AD722 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD722

RGB to NTSC/PAL Encoder

Analog Devices 

AD722

The IF strips used in TVs delay the chrominance by 170 ns

more than the luminance. To compensate for this, transmitted

video has the chrominance lead the luminance by 170 ns. The

term used for this is chrominance delay, and it is specified as

–170 ns, the negative (–) indicating that chrominance leads the

luminance. This correction to TV broadcasts was made in the

early days of TV and is the standard to this day.

The delay line used in the luminance path of the AD722 creates

a –170 ns chrominance delay. This will be realigned by the RF

section of a TV when it is used for receiving the signal.

However, for baseband inputs, the chrominance will lead the

luminance by a small amount. This will show up as a slight

color shadow to the left of objects. The physical offset can be

calculated by approximating the active horizontal line time of a

TV as 50 µs. Thus, the chrominance offset distance will be the

width of the screen times (50 µs/170 ns) or 0.0034. For a 13

inch monitor the screen width is about 10 in. (25 cm), so the

offset distance will be 0.034 in. (0.85 mm).

Dot Crawl

There are numerous distortions that are apparent in the presen-

tation of composite NTSC signals on TV monitors. These ef-

fects will vary in degree depending on the circuitry used by the

monitor to process the signal and on the nature of the image be-

ing displayed. It is generally not possible to produce pictures on

a composite monitor that are as high quality as those produced

by standard quality RGB, VGA monitors.

One well known distortion of composite video images is called

dot crawl. It shows up as a moving dot pattern at the interface

between two areas of different color. It is caused by the inability

of the monitor circuitry to adequately separate the luminance

and chrominance signals.

One way to prevent dot crawl is to use a video signal that has

separate luminance and chrominance. Such a signal is referred

to as S-video or Y/C video. Since the luminance and chromi-

nance are already separated, the monitor does not have to per-

form this function. The S-Video outputs of the AD722 can be

used to create higher quality pictures when there is an S-Video

input available on the monitor.

Flicker

In a VGA conversion application, where the software controlled

registers are correctly set, there are two techniques that are com-

monly used by VGA controller manufacturers to generate the

interlaced signal. Each of these techniques introduces a unique

characteristic into the display created by the AD722. The arti-

facts described below are not due to the encoder or its encoding

algorithm as all encoders will generate the same display when

presented with these inputs. They are due to the method used

by the controller display chip to convert a noninterlaced output

to an interlaced signal. The method used is a feature of the de-

sign of the VGA chip and is not programmable.

The first interlacing technique outputs a true interlaced signal

with odd and even fields (one each to a frame Figure 17a). This

provides the best picture quality when displaying photography,

CD video and animation (games, etc.). However, it will intro-

duce a defect commonly referred to as flicker into the display.

Flicker is a fundamental defect of all interlaced displays and is

caused by the alternating field characteristic of the interlace

technique. Consider a one pixel high black line which extends

horizontally across a white screen. This line will exist in only

one field and will be refreshed at a rate of 30 Hz (25 Hz for

PAL). During the time that the other field is being displayed the

line will not be displayed. The human eye is capable of detect-

ing this, and the display will be perceived to have a pulsating or

flickering black line. This effect is highly content sensitive and

is most pronounced in applications in which text and thin

horizontal lines are present. In applications such as CD video,

photography and animation, portions of objects naturally

occur in both odd and even fields and the effect of flicker is

imperceptible.

The second technique which is commonly used is to output an odd

and even field which are identical (Figure 17b). This ignores the

data which naturally occurs in one of the fields. In this case the

same one pixel high line mentioned above would either appear

as a two pixel high line, (one pixel high in both the odd and even

field) or will not appear at all if it is in the data which is ignored by

the controller. Which of these cases occurs is dependent on the

placement of the line on the screen. This technique provides a

stable (i.e., nonflickering) display for all applications, but small

text can be difficult to read and lines in drawings (or spread-

sheets) can disappear. As above, graphics and animation are not

particularly affected although some resolution is lost.

There are methods to dramatically reduce the effect of flicker and

maintain high resolution. The most common is to ensure that dis-

play data never exists solely in a single line. This can be accom-

plished by averaging/weighting the contents of successive/multiple

noninterlaced lines prior to creating a true interlaced output (Fig-

ure 17c). In a sense this provides an output which will lie between

the two extremes described above. The weight or percentage of one

line that appears in another and the number of lines used are vari-

ables that must be considered in developing a system of this type. If

this type of signal processing is performed, it must be completed

prior to the data being presented to the AD722 for encoding.

Vertical Scaling

In addition to converting the computer generated image from

noninterlaced to interlaced format, it is also necessary to scale

the image down to fit into NTSC or PAL format. The most

common vertical lines/screen for VGA display are 480 and 600

lines. NTSC can only accommodate approximately 400 visible

lines/frame (200 per field), PAL can accommodate 576 lines/

frame (288 per field). If scaling is not performed, portions of

the original image will not appear in the television display.

This line reduction can be performed by merely eliminating ev-

ery Nth (6th line in converting 480 lines to NSTC or every 25th

line in converting 600 lines to PAL). This risks generation of

jagged edges and jerky movement. It is best to combine the scaling

with the interpolation/averaging technique discussed above to

ensure that valuable data is not arbitrarily discarded in the scaling

process. Like the flicker reduction technique mentioned above,

the line reduction must be accomplished prior to the AD722 en-

coding operation.

There is a new generation of VGA controllers on the market

specifically designed to utilize these techniques to provide a

crisp and stable display for both text and graphics oriented ap-

plications. In addition these chips rescale the output from the

computer to fit correctly on the screen of a television. A list of

known devices is available through Analog Devices’ Applications

group, but the most complete and current information will be

available from the manufacturers of graphics controller ICs.

REV. 0

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