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In
computer mode,
the
chroma component from
the external video
signal
is
muted by
Q103
being
turned
on
when
VOVLY
is
changed
to
H
level.
The
reason for
this
is
to
prevent beating
between
the
chroma component
generated
internally
and
the
external
chroma component.
Since the external color burst
is
not
applied to
IC101, the
color subcarrier
oscillator oscillates at
the free-running
frequency
(4.
433618MHz),
result-
ing in
output of
color subcarrier
signals
(mutually
out of phase
by
90°)
at
pins
10 and 11
in
the
same
way
as
in external
and
superimpose modes.
And
since
no
color burst
is
applied to
pin
1,
on
the other hand, the pin
2
voltage
exceeds the
pin
3
reference
voltage, resulting
in
blanking of the color
subcarrier
outputs
on
pins
10 and
11
by
the
killer
detector
and
killer
blanking
circuits,
thereby
forcing
a lowering of the
voltage
on
pin 2
by IC102
VOVOUT
(L
when
in
computer mode)
via
R120.
Since the pin
10 and 11
color subcarrier
outputs
are
also
in
blank (IC101
function) during the
pin 7
PAL
pulse
intervals,
enlarging the pulse
width
will
prevent the generation of color bursts
in
the
carrier
color
signal
modulator. Therefore, the width
is
limited to
about 800ns
and
the pulse
position
is
set
near the front
porch of
the
internal/external
horizontal synchronizing
signal,
thereby
eliminat-
ing
the
influence
of the blanking
(see
Photo.
13-4
waveform
(b)).
If
the
nth
line
vector
of the
color subcarrier
outputs
on
pins
10 and 11
are
set
as indicated
in
Photo. 13-5
in
computer mode, and
the
PAL
pulse
on
pin 7
is
counted
up by
1,
the vecotr
at
the
(n+l)th
line
shown
in
Photo. 13-6
is
obtained,
and
the phase of the
color subcarrier
of
the
R-Y
axis
component
is
inverted.
Part
of the
oscillator
output
from
pin
13
of
the
color
subcarrier
oscillator
is
picked
up
by
the
reference clock for the synchronizing pulse genera-
tor
IC102,
and
is
then
amplified
by Q108
before
being applied
to pin
17 of IC102.
In addition
to controlling
the input
level
of the
carrier
color
signal
modulator
by
Q108
and Q109,
the color
subcarrier
output
from
pins
10 and
11 of
IC101
also inverts
the
B-Y
phase
at
Q108
to ensure
proper
polarity for
the
carrier
color
signal.
4.43MHz
AMP
Fig.
13-48
Color Subcarrier reproduction
circuit
05
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