Electrohome VistaGRAPHX 10K Guía de instalación descargar pdf (Pagina 8)

EXTRONEWS JAN/FEB 1999

8

with reasonable results. In many situa-

tions, that approach is used today. In

fact, most all of the digital decoders on

the market automatically switch back-

and-forth between notch/bandpass and

combing as required. Watching a VHS

tape? You’ll most likely be operating in

the notch/bandpass mode even if you

have a comb filter in your display. Why

is it called a notch/bandpass filter?

Figure 2 illustrates the basic topology of

this filter. The composite NTSC is input

to a system having two analog signal

pathways. One pathway substantially

passes frequencies from the region of the

3.58 MHz subcarrier and lower. Here, a

series-resonant type passive filter

(capacitor and inductor) is employed

whose resonant frequency is centered

about 3.58 MHz. The operation of the

filter is such that as energy approaches its

resonant point, energy is dissipated in the

filter; otherwise it is allowed to pass on.

Therefore, little energy within the color

subcarrier region passes by the filter. The

result is a severe “notch” created in the Y

channel bandpass centered about 3.58

MHz. This action substantially removes

the chroma information from the

incoming signal and we realize the luma

or Y channel. But, we sacrifice high

frequency information, or horizontal

picture detail, above the color subcarrier

frequency region.

Conversely, the second pathway passes

frequencies just within the region of the

3.58 MHz subcarrier. Here, a parallel-

resonant filter (capacitor and inductor) is

employed whose resonant frequency is…

can you guess? Yes, 3.58 MHz. A

parallel resonant filter acts in an opposite

manner in that it severely attenuates

signals that are not near its resonant

point. Signals near its resonant point are

passed through as shown in Figure 2.

The result is that the majority of the

chroma information makes it through the

filter and we realize the C channel.

Now, mentally overlay the images of each

filter characteristic shown in Figure 2 and

you’ll see that significant overlap occurs.

Regions exist where some luma energy

finds its way through the bandpass filter

and some chroma energy finds its way

through the notch filter. Hence, the

notch/bandpass approach is far from

ideal. Analog methods do not allow for

extremely precise control of filter charac-

teristics, or in other words, the sides of

these filters cannot be made very steep,

so as to block unwanted information.

These filter crossover regions are respon-

sible for most decoding artifacts seen in

color decoders using this methodology.

SIDE EFFECTS…

THEY MAKE MY CHROMA CRAWL

Visual image side effects are the result of

luma and chroma components alternately

making their way through the wrong

decoding channel. You have seen the

results. When chroma information is

present in the luma channel, it creates

“dot crawl” or “chroma crawl” on the

vertical edges of color details. Probably

Y/C

SEPARATOR

DELAY

COLOR

DIFFERENCE

DECODER

MATRIX

DECODER

Y

Y

R

G

B

R - Y

B - Y

NTSC

COMPOSITE

VIDEO

C

COLOR BURST

3.58

MHz

OSC

FIGURE 1. BASIC NTSC DECODING SYSTEM

Notch Filter Decoders –

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