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AD725 Datasheet(PDF) 15 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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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
220 F 75
220 F 75
220 F 75
68 H
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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