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On
the other hand,
since
the overlay
flag
is
in-
cluded
in
the color
difference
signal,
a correct
RGB
signal
(analog
signal)
cannot be obtained
from
the matrix
circuit
described
earlier
during
external
synchronization
mode.
Although
this
has
no
effect
on
the
potential
obtained
when
switching to
TTL
level
where
the
R
and
B
signals
are
concerned,
a
signal
which
appears to include
the
G
signal
is
obtained
if
a
flag
is
present
when
the
G
signal
is
concerned.
Hence,
to prevent the
generation
of a
G
signal in
the exte
rnal sign
al
section,
the
G
output
is
blocked while
OVLYF
is
at
H
level
(that
is,
whil e the exte
rnal
video
signal
is
displayed) as
a
result
of
OVLYF
being
inverted
and
added
to
the analog
signal
(pin
3 of IC104)
(see
Fig.
13-51).
The
OVLYF
signal exists in
the following
states.
Table 13-13
Mode
OVLYF
Superimpose
mode
External video
signal
display
section
Computer
vide
signal
section
L
External video
mode
Always
H
Computer
mode
Always L
Fig.
13-51
Waveforms
at
pins
3 and 5
of
IC104
during each
109
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