ADVANCE INFORMATION
VPC 323xD, VPC 324xD
2.2. Adaptive Comb Filter
HDG typically defines the comb strength on horizontal
edges. It determines the amount of the remaining
cross-luminance and the sharpness on edges respec-
tively. As HDG increases, the comb strength, e. g.
cross luminance reduction and sharpness, increases.
The 4H adaptive comb filter is used for high-quality
luminance/chrominance separation for PAL or NTSC
composite video signals. The comb filter improves the
luminance resolution (bandwidth) and reduces interfer-
ences like cross-luminance and cross-color. The adap-
tive algorithm eliminates most of the mentioned errors
without introducing new artifacts or noise.
VDG typically determines the comb filter behaviour on
vertical edges. As VDG increases, the comb strength,
e. g. the amount of hanging dots, decreases.
A block diagram of the comb filter is shown in Fig. 2–2.
The filter uses four line delays to process the informa-
tion of three video lines. To have a fixed phase relation-
ship 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.
After selecting the combfilter performance in horizontal
and vertical direction, the diagonal picture perfor-
mance may further be optimized by adjusting DDR. As
DDR increases, the dot crawl on diagonal colored
edges is reduced.
To enhance the vertical resolution of the picture, the
VPC provides a vertical peaking circuitry. The filter
gain is adjustable between 0 – +6 dB and a coring filter
suppresses small amplitudes to reduce noise artifacts.
In relation to the comb filter, this vertical peaking
widely contributes to an optimal two-dimensional reso-
lution homogeneity.
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.
2.3. Color Decoder
In this block, the standard luma/chroma separation and
multi-standard color demodulation is carried out. The
color demodulation uses an asynchronous clock, thus
allowing a unified architecture for all color standards.
The comb filter uses the middle line as reference,
therefore, the comb filter delay is two lines. 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 processing delay
is always two lines.
A block diagram of the color decoder is shown in Fig.
2–4. The luma as well as the chroma processing, is
shown here. The color decoder also provides several
special modes, e.g. wide band chroma format which is
intended for S-VHS wide bandwidth chroma. Also, filter
settings are available for processing a PAL+ helper sig-
nal.
In order to obtain the best-suited picture quality, the
user has the possibility to influence the behaviour of
the adaption algorithm going from moderate combing
to strong combing. Therefore, the following three para-
meters may be adjusted:
If the adaptive comb filter is used for luma chroma
separation, the color decoder uses the S-VHS mode
processing. The output of the color decoder is YCrCb
in a 4:2:2 format.
– HDG ( horizontal difference gain )
– VDG ( vertical difference gain )
– DDR ( diagonal dot reducer )
Bandpass
Filter
CVBS Input
Luma Output
Bandpass/
Notch Filter
2H Delay Line
2H Delay Line
Chroma Output
Bandpass
Filter
Chroma Input
Fig. 2–2: Block diagram of the adaptive comb filter (PAL mode)
Micronas
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MICRONAS [ MICRONAS ]
