a
RGB to NTSC/PAL Encoder
AD722
Analog Devices, Inc., 1995
One Technology Way, P.O. Box 9106, Norwood. MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
FEATURES
Low Cost, Integrated Solution
+5 V Operation
Accepts FSC Clock or Crystal, or 4FSC Clock
Composite Video and Separate Y/C (S-Video) Outputs
Minimal External Components:
No External Filters or Delay Lines Required
Onboard DC Restoration
Accepts Either HSYNC & VSYNC or CSYNC
Phase Lock to External Subcarrier
Drives 75
Reverse-Terminated Loads
Logic Selectable NTSC or PAL Encoding Modes
Compact 16-Pin SOIC
APPLICATIONS
RGB to NTSC or PAL Encoding
PRODUCT DESCRIPTION
The AD722 is a low cost RGB to NTSC/PAL Encoder that
converts red, green and blue color component signals into their
corresponding luminance (baseband amplitude) and chromi-
nance (subcarrier amplitude and phase) signals in accordance
with either NTSC or PAL standards. These two outputs are
also combined to provide composite video output. All three out-
puts can simultaneously drive 75
, reverse-terminated cables.
All logical inputs are CMOS compatible. The chip operates
from a single +5 V supply. No external delay lines or filters are
required. The AD722 may be powered down when not in use.
The AD722 accepts either FSC or 4FSC clock. When a clock is
not available, a low cost parallel-resonant crystal (3.58 MHz
(NTSC) or 4.43 MHz (PAL)) and the AD722's on-chip oscilla-
tor generate the necessary subcarrier clock. The AD722 also ac-
cepts the subcarrier clock from an external video source.
The interface to VGA Controllers and MPEG Video Decoders
is simple: an on-chip logic "XNOR" accepts the available verti-
cal (VSYNC) and horizontal sync (HSYNC) signals and creates
the composite sync (CSYNC) signal on-chip. If available, the
AD722 will also accept a standard CSYNC signal by connecting
VSYNC to +5 V and applying CSYNC to HSYNC pin. The
AD722 contains decoding logic to identify valid HSYNC pulses
for correct burst insertion.
Delays in the U and V chroma filters are matched by an on-chip
sampled-data delay line in the Y signal path. To prevent
aliasing, a prefilter at 5 MHz is included ahead of the delay line
and a post-filter at 5 MHz is added after the delay line to sup-
press harmonics in the output. These low-pass filters are opti-
mized for minimum pulse overshoot. The overall luma delay,
relative to chroma, has been designed to be 170 ns, which
precompensates for delays in the filters used in the IF section of
a television receiver. This precompensation delay is already
present in TV broadcasts. The AD722 comes in a space-saving
SOIC and is specified for the 0
C to +70
C commercial tem-
perature range.
FUNCTIONAL BLOCK DIAGRAM
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
2-POLE
LP POST-
FILTER
X2
X2
X2
LUMINANCE
OUTPUT
COMPOSITE
OUTPUT
CHROMINANCE
OUTPUT
U
CLAMP
V
CLAMP
NTSC/PAL
CLOCK
AT 4FSC
DC
CLAMP
DC
CLAMP
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
AD722SPECIFICATIONS
REV. 0
2
(Unless otherwise noted, V
S
= +5, T
A
= +25
C, using FSC synchronous clock. All loads are
150
5% at the IC pins. Outputs are measured at the 75
reverse terminated load.)
