VDP3108B datasheet(9/72 Pages) MICRONAS

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PRELIMINARY DATA SHEET

VDP 31xxB

Micronas

2.2. 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 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.

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.

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

the luminance/chrominance signal from the 4 bandpass/

notch filter signals. By using soft mixing of the 4 signals

switching artifacts of the adaption algorithm are com-

pletely suppressed.

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.

Various parameters of the comb filter are adjustable,

hence giving to the user the ability to adjust his own de-

sired picture quality.

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 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

increase the “hanging dots” amount for some given color

transitions is higher.

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.

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

set by the parameter CLIM, ranging from 0 (no limitation)

to 31 (max. limitation).

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.

1 H Line Delay

CVBS Input

Chroma Input

Bandpass

Filter

Bandpass

Filter

Bandpass/

Notch

Filter

Luma Output

Chroma Output

Fig. 2–3: Block diagram of the adaptive comb filter

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