a
RGB to NTSC/PAL Encoder
AD724
operates from a single +5 V supply. No external delay lines or
filters are required. The AD724 may be powered down when
not in use.
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
Luma and Chroma Outputs Are Time Aligned
Minimal External Components:
No External Filters or Delay Lines Required
Onboard DC Clamp
Accepts Either HSYNC and VSYNC or CSYNC
Phase Lock to External Subcarrier
Drives 75 ⍀ Reverse-Terminated Loads
Logic Selectable NTSC or PAL Encoding Modes
Compact 16-Lead SOIC
The AD724 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 AD724’s on-chip oscillator generate the necessary subcarrier clock. The AD724 also
accepts the subcarrier clock from an external video source.
The interface to graphics controllers is simple: an on-chip logic
“XNOR” accepts the available vertical (VSYNC) and horizontal sync (HSYNC) signals and creates the composite sync
(CSYNC) signal on-chip. If available, the AD724 will also
accept a standard CSYNC signal by connecting VSYNC to
Logic HI and applying CSYNC to the HSYNC pin. The
AD724 contains decoding logic to identify valid horizontal sync
pulses for correct burst insertion.
APPLICATIONS
RGB to NTSC or PAL Encoding
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 suppress
harmonics in the output. These low-pass filters are optimized
for minimum pulse overshoot. The overall luma delay, relative
to chroma, has been designed to be time aligned for direct input to
a television’s baseband. The AD724 comes in a space-saving
SOIC and is specified for the 0°C to +70°C commercial temperature range.
PRODUCT DESCRIPTION
The AD724 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 chrominance (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 outputs can
simultaneously drive 75 Ω, reverse-terminated cables. All logical inputs are TTL, 3 V and 5 V CMOS compatible. The chip
FUNCTIONAL BLOCK DIAGRAM
SUBCARRIER
4FSC
NTSC/PAL
HSYNC
VSYNC
PHASE
DETECTOR
FSC
CHARGE
PUMP
XOSC
4FSC
FSC
LOOP
FILTER
4FSC
VCO
CSYNC
SYNC
SEPARATOR
BURST
XNOR
NTSC/PAL
CSYNC
4FSC
FSC 90°
QUADRATURE
+4
DECODER
±180°
(PAL ONLY)
SC 90°/270°
FSC 0°
CSYNC
RED
GREEN
DC
CLAMP
Y
DC
CLAMP
RGB-TO-YUV
ENCODING
MATRIX
CLOCK
AT 8FSC
3-POLE
LP PREFILTER
SAMPLEDDATA
DELAY LINE
U
4-POLE
LPF
BALANCED
MODULATORS
BLUE
DC
CLAMP
V
4-POLE
LPF
X2
LUMINANCE
OUTPUT
NTSC/PAL
U
CLAMP
2-POLE
LP POSTFILTER
X2
COMPOSITE
OUTPUT
4-POLE
LPF
X2
CHROMINANCE
OUTPUT
V
CLAMP
BURST
REV. B
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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1999
