Mitsubishi VS-6043 Uživatelský manuál stáhnout pdf (Strana 5)

EXTRONEWS MAY/JUNE 1999

5

STEVE SOMERS

Comb Filters:

NTSC Decoding Basics

by Steve Somers, V.P. Engineering

Adaptive Comb Filter

Decoders (Part 4)

This is the last installment of our

series on NTSC decoding basics, and

Y/C separation in particular.

Meanwhile the quest for high quality

decoders continues on into the sunset

of NTSC. The emphasis here is on

the first step in the process known as

Y/C (luma/chroma) separation since it

does represent the most crucial step in

realizing the full potential of the

transmission medium. The earlier

installments covered NTSC signal

creation, notch/bandpass filtering, and

line comb filters.

Implementation of high-speed digital

processing and low cost memory

components facilitated many improve-

ments in Y/C separation. These strides

in decoding performance involve

decision-making, called adaption,

based upon image content. Moreover,

having more image memory available

means that video processing can now

take place beyond the original 2D

boundaries (i.e. the information within

a picture field). Processing decisions

now extend into the temporal realm, or

from picture frame to picture frame.

This means more intelligence in Y/C

separation as subject matter moves

across the screen from frame to frame.

2D OR NOT 2D?

Since conventional (line type) comb

filters have problems handling diagonal

lines and vertical color changes, it

seems the first order of business would

be to tackle these tricky situations.

Remember that when processing

diagonal lines, succeeding luma (Y)

information is shifted in time and does

(continued on page 18–

See “Comb Filters”)

not properly cancel line-to-line or field-

to-field. This results in Y information

being interpreted as chroma (C) infor-

mation. The outcome is “cross color”

or the rainbow effect in the region of

the diagonal lines. The vertical color

changes, which do not match in time at

the transition point, result in chroma

information being interpreted as Y

information. Hence, you see the

“hanging dots” at the color boundary.

Now, suppose we take the two-line

comb design discussed previously

(see Figure 1). This design is

comparing data between two stored

lines and the incoming data. It utilizes

averaging to soften the transition

between colors. If we add a system

called a correlator, data between the

three lines can be tested before any

actual averaging or summation takes

place (see Figure 2). Here, if there is

significant correlation of data value

between Line 1 and Line 2, then CF= 0

and a difference between Line 2 and

Line 1 is used. But, if Line 2 and Line

3 correlate to a higher degree, CF= 1

and the difference between Line 2 and

Line 3 is used. If there is no correlation

1-H

Delay

Y CHANNEL

C CHANNEL

Composite

NTSC

+

-

1-H

Delay

1

4

/

1

4

/

1

4

/

1

4

/

1

2

/

1

2

/

FIGURE 1. 2-LINE COMB FILTER

1 2 3 4 5 6 7 8 9 10 ... 19 20

Komentáře k této Příručce

Žádné komentáře