Parameter
Conditions
Min
Typ
Max
Units
SIGNAL INPUTS (RDIN, GRIN, BLIN)
Input Amplitude
NTSC
714
mV p-p
PAL
700
mV p-p
Black Level
0
3
V
Input Resistance
1
Red, Green, Blue
1
M
Input Capacitance
5
pF
LOGIC INPUTS (SYNC, FSC, ENCD, NTSC)
CMOS Logic Levels
Logic LO Input Voltage
1
V
Logic HI Input Voltage
4
V
Logic LO Input Current (DC)
<1
A
Logic HI Input Current (DC)
<1
A
VIDEO OUTPUTS
2
Luminance (LUMA)
Roll-off @ 5 MHz
NTSC
10
dB
PAL
7
dB
Gain Error
15
5
+15
%
Linearity
0.6
%
Sync Level
NTSC
243
286
329
mV
PAL
300
mV
DC Black Level
1.3
V
Chrominance (CRMA)
Bandwidth
NTSC
3.6
MHz
PAL
4.4
MHz
Color Burst Amplitude
NTSC
170
240
330
mV p-p
PAL
252
mV
Color Signal to Burst Ratio Error
15
3
15
%
Color Burst Width
NTSC
2.51
s
PAL
2.28
s
Phase Error
3
3
Degrees
DC Black Level
2.1
V
Chroma Feedthrough
R, G, B = 0
10
40
mV p-p
Chroma/Luma Time Alignment
140
ns
Composite (COMP)
Absolute Gain Error
5
1
5
%
Differential Gain
With Respect to Chroma
0.5
%
Differential Phase
With Respect to Chroma
2.0
%
DC Black Level
1.6
V
POWER SUPPLIES
Recommended Supply Range
Single Supply
+4.75
+5.25
V
Quiescent Current--Encode Mode
30
40
mA
Quiescent Current--Power Down
1
mA
NOTES
1
R, G, and B signals are inputted to an on-chip AC coupling capacitor.
2
All outputs measured at a 75
reverse-terminated load; voltages at the IC output pins are twice those specified here.
3
Difference between ideal color-bar phases and the actual values.
Specifications are subject to change without notice.
AD722
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3
ORDERING GUIDE
Temperature
Package
Package
Model
Range
Description
Option
AD722JR-16
0
C to +70
C
16-Pin SOIC R-16
AD722JR-16-Reel
0
C to +70
C
16-Pin SOIC R-16
AD722JR-16-Reel7
0
C to +70
C
16-Pin SOIC R-16
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage V
S
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +6 V
Internal Power Dissipation . . . . . . . . . . . . . . . . . . . . . . 600 mW
Operating Temperature Range . . . . . . . . . . . . . . 0
C to +70
C
Storage Temperature Range . . . . . . . . . . . . . 65
C to +125
C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300
C
NOTE
*Stresses above those listed under "Absolute Maximum Ratings" may cause
permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other conditions above those indicated in the
operational section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
Thermal Characteristics: 16-Pin SOIC Package:
JA
= 100
C/W.
PIN CONFIGURATION
16-Pin Small Outline Package (Wide Body)
(R-16)
AGND
ENCD
RIN
GIN
FIN
APOS
BIN
STND
HSYNC
VSYNC
SELECT
LUMA
COMP
DPOS
DGND
CRMA
1
2
16
15
5
6
7
12
11
10
3
4
14
13
8
9
TOP VIEW
(Not to Scale)
AD722
WARNING!
ESD SENSITIVE DEVICE
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD722 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
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AD722
PIN DESCRIPTIONS
Pin
Mnemonic
Description
Equivalent Circuit
1
STND
A Logical HIGH input selects NTSC encoding.
Circuit A
A Logical LOW input selects PAL encoding.
CMOS Logic Levels.
2
AGND
Analog Ground Connection.
3
FIN
FSC clock or parallel-resonant crystal, or 4FSC clock input.
Circuit B
For NTSC: 3.579 545 MHz or 14.318 180 MHz.
For PAL: 4.433 619 MHz or 17.734 480 MHz.
CMOS Logic Levels for subcarrier clocks.
4
APOS
Analog Positive Supply (+5 V
5%).
5
ENCD
A Logical HIGH input enables the encode function.
Circuit A
A Logical LOW input powers down chip when not in use.
CMOS Logic Levels.
6
RIN
Red Component Video Input.
Circuit C
0 to 714 mV for NTSC; 0 to 700 mV for PAL.
7
GIN
Green Component Video Input.
Circuit C
0 to 714 mV for NTSC.
0 to 700 mV for PAL.
8
BIN
Blue Component Video Input.
Circuit C
0 to 714 mV for NTSC.
0 to 700 mV for PAL.
9
CRMA
Chrominance Output (Subcarrier Only).*
Circuit D
Approximately 1.8 V peak-to-peak for both NTSC and PAL.
10
COMP
Composite Video Output.*
Circuit D
Approximately 2.5 V peak-to-peak for both NTSC and PAL.
11
LUMA
Luminance plus SYNC Output.*
Circuit D
Approximately 2 V peak-to-peak for both NTSC and PAL.
12
SELECT
A Logical LOW input selects the FSC operating mode.
