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AD725 Datasheet(PDF) 14 Page - Analog Devices

Part No. AD725
Description  Low Cost RGB to NTSC/PAL Encoder with Luma Trap Port
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD725
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
every 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 men-
tioned above, the line reduction must be accomplished prior to
the AD725 encoding 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
applications. 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’ Applica-
tions group, but the most complete and current information will
be available from the manufacturers of graphics controller ICs.
Synchronous vs. Asynchronous Operation
The source of RGB video and synchronization used as an input
to the AD725 in some systems is derived from the same clock
signal as used for the AD725 subcarrier input (4FSC). These
systems are said to be operating synchronously. In systems
where two different clock sources are used for these signals, the
operation is called asynchronous.
The AD725 supports both synchronous and asynchronous
operation, but some minor differences might be noticed be-
tween them. These can be caused by some details of the inter-
nal circuitry of the AD725.
There is an attempt to process all of the video and synchroniza-
tion signals totally asynchronous with respect to the subcarrier
signal. This was achieved everywhere except for the sampled
delay line used in the luminance channel to time align the lumi-
nance and chrominance. This delay line uses a signal at eight
times the subcarrier frequency as its clock.
The phasing between the delay line clock and the luminance
signal (with inserted composite sync) will be constant during
synchronous operation, while the phasing will demonstrate a
periodic variation during asynchronous operation. The jitter of
the asynchronous video output will be slightly greater due to
these periodic phase variations.
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=+
NONINTERLACED
ODD FIELD
EVEN FIELD
a. Conversion of Noninterlace to Interlace
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NONINTERLACED
ODD FIELD
EVEN FIELD
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b. Line Doubled Conversion Technique
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NONINTERLACED
ODD FIELD
EVEN FIELD
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c. Line Averaging Technique
Figure 21.
LUMA TRAP-THEORY
The composite video output of the AD725 can be improved for
some types of images by incorporating a luma trap (or Y-Trap)
in the encoder circuit. The basic configuration for such a circuit
is a notch or band elimination filter that is centered at the
subcarrier frequency. The luma trap is only functional for the
composite video output of the AD725; it has no influence on
the S-Video (or Y/C-Video) output.
The need for a luma trap arises from the method used by com-
posite video to encode the color part (chrominance or chroma)
of the video signal. This is performed by amplitude and phase
modulation of a subcarrier. The saturation (or lack of dilution of
a color with white) is represented in the subcarrier’s amplitude
modulation, while the hue (or color as thought of as the sections
of a rainbow) information is contained in the subcarrier’s phase
modulation. The modulated subcarrier occupies a bandwidth
somewhat greater than 1 MHz depending on the video standard.
For a composite signal, the chroma is linearly added to the
luminance (luma or brightness) plus sync signal to form a single
composite signal with all of the picture information. Once this
addition is performed, it is no longer possible to ascertain which
component contributed which part of the composite signal.
At the receiver, this single composite signal must be separated
into its various parts to be properly processed. In particular, the
chroma must be separated and then demodulated into its or-
thogonal components, U and V. Then, along with the luma
signal, the U and V signals generate the RGB signals that con-
trol the three video guns in the monitor.
A basic problem arises when the luma signal (which contains no
color information) contains frequency components that fall




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