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AD725 Datasheet(PDF) 9 Page - Analog Devices

Part No. AD725
Description  Low Cost RGB to NTSC/PAL Encoder with Luma Trap Port
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com

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The reconstructed luma signal is then smoothed with a two pole
Bessel low-pass filter. This filter has a –3 dB bandwidth of
5.25 MHz for NTSC, 6.5 MHz for PAL. A final buffer pro-
vides current drive for the LUMA output pin.
Chrominance Signal Path
The chrominance path begins with the U and V color-difference
matrices. The AD725 uses U and V modulation vectors for
NTSC and PAL (+U being defined as 0 degrees phase), simpli-
fying the design compared to I and Q designs. The U and V ma-
trices combine the RGB inputs by the standard transformations:
U = 0.493
× (B – Y)
V = 0.877
× (R – Y)
The Y signal in these transformations is provided by the lumi-
nance matrix.
Before modulation, the U and V signals are prefiltered to pre-
vent aliasing. These four-pole modified Bessel low-pass filters
have a –3 dB bandwidth of 1.2 MHz for NTSC and 1.5 MHz
for PAL.
Between the prefilters and the modulators, the colorburst vec-
tors are added to the U and V signals. The colorburst levels are
defined according to the encoding standard. For NTSC, the
colorburst is in the –U direction (with no V component) with a
resultant amplitude of 286 mV (40 IRE) at 180 degrees phase.
For PAL, the colorburst has equal parts of –U and
±V vectors
(changing V phase every line) for a resultant amplitude of
300 mV alternating between 135 and 225 degrees phase (volt-
ages at the pin will be twice these levels).
The burst gate timing is generated by waiting for a certain num-
ber of reference clock cycles following the falling sync edge. If
the sync pulse width is measured to be outside the standard
horizontal width, it is assumed that the device is in an h/2 period
(vertical blanking interval) and the burst is suppressed.
The U and V signals are used to modulate a pair of quadrature
clocks (sine and cosine) at one-fourth the reference frequency
input (3.579 545 MHz for NTSC, 4.433618 MHz for PAL).
For PAL operation, the phase of the cosine (V) clock is changed
after each falling sync edge is detected. This will change the
V-vector phase in PAL mode every horizontal line. By driving
the AD725 with an odd number of sync edges per field, any
individual line will flip phase each field as required by the standard.
In order to suppress the carriers in the chrominance signal, the
U and V modulators are balanced. Once per horizontal line the
offsets in the modulators are cancelled in order to minimize
residual subcarrier when the RGB inputs are equal. This offset
cancellation also provides a dc restore for the U and V signal
paths, so it is important that the RGB inputs be held at black
level during this time. The offset cancellation occurs after each
falling sync edge, approximately 350 ns after the falling sync
edge, lasting for a period of 140 ns. If the inputs are unbalanced
during this time (for example, if a sync-on-green RGB input
were used), there will be an offset in this chrominance response
of the inputs during the remainder of the horizontal line, includ-
ing the colorburst.
The U signal is sampled by the sine clock and the V signal is
sampled by the cosine clock in the modulators, after which they
are summed to form the chrominance (C) signal.
The chrominance signal then passes through a final four-pole
modified Bessel low-pass filter to remove the harmonics of the
switching modulation. This filter has a –3 dB frequency of
4.4 MHz for NTSC and 5.9 MHz for PAL. A final buffer pro-
vides current drive for the CRMA output pin.
Composite Output
To provide a composite video output, the separate (S-Video)
luminance and chrominance signal paths are summed. Prior to
summing, however, a filter tap for removing cross-color artifacts
in the receiver is provided.
The luminance path contains a resistor, output pin (YTRAP),
and buffer prior to entering the composite summer. By connecting
an inductor and capacitor on this pin, an R-L-C series-resonant
circuit can be tuned to null out the luminance frequency
response at the chrominance subcarrier frequency (3.579 545 MHz
for NTSC, 4.433 618 MHz for PAL). The center frequency (fC)
of this filter will be determined by the external inductor and
capacitor by the equation:
f C =
π LC
It can be seen from this equation that the center frequency of
the trap is entirely dependent on external components.
The ratio of center frequency to bandwidth of the notch (Q =
fC/BW) can be described by the equation:
When choosing the Q of the filter, it should be kept in mind that
the sharper the notch, the more critical the tolerance of the
components must be in order to target the subcarrier frequency.
Additionally, higher Q notches will exhibit a transient response
with more ringing after a luminance step. The magnitude of this
ringing can be large enough to cause visible shadowing for Q
values much greater than 1.5.

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