Mitsubishi VS-6043 Uživatelský manuál stáhnout pdf (Strana 18)

EXTRONEWS MAY/JUNE 1999

at all, then CF= 0 and the difference

between Line 2 and Line 1 is used.

This type of adaptive system works

only within a picture field and is called

intrafield filtering. Additional logic is

usually added to detect whether the

system sampling of data is stable

enough to perform proper combing. If

not, the system switches back to the

notch/bandpass filtering method. This

is quite common when decoding video

from the typical VHS VCR.

The term “2D” indicates that the filter

implements detection of both hori-

zontal transitions (along horizontal

lines) and vertical transitions (between

horizontal lines) within the picture

field of interest. The term adaptive

carries wide meaning in that comb

filter manufacturers create unique

methods that enable the filter to make

better decisions as to which process

algorithms to use. Adaptive processing

can lead to increased noise or graini-

ness in the image. To combat this, a

system function may be added called

coring. The coring function modifies

(or outputs the equivalent of zeroes)

the data values near a transition so as

to remove the random artifacts. This,

in conjunction with a contouring

circuit, adjusts the values to provide a

much more pleasing image transition.

So, the 2D adaptive filter attempts to

eliminate the hanging dots at color

transitions. If a situation occurs where

there are different colors on three

successive lines, the filter fails and

artifacts appear. The problem will be

most noticeable on specific, abrupt

color changes and not so obvious on

gradual color transitions. On diagonal

lines, the 2D adaptive filter is less

effective. There is typically not much

luma correlation from one line to

another. Although some averaging

occurs, this filter type is only marginally

better than a regular line comb.

CURE FOR MOTION SICKNESS?

3D motion adaptive comb filters represent

the most sophisticated comb technology

available. While still pricey compared to

previously discussed filter types, they can

provide near perfect separation of Y and

C for still frame images. In 3D comb

filtering, picture information is taken and

compared to information in successive

frames (called inter-frame filtering), as

opposed to 2D filters which process data

taken from successive lines within a field

or frame (intra-frame filtering).

For still images, the picture data from one

frame to the next is essentially identical.

Since it has a high degree of correlation,

making comparisons (similar in topology

to the line comb examples) provides near

perfect output of correlated Y and C. In

this case, diagonal lines and color transi-

tions can be matched very well. This is

the key advantage over the 2D filter.

However, if there is picture movement or

color changes between frames, the 3D

system will produce noticeable errors.

Here is where the motion adaptation

comes into play. The 3D filter compares

the data of several video frames to

determine correlation (still image) or lack

of it (motion present). If motion or

serious color changes are detected, the

system switches to 2D adaptive filter

operation. Under this condition the 3D

filter may perform no better than the 2D

filter. Different implementations of the

filter will use different algorithms for the

determination of motion or color changes.

This motion detection is not a trivial

pursuit and will separate the good 3D

comb filter from the not-so-good one.

YOU TOO CAN BECOME AN EXPERT

After all the foregoing, what does it mean

to you? How can you tell one filter from

another? That is probably the most

important question on your mind.

Further, it’s one thing to recognize the

type of comb filter design and quite

another to determine if it’s performing

correctly. Let’s see if I can provide you

with “something to walk away with”.

Refer to the table on the opposite page for

a compilation of features and problems

with each of the topologies discussed in

this series on Y/C separation in the NTSC

decoding process. While you may not

find that memorizing the various features

is any advantage to you, pay particular

attention to those features that help

identify the comb filter type of most

interest to you. You never know when

and where you’ll see a good comb filter.

1-H

Delay

Y CHANNEL

C CHANNEL

CF (Correlation Factor)

Composite

NTSC

1-H

Delay

MULTIPLEXER

Line 1

Line 2

Line 3

Line 2

Line 1

Line 3

CORRELATION

DETECTOR

FIGURE 2.

2D Adaptive comb filter

Comb Filters–

Continued from page 5

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