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AD722 Datasheet(PDF) 11 Page - Analog Devices

Part No. AD722
Description  RGB to NTSC/PAL Encoder
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Maker  AD [Analog Devices]
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AD722
REV. 0
–11–
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.




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