Electrohome VistaGRAPHX 10K Guía de instalación descargar pdf (Pagina 9)

EXTRONEWS JAN/FEB 1999

9

the most noticeable examples are on the

vertical edges of red lettering or other

similar color objects. This artifact is

referred to as cross-luminance.

During moments when high frequency

information, or horizontal detail, is

present, some of this energy will find its

way through the chroma channel. It

presents itself as a “rainbow effect” in

regions of the image having the high

detail. Remember seeing this effect on

people wearing finely textured suits or

clothes with narrow stripes? This

phenomenon is called cross-color. It can

only occur within frequencies that “fit”

within the bandpass of the chroma signal

path… or, about 3.0 to 4.0 MHz.

These two common effects primarily

characterize an analog notch/bandpass

decoder scheme in a color display. And,

of course, the other most noticeable effect

is the loss of high frequency details in the

image.

HEY, WE ONLY OPENED ONE BOX

Well, we’ve only ventured through one

box in the decoder diagram (Figure 1).

Unfortunately, there isn’t space to cover

the whole process here [amen. – ed.].

Each of the remaining boxes deserves its

own article (let the editor or me know if

you are interested). However, let’s just

wrap it up in general.

That first box in the chain is the most

critical. After Y and C are obtained, the

Y information must be delayed to account

for the longer processing time required to

synchronously detect the phase and level

differences in the color subcarrier in order

to derive R-Y and B-Y, the color differ-

ence signals. The color burst sample that

is sent on the backporch of the video

signal is used to key the local 3.58

MHz oscillator so as to synchronize or

“genlock” it to the incoming signal.

When this local reference oscillator

frequency is compared to the incoming

C-channel information within the Color

Difference Decoder, the result is the R-

Y and B-Y signals.

Once the color difference signals are

derived, they are input along with the Y

channel to the Matrix Decoder.

Remember that we can derive G-Y

algebraically if we have two of the

three components. The Matrix Decoder

derives the G-Y signal from the two

difference signals. In addition, it alge-

braically adds each of the difference

signals to the Y channel. The result is

the red, green, and blue video signals

used to drive the display device.

COMPOSITE

NTSC IN

BUFFER

BUFFER

BUFFER

Y CHANNEL

3.58 MHz

SERIES

RESONANT "NOTCH"

FILTER

3.58 MHz

PARALLEL

RESONANT "BANDPASS"

FILTER

C CHANNEL

BUFFER

f

Characteristic

Signal

3.58 MHz

f

Characteristic

Signal

3.58 MHz

FIGURE 2.

BASIC NOTCH/BANDPASS

FILTER SYTEM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 19 20

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