AD720/AD721
(continued from page 1)
Y = 0.299R + 0.587G + 0.114B
U = 0.493 (B-Y)
V = 0.877 (R-Y)
All required low-pass filters are on chip. After the input signals
pass through a precision RGB to YUV encoding matrix, two on-
chip low-pass filters limit the bandwidth of the U and V color
difference signals to 1.2 MHz prior to quadrature modulation of
the color subcarrier; a third low-pass filter at 3.6 MHz (NT SC)
or 4.4 MHz (PAL) follows the modulators to limit the harmonic
content of the output.
For NTSC operation, the chroma amplitude is increased by the
factor 1.06 prior to summation with the luminance output. The
burst signal is inserted into the Y channel in the encoding matrix.
The three outputs of the encoding matrix, now transformed into Y,
U, and V components, take two paths. The Y (luminance) signal is
passed through a delay line consisting of a prefilter, a sampled-data
delay line, and a post filter. The pre- and post-filters prevent
aliasing of harmonics back into the baseband video. The overall de-
lay is a nominal –170 ns relative to the chrominance signal, in
keeping with broadcast requirements to compensate for delays in-
troduced by the filters in the decoding process.
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,
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 harmon-
ics in the output. T hese low-pass filters are optimized for mini-
mum pulse overshoot. T he overall delay is about 170 ns, which
precompensates for delays in the filters used to decode the
NT SC or PAL signal in a television receiver. (T his precompen-
sation delay is already present in T V broadcasts.)
T he U and V components pass through 4-pole modified Bessel
low-pass filters with a 1.2 MHz –3 dB frequency to prevent
aliasing in the balanced modulators, where they modulate a
3.579 545 000 MHz (NT SC) or 4.433 618 750 MHz (PAL)
signal via a pair of balanced modulators driven in quadrature by
the color subcarrier.
The AD720 and AD721 are available in a 28-pin plastic leaded
chip carrier for the 0°C to +70°C commercial temperature range.
TH EO RY O F O P ERATIO N
T he AD720/AD721 4FSC input drives a digital divide-by-4 cir-
cuit (two flip-flops) to create the quadrature signal. T he refer-
ence phase 0° is used for the U signal. In the NT SC mode, the
V signal is modulated at 90°, but in the PAL mode, the V
modulation input alternates between 90° and 270° at half the
line rate as required by the PAL standard. T he outputs of the
balanced modulators are summed and low-pass filtered to re-
move harmonics.
Referring to the AD720/AD721 block diagram (Figure 8), the
RGB inputs (each 0 mV to 714 mV in NT SC or 0 mV to
700 mV in PAL) are first encoded into luminance and color
difference signals. T he luminance signal is called the “Y”
signal and the color-difference signals are called U and V. T he
RGB inputs are encoded into the YUV format using the
transformation
NTSC/PAL
ASNC
DELAYED C-SYNC
C-SYNC
DELAY
POWER AND GROUNDS
SYNC
DECODER
NTSC/PAL
LOGIC
+5V
+5V
C-SYNC
BURST
ANALOG
ANALOG ONLY
ANALOG
LOGIC
–5V
AGND
DGND
±180
°
SC 90
°
/270
NTSC/PAL
CLOCK
AT 8FSC
°
(PAL ONLY)
SC 90
°
4FSC
ENCD
QUADRATURE
DECODER
SC 0
Y
°
BURST
5MHz
DC
LUMINANCE OUTPUT*
–0.572V TO 1.43V NTSC
–0.6V TO 1.4V PAL
5MHz
4-POLE LP
SAMPLED-
DATA
DELAY LINE
2-POLE
LP POST-
FILTER
RESTORE
AND C-SYNC
INSERTION
X2
RED
PRE-FILTER
COMPOSITE OUTPUT*
–0.572V TO 2V NTSC
–0.6V TO 2V PAL
RGB-TO-YUV
ENCODING
MATRIX
1.2MHz
4-POLE
LPF
NTSC/PAL
∑
X2
X2
U
V
GREEN
BLUE
3.6MHz (NTSC)
4.4MHz (PAL)
3-POLE LPF
BALANCED
MODULATORS
CHROMINANCE OUTPUT*
572mVp-p NTSC
600mVp-p PAL
∑
1.2MHz
4-POLE
LPF
*NOTE:
ROUT
1.5Vp-p
X2
X2
X2
THE LUMINANCE, COMPOSITE, AND CHROMINANCE
OUTPUTS ARE AT TWICE NORMAL LEVELS FOR
DRIVING 75Ω REVERSE-TERMINATED LINES.
AD721
(ONLY)
GOUT
1.5Vp-p
BOUT
1.5Vp-p
Figure 8. AD720/AD721 Functional Block Diagram
REV. 0
–5–
5秒后页面跳转
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