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AD722 Datasheet(PDF) 7 Page - Analog Devices

Part No. AD722
Description  RGB to NTSC/PAL Encoder
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD722
REV. 0
–7–
9.72µs
5.49µs
4.59µs
39.7 IRE
33.8 IRE
9.0 CYCLES
H TIMING MEASUREMENT RS–170A (NTSC)
FIELD = 1 LINE = 22
AVERAGE 256
≥ 256
124ns
100ns
Figure 12. Horizontal Timing, NTSC
5.59µs
AVERAGE 256
≥ 256
102ns
94ns
2.28µs
4.60µs
293.5mV
249.0mV
H TIMING (PAL)
LINE = 25
Figure 13. Horizontal Timing, PAL
PHASE
DETECTOR
CHARGE
PUMP
FILTER
LOOP
XOSC
4FSC
VCO
SYNC
SEPARATOR
XNOR
XOSC
CHARGE
PUMP
FILTER
LOOP
4FSC
VCO
XNOR
4FSC
4FSC
FSC
SUB-
CARRIER
NTSC/PAL
HSYNC
VSYNC
BURST
QUADRATURE
+4
DECODER
±180°
(PAL ONLY)
NTSC/PAL
FSC 90
°
FSC 0
°
4FSC
SC 90
°/270°
FSC
CSYNC
CSYNC
DC
CLAMP
RED
GREEN
BLUE
RGB-TO-YUV
ENCODING
MATRIX
BURST
Y
3-POLE
LP PRE-
FILTER
U
V
4-POLE
LPF
4-POLE
LPF
BALANCED
MODULATORS
3-POLE LPF
3.6MHz (NTSC)
4.4MHz (PAL)
NTSC/PAL
CSYNC
INSERTION
SAMPLED-
DATA
DELAY
LINE
5 MHz
2-POLE
LP POST-
FILTER
X2
X2
X2
LUMINANCE
OUTPUT
COMPOSITE
OUTPUT
CHROMINANCE
OUTPUT
U
CLAMP
V
CLAMP
NTSC/PAL
CLOCK
AT 4FSC
POWER AND GROUNDS
+5V
+5V
AGND
DGND
LOGIC
ANALOG
ANALOG
LOGIC
DC
CLAMP
DC
CLAMP
NOTE:
THE LUMINANCE, COMPOSITE, AND
CHROMINANCE OUTPUTS ARE AT
TWICE NORMAL LEVELS FOR DRIVING
75
Ω REVERSE-TERMINATED LINES.
Figure 14. Functional Block Diagram
THEORY OF OPERATION
The AD722 was designed to have three allowable modes of ap-
plying a clock via the FIN pin. These are FSC (frequency of
subcarrier, 3.579545 MHz for NTSC or 4.433618 MHz for
PAL) mode with CMOS clock applied, FSC mode using on-
chip crystal oscillator, and 4FSC mode with CMOS clock ap-
plied. To use FSC mode SELECT is pulled low and then either
a CMOS FSC clock is applied to FIN, or a parallel-resonant
crystal and appropriate tuning capacitor is placed between FIN
pin and GND to utilize the on-chip oscillator. The on-chip
Phase Locked Loop (PLL) is used in these modes to generate an
internal 4FSC which is divided to perform the digital clocking
as well as to create the quadrature subcarrier signals for the
chrominance modulation. In 4FSC mode the PLL is bypassed.
Referring to the AD722 block diagram (Figure 14), the RGB in-
puts (each 714 mV p-p max for NTSC or 700 mV p-p max for
PAL) are ac-coupled and then pass through dc clamps. These
clamps allow the user to have a black level which is not at 0 V.
The clamps will clamp to a black input signal level between 0 V
and 3 V. The clamping occurs just after the falling edge of
HSYNC.
The RGB inputs then pass into an analog encoding matrix
which creates the luminance (“Y”) signal and the chrominance
color difference (“U” and “V”) signals. The RGB to YUV en-
coding is performed using the following standard transformation:
Y = 0.299
× R + 0.587 × G + 0.114 × B
U = 0.493
× (B–Y)
V = 0.877
× (R–Y)
After the encoding matrix, the AD722 has two parallel analog
paths. The Y (luminance) signal is first passed through a 3-pole
4.85 MHz/6 MHz (NTSC/PAL) Bessel low-pass filter to pre-
vent aliasing in the sampled-data delay line. This first low-pass
filter is also where the unclocked analog sync signal is injected
into the Y signal (more on the creation of this sync signal to fol-
low). The Y signal then passes through the sampled-data delay
line, which is clocked at 4FSC. The delay line was designed to
give an overall chrominance to luminance delay of –170 ns. Fol-
lowing the sampled-data delay line is a 5.25 MHz/6.5 MHz
(NTSC/PAL) 2-pole low-pass Bessel filter to smooth the recon-
structed luminance signal.




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