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AD722 Datasheet(PDF) 8 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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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
Ω 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
Ω 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.




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