VDP313XY datasheet(8/76 Pages) MICRONAS

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VDP 313xY

ADVANCE INFORMATION

Micronas

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

cross-luminance and cross-color artifacts. The adap-

tive 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. 2–1.

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.

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 one line. If the comb

filter is switched off, the delay lines are used to pass

the luminance/ chrominance signals from the A/D con-

verters to the luminance/ chrominance 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

desired picture quality.

Two parameters (KY, KC) set the global gain of lumi-

nance and chrominance comb separately; these val-

ues directly weigh the adaption algorithm output. In

this way, it is possible to obtain a luminance/chromi-

nance separation ranging from standard notch/band-

pass 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.

Using the default setting, the comb filter has separate

luminance and chrominance decision algorithms; it is

however possible to switch the chrominance comb fac-

tor to the current luminance adaption output by setting

CC to 1.

Another interesting feature is the programmable limita-

tion of the luminance comb amount; proper limitation,

associated to adequate luminance peaking, gives rise

to an enhanced 2-D resolution homogeneity. This limi-

tation 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/

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.

Fig. 2–1: Block diagram of the adaptive comb filter (PAL mode)

1H Delay Line

CVBS Input

Chroma Input

Luma Output

Chroma Output

Bandpass

Filter

Bandpass/

Notch

Filter

Bandpass

Filter

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