Circuit A
A Logical HIGH input selects the 4FSC operating mode.
CMOS Logic Levels.
13
DGND
Digital Ground Connections.
14
DPOS
Digital Positive Supply (+5 V
5%) .
15
VSYNC
Vertical Sync Signal (if using external CSYNC set at +5 V).
Circuit A
16
HSYNC
Horizontal Sync Signal (or CSYNC signal).
Circuit A
*The Luminance, Chrominance, and Composite Outputs are at twice normal levels for driving 75
reverse-terminated lines.
Equivalent Circuits
5
1
12
15
16
DPOS
3
V
BIAS
APOS
10
9
11
APOS
7
V
CLAMP
6
8
APOS
4pF
TYP
Circuit B
Circuit D
Circuit C
Circuit A
REV. 0
5
IRE
s
0.10
0.5
0.5
0
60
10
20
30
40
50
0.0
APL = 50.8%
525 LINE NTSC NO FILTERING
SLOW CLAMP TO 0.00V @ 6.63s
PRECISION MODE OFF
SYNCHRONOUS SYNC = SOURCE
FRAMES SELECTED : 1 2
100
50
0
50
VOLTS
Figure 4. 100% Color Bars, NTSC
s
0.10
0.5
0.5
0
60
10
20
30
40
50
0.0
VOLTS
APL = 50.6%
625 LINE PAL NO FILTERING
SLOW CLAMP TO 0.00V @ 6.72s
PRECISION MODE OFF SOUND-IN-SYNC OFF
SYNCHRONOUS SYNC = SOURCE
FRAMES SELECTED : 1 2 3 4
Figure 5. 100% Color Bars, PAL
Figure 6. 100% Color Bars on Vector Scope, NTSC
RGB
3
75
75
TEKTRONIX
TSG 300
COMPONENT
VIDEO
WAVEFORM
GENERATOR
AD722
RGB TO
NTSC/PAL
ENCODER
SONY
MONITOR
MODEL
1342
COMPOSITE
VIDEO
COMPOSITE
SYNC
TEKTRONIX
VM700A
WAVEFORM
MONITOR
TEKTRONIX
1910
COMPOSITE
VIDEO
WAVEFORM
GENERATOR
FSC
GENLOCK
Figure 1. Evaluation Setup
s
0.10
0.5
0.5
0
60
10
20
30
40
50
0.0
APL = 49.5%
525 LINE NTSC NO FILTERING
SLOW CLAMP TO 0.00V @ 6.63s
PRECISION MODE OFF
SYNCHRONOUS SYNC = SOURCE
FRAMES SELECTED : 1 2
VOLTS
100
50
0
IRE
50
Figure 2. Modulated Pulse and Bar, NTSC
s
0.10
0.5
0.5
0
60
10
20
30
40
50
0.0
APL = 49.7%
625 LINE PAL NO FILTERING
SLOW CLAMP TO 0.00V @ 6.72s
PRECISION MODE OFF SOUND-IN-SYNC OFF
SYNCHRONOUS SYNC = SOURCE
FRAMES SELECTED : 1 2 3 4
VOLTS
Figure 3. Modulated Pulse and Bar, PAL
Typical CharacteristicsAD722
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6
AD722Typical Characteristics
1ST
2ND
3RD
4TH
5TH
6TH
0.00
0.16
0.49
0.53
0.52
0.38
0.00
0.44
1.14
1.01
0.53
0.01
DG DP (NTSC) (SYNC = EXT)
FIELD = 1 LINE = 27, 100 IRE RAMP
DIFFERENTIAL GAIN (%)
DIFFERENTIAL PHASE (deg)
MIN = 0.53
MAX = 0.00
pp/MAX = 0.53
MIN = 1.14
MAX = 0.00
pkpk = 1.14
0.2
0.0
0.2
0.4
0.6
0.8
0.5
0.0
0.5
1.0
1.5
Figure 10. Composite Output
Differential Phase and Gain, NTSC
1ST
2ND
3RD
4TH
5TH
6TH
0.00
0.14
0.32
0.16
0.04
0.10
0.00
1.01
1.18
0.44
0.42
0.70
0.3
0.2
0.1
0.0
0.1
0.2
0.3
0.4
0.5
1.5
1.0
0.5
0.0
0.5
1.0
1.5
2.0
DG DP (PAL) (SYNC = EXT)
LINE = 25, 700mV RAMP
DIFFERENTIAL GAIN (%)
DIFFERENTIAL PHASE (deg)
MIN = 0.32
MAX = 0.10
pkpk = 0.42
MIN = 1.18
MAX = 0.70
pkpk = 1.89
Figure 11. Composite Output
Differential Phase and Gain, PAL
Figure 7. 100% Color Bars on Vector Scope, PAL
IRE
s
0.10
0.5
0.5
0
60
10
20
30
40
50
0.0
VOLTS
100
50
0
50
APL = 11.6%
525 LINE NTSC NO FILTERING
SLOW CLAMP TO 0.00V @ 6.63s
PRECISION MODE OFF
SYNCHRONOUS SYNC = SOURCE
FRAMES SELECTED : 1 2
Figure 8. Multipulse, NTSC
s
0.10
0.5
0.5
0
60
10
20
30
40
50
0.0
VOLTS
APL = 11.4%
625 LINE PAL NO FILTERING
SLOW CLAMP TO 0.00V @ 6.72s
PRECISION MODE OFF SOUND-IN-SYNC OFF
SYNCHRONOUS SYNC = SOURCE
FRAMES SELECTED : 1 2 3 4
Figure 9. Multipulse, PAL
AD722
REV. 0
7
9.72s
5.49s
4.59s
39.7 IRE
33.8 IRE
9.0 CYCLES
