AD725 datasheet(15/20 Pages) AD

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AD725

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within the chroma band. All signals in this band are processed

as chroma information since the chroma processing circuit has

no knowledge as to where these signals originated. Therefore,

the color that results from the luma signals in the chroma band

is a false color. This effect is referred to as cross chrominance.

The cross chrominance effect is sometimes evident in white text

on a black background as a moving rainbow pattern around the

characters. The sharp transitions from black to white (and vice

versa) that comprise the text dots contain frequency compo-

nents across the whole video band, and those in the chroma

band create cross chrominance. This is especially pronounced

when the dot clock used to generate the characters is an integer

multiple of the chroma subcarrier frequency.

Another common contributor to cross chrominance effects is

certain striped clothing patterns that are televised. At a specific

amount of zoom, the spatial frequency of vertical stripe patterns

will generate luma frequencies in the chroma band. These fre-

quency components will ultimately get turned into color by the

video monitor. Since the phase of these signals is not coherent

with the subcarrier, the effect shows up as random colors. If the

zoom of a TV camera is modified or there is motion of the

striped pattern, the false colors can vary quite radically and

produce a quite objectionable “moving rainbow” effect. Most

TV-savvy people have learned to adapt by just not wearing

certain patterns when appearing on TV.

An excellent way to eliminate virtually all cross chrominance

effects is to use S-video. Since the luma and chroma are carried

on two separate circuits, there is no confusion as to which cir-

cuit should process which signals. Unfortunately, not all TVs

that exist today, and probably still not even half of those being

sold, have a provision for S-video input.

To ensure compatibility with the input capabilities of the major-

ity of TVs in existence, composite video must be supplied.

Many more TVs have a composite baseband video input port

than have an S-video port to connect cameras and VCRs.

However, still the only common denominator for virtually all

TVs is an RF input. This requires modulating the baseband

video onto an RF carrier that is usually tuned to either Channel

3 or 4 (for NTSC). Most video games that can afford only a

single output use an RF interface because of its universality.

Sound can also be carried on this channel.

Since it is not practical to rely exclusively on S-video to improve

the picture quality by eliminating cross chrominance, a luma

trap can be used to minimize this effect for systems that use

composite video. The luma trap notches out or “traps” the

offending frequencies from the luma signal before it is added to

the chroma. The cross chrominance that would be generated by

these frequencies is thereby significantly attenuated.

The only sacrifice that results is that the luma response has a

“hole” in it at the chroma frequency. This will lower the lumi-

nance resolution of details whose spatial frequency causes

frequency components in the chroma band. However, the

attenuation of cross chrominance outweighs this in the picture

quality. S-video will not just eliminate cross chrominance, but

will also not have this notch in the luma response.

Implementing a Luma Trap

The AD725 implementation of a luma trap uses an on-chip

resistor along with an off-chip inductor and capacitor to create

an RLC notch filter. The filter must be tuned to the center

frequency of the video standard being output by the AD725,

3.58 MHz for NTSC or 4.43 MHz for PAL.

The circuit is shown in Figure 22. The 1 k

Ω series resistor in

the composite video luma path on the AD725 works against the

impedance of the off-chip series LC to form a notch filter. The

frequency of the filter is given by:

π LC

1.0k

14.318180MHz

YTRAP

LUMA

COMP

220 F 75

CRMA

220 F 75

4FSC

17.734475MHz

STND

68 H

18pF

47k

1N4148

NTSC/

PAL

AD725

9pF

Figure 22. Luma Trap Circuit for NTSC and PAL Video

Dual-Standard Luma Trap

For a filter that will work for both PAL and NTSC a means is

required to switch the tuning of the filter between the two

subcarrier frequencies. The PAL standard requires a higher

frequency than NTSC. A basic filter can be made that is tuned

to the PAL subcarrier and a simple diode circuit can then be

used to switch in an extra parallel capacitor that will lower the

filter’s frequency for NTSC operation.

Figure 22 shows how the logic signal that drives STND (Pin 1)

can also be used to drive the circuit that selects the tuning of the

luma trap circuit. When the signal applied to STND (Pin 1) is

low (ground), the PAL mode is selected. This results in a bias of

0 V across D1, which is an off condition. As a result, C2 is out

of the filter circuit and only C1 tunes the notch filter to the PAL

subcarrier frequency, 4.43 MHz.

On the other hand, when STND is high (+5 V), NTSC is se-

lected and there is a forward bias across D1. This turns the

diode on and adds C2 in parallel with C1. The notch filter is

now tuned to the NTSC subcarrier frequency, 3.58 MHz.

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