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參數(shù)資料
| 型號(hào)： | AD721 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | RGB to NTSC/PAL Encoders(RGB-NTSC/PAL 編碼器) |
| 中文描述： | RGB到的NTSC / PAL編碼器（RGB-NTSC/PAL編碼器） |
| 文件頁(yè)數(shù)： | 5/8頁(yè) |
| 文件大小： | 146K |
| 代理商： | AD721 |
AD720/AD721
REV. 0
–5–
(continued from page 1)
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.
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 AD720 and AD721 are available in a 28-pin plastic leaded
chip carrier for the 0
°
C to +70
°
C commercial temperature range.
T HE ORY OF OPE RAT ION
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
Y = 0.299R + 0.587G + 0.114B
U = 0.493 (B-Y)
V = 0.877 (R-Y)
For NT SC operation, the chroma amplitude is increased by the
factor 1.06 prior to summation with the luminance output. T he
burst signal is inserted into the Y channel in the encoding matrix.
T he three outputs of the encoding matrix, now transformed into Y,
U, and V components, take two paths. T he Y (luminance) signal is
passed through a delay line consisting of a prefilter, a sampled-data
delay line, and a post filter. T he pre- and post-filters prevent
aliasing of harmonics back into the baseband video. T he 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.
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.
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.
SYNC
DECODER
BURST
C-SYNC
4FSC
ENCD
RED
GREEN
BLUE
QUADRATURE
DECODER
DELAYED C-SYNC
SC 90
°
SC 0
°
CLOCK
AT 8FSC
DC
RESTORE
AND C-SYNC
INSERTION
5MHz
2-POLE
LP POST-
FILTER
COMPOSITE OUTPUT*
–0.572V TO 2V NTSC
–0.6V TO 2V PAL
LUMINANCE OUTPUT*
–0.572V TO 1.43V NTSC
–0.6V TO 1.4V PAL
CHROMINANCE OUTPUT*
572mVp-p NTSC
600mVp-p PAL
NTSC/
PAL
C-SYNC
DELAY
±
180
°
(PAL ONLY)
RGB-TO-YUV
MATRIX
BURST
Y
U
V
SC 90
°
/270
°
5MHz
4-POLE LP
PRE-FILTER
1.2MHz
4-POLE
LPF
1.2MHz
4-POLE
LPF
SAMPLED-
DATA
DELAY LINE
∑
3.6MHz (NTSC)
4.4MHz (PAL)
3-POLE LPF
∑
X2
X2
X2
POWER AND GROUNDS
+5V
AGND
DGND
LOGIC
ANALOG
ANALOG ONLY
ANALOG
LOGIC
+5V
–5V
*NOTE:
THE LUMINANCE, COMPOSITE, AND CHROMINANCE
OUTPUTS ARE AT TWICE NORMAL LEVELS FOR
DRIVING 75
REVERSE-TERMINATED LINES.
ASNC
NTSC/
PAL
BALANCED
MODULATORS
NTSC/
PAL
X2
X2
X2
ROUT
1.5Vp-p
GOUT
1.5Vp-p
BOUT
1.5Vp-p
AD721
(ONLY)
NTSC/
PAL
Figure 8. AD720/AD721 Functional Block Diagram
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