H TIMING MEASUREMENT RS170A (NTSC)
FIELD = 1 LINE = 22
AVERAGE 256
256
124ns
100ns
Figure 12. Horizontal Timing, NTSC
5.59s
AVERAGE 256
256
102ns
94ns
2.28s
4.60s
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
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
(BY)
V = 0.877
(RY)
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.
REV. 0
8
AD722
The other analog path is the chrominance path which is where
the U and V color difference signals are processed. The U and V
signals first pass through 4-pole modified Bessel low-pass filters
with 3 dB frequencies of 1.2 MHz/1.5 MHz (NTSC/PAL) to
prevent aliasing in the modulators. The color burst signal is in-
jected into the U and V channels in these premodulation filters.
The U and V signals are then modulated independently by a
pair of balanced switching modulators driven in quadrature by
the color subcarrier.
The bandwidths of all the on-chip filters are tuned using propri-
etary auto-tuning circuitry. The basic principle is to match an
RC time constant to a reference time period, that time being
one cycle of a subcarrier clock. The auto-tuning is done during
the vertical blanking interval and has some added hysteresis so
that once an acceptable tuning value is reached the part won't
toggle between tuning values from field to field. The band-
widths stated in the above discussion are the design target band-
widths for NTSC and PAL.
The AD722's 4FSC clock (either produced by the on-chip PLL
or user supplied) drives a digital divide-by-4 circuit to create the
quadrature signals for modulation. The reference phase 0
is
used for the U signal. In the NTSC mode, the V signal is modu-
lated at 90
, but in PAL mode, the V modulation alternates be-
tween 90
and 270
at half the line rate as required by the PAL
standard. The outputs of the U and V balanced modulators
are summed and passed through a 3-pole low-pass filter with
3.6 MHz/4.4 MHz bandwidths (NTSC/PAL) in order to re-
move the harmonics generated during the switching modulation.
The filtered chrominance signal is then summed with the fil-
tered luminance signal to create the composite video signal. The
separate luminance, chrominance, and composite video signals
are amplified by a factor of two in order to drive 75
reverse-
terminated lines. The separate luminance and chrominance out-
puts together are known as S-video. The composite and S-video
outputs are simultaneously available.
The two sync inputs HSYNC and VSYNC are fed into an
XNOR gate to create a CSYNC signal for the AD722. If the
user produces, or has access to, a true composite sync signal, it
can be input to the HSYNC pin while the VSYNC pin is held
high. In either case the CSYNC signal which is present after the
XNOR gate, is used to generate the sync and burst signals
which ultimately get injected into the analog signal chain. The
unclocked CSYNC signal is sent to a reference cell on the chip
which, when CSYNC is low, allows a reference voltage (based
on a power supply division) to be injected into the luminance
chain. The width of the injected sync is the same as the width of
the supplied sync signal.
The CSYNC signal (after the XNOR gate) also goes to the digi-
tal section of the AD722 where it is clocked in by a 2FSC clock.
