VPC3230D datasheet(9/78 Pages) MICRONAS

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ADVANCE INFORMATION

VPC 323xD, VPC 324xD

Micronas

2.2. Adaptive Comb Filter

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.

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.

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 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.

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:

– HDG ( horizontal difference gain )

– VDG ( vertical difference gain )

– DDR ( diagonal dot reducer )

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.

VDG typically determines the comb filter behaviour on

vertical edges. As VDG increases, the comb strength,

e. g. the amount of hanging dots, decreases.

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.

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.

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.

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.

Fig. 2–2: Block diagram of the adaptive comb filter (PAL mode)

2H Delay Line

CVBS Input

Chroma Input

Bandpass

Bandpass/

Luma Output

Chroma Output

Filter

Notch Filter

Bandpass

Filter

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