting feature is the programmable limita-
tion of the luma comb amount; proper limitation,
associated to adequate luma peaking, gives rise to an
enhanced 2-D resolution homogeneity. This limitation is
setbytheparameterCLIM, rangingfrom0(nolimitation)
to 31 (max. limitation).
The comb filter uses the middle line as reference, there-
fore, 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 DAA parameter (1:off , 0:on) is used to disable/en-
able 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-looking phe-
nomenon is mostly visible on tuner images, since they
are always corrupted by some noise; and this looks like
rain.
Various parameters of the comb filter are adjustable,
hence giving to the user the ability to adjust his own de-
sired picture quality.
Bandpass
Filter
CVBS Input
Luma Output
Bandpass/
1 H Line Delay
1 H Line Delay
Notch
Filter
Chroma Output
Bandpass
Filter
Chroma Input
Fig. 2–3: Block diagram of the adaptive comb filter
Micronas
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MICRONAS [ MICRONAS ]