The digital section then measures the width of the CSYNC
pulses to separate horizontal pulses from vertical equalizing and
serration pulses. A burst flag is generated only after valid hori-
zontal sync pulses and is timed from the falling edge of the
clocked-in CSYNC signal. In synchronous systems (those in
which the subcarrier clock, sync signals, and RGB signals are all
synchronous) this will give a fixed burst position relative to the
falling edge of the output sync. However, in asynchronous sys-
tems the sync to burst position can change line to line by as
much as 140 ns (the period of a 2FSC clock cycle) due to the
fact that the burst flag is generated from a clocked CSYNC while
the sync is injected unclocked. This phenomenon may or may not
create visual artifacts in some high-end video systems. The burst
flag which is generated goes to the reference cell and allows a refer-
ence voltage to be inserted to the U and V low-pass filters.
APPLYING THE AD722
Inputs
RIN, BIN, GIN are analog inputs that should be terminated to
ground with 75
in close proximity to the IC. When properly
terminated the peak-to-peak voltage for a maximum input level
should be 714 mV p-p for NTSC or 700 mV p-p for PAL. The
horizontal blanking interval should be the most negative part of
each signal.
The signal should be flat during the horizontal blanking interval.
Internal circuitry will clamp this level during HSYNC to a refer-
ence that is used internally as the black level. The horizontal
blanking level at the input pins can range between 0 V and 3 V
with respect to the ground level of the AD722.
HSYNC and VSYNC are two logic level inputs that are com-
bined internally to produce a composite sync signal. If a com-
posite sync signal is to be used, it can be input to HSYNC while
VSYNC is pulled to logic HI (+5 V).
The form of the input sync signal(s) will determine the form of
the composite sync on the composite video (COMP) and lumi-
nance (LUMA) outputs. If no equalization or serration pulses
are included in the HSYNC input there won't be any in the out-
puts. Although sync signals without equalization and serration
pulses do not technically meet the video standards' specifica-
tions, many monitors do not require these pulses in order to
display good pictures. The decision whether to include these
signals is a system tradeoff between cost and complexity and
adhering strictly to the video standards.
The SELECT input is a CMOS logic level that programs the
AD722 to use a subcarrier at a 1FSC (LO) frequency or a
4FSC (HI) frequency for the appropriate standard being used.
A 4FSC clock is used directly, while a 1FSC input is multiplied
up to 4FSC by an internal phase locked loop.
The FIN input can be a logic level clock at either FSC or 4FSC
frequency or can be a parallel resonant crystal at 1FSC fre-
quency. An on-chip oscillator will drive the crystal. Most crys-
tals will require a shunt capacitance of between 10 pF and 30
pF for reliable start up and proper frequency of operation.
The NTSC specification calls for a frequency accuracy of
10 Hz
from the nominal subcarrier frequency of 3.579545 MHz. While
maintaining this accuracy in a broadcast studio might not be a
severe hardship, it can be quite expensive in a low cost con-
sumer application.
The AD722 will operate with subcarrier frequencies that deviate
quite far from those specified by the TV standards. However,
the monitor will in general not be quite so forgiving. Most moni-
tors can tolerate a subcarrier frequency that deviates several hun-
dred Hz from the nominal standard without any degradation in
picture quality. These conditions imply that the subcarrier fre-
quency accuracy is a system specification and not a specification
of the AD722 itself.
The STND pin is used to select between NTSC and PAL opera-
tion. Various blocks inside the AD722 use this input to program
their operation. Most of the more common variants of NTSC
and PAL are supported. There are, however, two known specific
standards which are not supported. These are NTSC 4.43 and
M-PAL.
AD722
REV. 0
9
resistor is required close to each AD722 output, while 75
to
ground should terminate the far end of each line.
The outputs have a dc bias and must be ac coupled for proper
operation. The COMP and LUMA outputs have information
down to 30 Hz that must be transmitted. Each output requires a
220
F series capacitor to work with the 75
resistance to pass
these low frequencies. The CRMA signal has information
mostly up at the chroma frequency and can use a smaller ca-
pacitor if desired, but 220
F can be used to minimize the num-
ber of different components used in the design.
