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AD724 Datasheet(PDF) 11 Page - Analog Devices

Part No. AD724
Description  RGB to NTSC/PAL Encoder
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD724
REV. B
–11–
video from an MPEG decoder and creating both analog RGB
video and composite video.
The 24-bit wide RGB video is converted to analog RGB by
the ADV7120 (Triple 8-bit video DAC—available in 48-lead
LQFP). The analog current outputs from the DAC are termi-
nated to ground at both ends with 75
Ω as called for in the data
sheet. These signals are ac coupled to the analog inputs of the
AD724. The HSYNC and VSYNC signals from the MPEG
Controller are directly applied to the AD724.
If the set of termination resistors closest to the AD724 are re-
moved, an RGB monitor can be connected to these signals and
will provide the required second termination. This is acceptable
as long as the RGB monitor is always present and connected. If
it is to be removed on occasion, another termination scheme is
required.
The AD8013 or AD8073 triple video op amp can provide buff-
ering for such applications. Each channel is set for a gain of two
while the outputs are back terminated with a series 75
Ω resis-
tor. This provides the proper signal levels at the monitor, which
terminates the lines with 75
Ω.
AD724 APPLICATION DISCUSSION—NTSC/PAL
CRYSTAL SELECT CIRCUIT
For systems that support both NTSC and PAL, and will use a
crystal for the subcarrier, a low cost crystal selection circuit can
be made that, in addition to the two crystals, requires two low
cost diodes, two resistors and a logic inverter gate. The circuit
selection can be driven by the STND signal that already drives
Pin 1 to select between NTSC and PAL operation for the AD724.
A schematic for such a circuit is shown in Figure 17. Each crys-
tal ties directly to FIN (Pin 3) with one terminal and has the
other terminal connected via a series diode to ground. Each
diode serves as a switch, depending on whether it is forward
biased or has no bias.
FIN
AGND
DGND
HSYNC
VSYNC
HSYNC
VSYNC
COMP
75
220 F
COMPOSITE
VIDEO
CRMA
LUMA
75
220 F
75
220 F
S-VIDEO
10k
AD724
ENCD
SELECT
STND
APOS
DPOS
0.1 F
0.01 F
10–30pF
L1 (FERRITE BEAD)
+5V (VAA)
10 F
33 F
+5V (VCC)
GND
ADV7120
SYNC
CLOCK
BLANK
GND
24
DATA IN
HSYNC
VSYNC
+5V (VAA)
10k
+5V
10k
0.1 F
0.01 F
RSET
550
0.1 F
+5V
VAA
VREF
FSADJ
COMP
0.1 F
+5V (VAA)
RIN
GIN
BIN
75
75
75
IOG
IOR
IOB
+5V
10k
AD589
(1.2V REF)
+5V
0.1 F
+5V
CRYSTAL
* PARALLEL–RESONANT CRYSTAL; 3.579545MHz (NTSC)
OR 4.433620MHz (PAL) CAPACITOR VALUE DEPENDS
ON CRYSTAL CHOSEN
**
*
75
75
75
***
*** 0.1 F CAPACITORS RECOMMENDED
**FSC OR 4FSC CLOCK; 3.579545MHz, 14.31818MHz (NTSC)
OR 4.433620MHz, 17.734480MHz(PAL)
OSC
MPEG
DECODER
0.1 F
0.1 F
0.1 F
Figure 16. AD724 and ADV7120/ADV7122 Providing MPEG Video Solution
Figure 15 shows a circuit for connection to the VGA port of a
PC. The RGB outputs are ac coupled to the respective inputs of
the AD724. These signals should each be terminated to ground
with 75
Ω.
The standard 15-pin VGA connector has HSYNC on Pin 13
and VSYNC on Pin 14. These signals also connect directly to
the same name signals on the AD724. The FIN signal can be
provided by any of the means described elsewhere in the data
sheet. For a synchronous NTSC system, the internal 4FSC
(14.31818 MHz) clock that drives the VGA controller can be
used for FIN on the AD724. This signal is not directly accessible
from outside the computer, but it does appear on the VGA card.
If a separate RGB monitor is also to be used, it is not possible to
simply connect it to the R, G and B signals. The monitor pro-
vides a termination that would double terminate these signals.
The R, G and B signals should be buffered by three amplifiers
with high input impedances. These should be configured for a
gain of two, which is normalized by the divide-by-two termina-
tion scheme used for the RGB monitor.
The AD8013 is a triple video amplifier that can provide the
necessary buffering in a single package. It also provides a disable
pin for each amplifier, which can be used to disable the drive to
the RGB monitor when interlaced video is used (SELECT = LO).
When the RGB signals are noninterlaced, setting SELECT HI will
enable the AD8013 to drive the RGB monitor and disable the
encoding function of the AD724 via Pin 5. HSYNC and VSYNC
are logic level signals that can drive both the AD724 and RGB
monitor in parallel. If the disable feature is not required, the
AD8073 triple video op amp can provide a lower cost solution.
AD724 Used with an MPEG Decoder
MPEG decoding of compressed video signals is becoming a
more prevalent feature in many PC systems. To display images
on the computer monitor, video in RGB format is required.
However, to display the images on a TV monitor, or to record
the images on a VCR, video in composite format is required.
Figure 16 shows a schematic for taking the 24-bit wide RGB




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