Displaying VGA Output on a TV
The AD722 can be used to convert the analog RGB output from a
personal computer's VGA card to the NTSC or PAL television
standards. To accomplish this it is important to understand that
the AD722 requires interlaced RGB video and clock rates that
are consistent with those required by the television standards. In
most computers the default output is a noninterlaced RGB sig-
nal at a frame rate higher than used by either NTSC or PAL.
Most VGA controllers support a wide variety of output modes
that are controlled by altering the contents of internal registers.
It is best to consult with the VGA controller manufacturer to
determine the exact configuration required to provide an inter-
laced output at 60 Hz (50 Hz for PAL).
Basically these two standards use most of the features of the
standard that their names imply, but use the subcarrier that is
equal to or approximately equal to the frequency of the other
standard. Because of the automatic programming of the filters in
the chrominance path and other timing considerations, it is not
possible to support these standards.
Layout Considerations
The AD722 is an all CMOS mixed signal part. It has separate
pins for the analog and digital +5 V and ground power supplies.
Both the analog and digital ground pins should be tied to the
ground plane by a short, low inductance path. Each power
supply pin should be bypassed to ground by a low inductance
0.1
F capacitor and a larger tantalum capacitor of about 10
F.
The three analog inputs (RIN, GIN, BIN) should be terminated
with 75
to ground close to the respective pins. However, as
these are high impedance inputs, they can be in a loop-thru
configuration. This technique is used to drive two or more
devices with high frequency signals that are separated by some
distance. A connection is made to the AD722 with no local
termination, and the signals are run to another distant device
where the termination for these signals is provided.
The output amplitudes of the AD722 are double that required
by the devices that it drives. This compensates for the halving of
the signal levels by the required terminations. A 75
series
Figure 15. Interfacing the AD722 to the (Interlaced) VGA Port of a PC
+5V (V
AA
)
0.1F
75
649
649
75
649
649
75
649
649
AD813
5
6
7
11
12
13
14
9
10
8
1/3
1/3
5V
1
2
3
4
13
2
FIN
OSC
0.1F
+5V
**
*
CRYSTAL
AGND
DGND
14
15
7
9
10
11
1
4
5
AD722
ENCD
RIN
GIN
BIN
HSYNC
VSYNC
SELECT
STND
CRMA
LUMA
CMPS
APOS
DPOS
+
1030pF
0.1F
10F
0.1F
10F
75
75
JMP
*
PARALLELRESONANT
CRYSTAL; 3.579545MHz (NTSC)
OR 4.433620MHz (PAL)
CAPACITOR VALUE DEPENDS ON
CRYSTAL CHOSEN
**
FSC OR 4FSC CLOCK; 3.579545MHz,
14.31818MHz (NTSC) OR 4.433620MHZ,
17.734480MHz(PAL)
+
75
220F
COMPOSITE
VIDEO
+
75
220F
+
75
220F
Y
C
S-VIDEO
(Y/C VIDEO)
75
75
75
B
G
R
RGB MONITOR
VSYNC
HSYNC
FROM VGA PORT
+5V
+5V
10k
+5V
10k
SELECT
75
1/3
JMP
12
3
16
8
6
6
7
8
9
10
11
12
13
14
15
+
+5V
1
2
3
4
5
VGA OUTPUT
CONNECTOR
REV. 0
10
AD722
Figure 15 shows a circuit for connection to the VGA port of a
PC. The RGB outputs connect directly to the respective inputs
of the AD722. These signals should each be terminated to
ground with 75
.
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 AD722. 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 AD722. 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 AD813 is a triple video amplifier that can provide the nec-
essary 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 AD813 to drive the RGB monitor and disable the en-
coding function of the AD722 via Pin 5. HSYNC and VSYNC
are logic level signals that can drive both the AD722 and RGB
monitor in parallel.
AD722 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
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 DACavailable in 48-pin TQFP).
The analog current outputs from the DAC are terminated to
ground at both ends with 75
as called for in the data sheet.
These signals directly feed the analog inputs of the AD722. The
HSYNC and VSYNC signals from the MPEG Controller are di-
rectly applied to the AD722.
If the set of termination resistors closest to the AD722 are re-
moved, an RGB monitor can be connected to these signals and
it will provide the required second termination. This scenario is
acceptable as long as the RGB monitor is always present and
connected. If it is to be removed on occasion, another termina-
tion scheme is required.
The AD813 triple video op amp can provide buffering for such
applications. Each channel is set for a gain of two while the out-
puts are back terminated with a series 75
resistor. This pro-
vides the proper signal levels at the monitor which terminates
the lines with 75
.
AD722 APPLICATION DISCUSSION
Chrominance and Luminance Alignment
Inside the AD722 the chrominance and luminance signals are pro-
cessed by separate paths. They both are either output separately
(Y/C), or they are added together by the composite video ampli-
fier. Although both channels are filtered, the chrominance signal
experiences a greater filtering delay due to the higher order of
the chrominance path filters. To compensate, a sampled delay
line is used in the luminance path.
For baseband video it is desirable for the chrominance and lu-
minance to be accurately aligned with zero offset. However, the
situation for modulated RF video is a bit more complicated due
to the effects of the IF circuitry used in TV sets.
Figure 16. AD722 and ADV7120/ADV7122 Providing MPEG Video Solution
13
2
FIN
AGND
DGND
15
16 HSYNC
VSYNC
HSYNC
VSYNC
9
COMP
+
75
220F
COMPOSITE
VIDEO
10
11
CRMA
LUMA
+
75
220F
+
75
220F
S-VIDEO
12
1
10k
5
AD722
ENCD
SELECT
STND
APOS
DPOS
0.1F
0.01F
1030pF
L1 (FERRITE BEAD)
+5V (V
AA
)
10F
33F
+5V (V
CC
)
GND
ADV7120
SYNC
CLOCK
BLANK
GND
24
DATA IN
HSYNC
VSYNC
+5V (V
AA
)
10k
+5V
10k
0.1F
0.01F
R
SET
550
0.1F
+5V
V
AA
V
REF
FS
ADJ
COMP
0.1F
+5V (V
AA
)
8
RIN
GIN
BIN
75
75
75
IOG
IOR
IOB
3
MPEG
DECODER
6
7
+5V
10k
AD589
(1.2V REF)
14
4
+5V
OSC
0.1F
+5V
CRYSTAL
75
75
75
*
PARALLELRESONANT
CRYSTAL; 3.579545MHz (NTSC)
OR 4.433620MHz (PAL)
CAPACITOR VALUE DEPENDS ON
CRYSTAL CHOSEN
**
FSC OR 4FSC CLOCK; 3.579545MHz,
14.31818MHz (NTSC) OR 4.433620MHZ,
17.734480MHz(PAL)
**
*
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
s. Thus, the chrominance offset distance will be the
width of the screen times (50
s/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.
REV. 0
12
AD722
Synchronous vs. Asynchronous Operation
The source of RGB video and synchronization used as an input
to the AD722 in some systems is derived from the same clock
signal as used for the AD722 subcarrier input (FIN). 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 AD722 supports both synchronous and asynchronous op-
eration, but some minor differences might be noticed between
them. These can be caused by some details of the internal
circuitry of the AD722.
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 de-
lay line used in the luminance channel to time align the lumi-
nance and chrominance. This delay line uses a signal at twice
the subcarrier frequency as its clock.
The phasing between the delay line clock and the luminance sig-
nal (with inserted composite sync) will be constant during syn-
chronous 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.
C2031105/95
PRINTED IN U.S.A.
1
2
2
1
3
3
4
4
5
6
5
6
7
7
=
+
NONINTERLACED
ODD FIELD
EVEN FIELD
a. Conversion of Noninterlace to Interlace
2
1
3
4
5
6
7
=
+
NONINTERLACED
ODD FIELD
EVEN FIELD
1
2
3
4
5
6
7
b. Line Doubled Conversion Technique
2
1
3
4
5
6
7
=
+
NONINTERLACED
ODD FIELD
EVEN FIELD
1
2
3
4
5
6
7
c. Line Averaging Technique
Figure 17.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
16-Lead SOIC (R-16) (Wide Body)
PIN 1
0.2992 (7.60)
0.2914 (7.40)
0.4193 (10.65)
0.3937 (10.00)
1
16
9
8
0.0192 (0.49)
0.0138 (0.35)
0.0500 (1.27)
BSC
0.1043 (2.65)
0.0926 (2.35)
0.4133 (10.50)
0.3977 (10.00)
0.0118 (0.30)
0.0040 (0.10)
0.0125 (0.32)
0.0091 (0.23)
0.0500 (1.27)
0.0157 (0.40)
8
0
0.0291 (0.74)
0.0098 (0.25)
x 45
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