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Fig.
13-47
a
Horizontal synchronization
50(is/div
a:
VIDEO
input
500mV/div
b:
Waveform
(1)
(EXSO)
2V/div
c:
Waveform
(4)
(EXVS)
2V/div
Photo
13-2
13.
14 Bandpass
Filter,
Phase
Shifter,
and Color
Subcarrier
Reproduction
Circuits
•
Bandpass
filter
(see Fig.
13-48)
The
bandpass
filter
is
a
series
connected
reso-
nance
circuit
used
to separate the
chroma com-
ponent from
external video
signals (see Fig.
13-45
wavefrom.
(4)).
•
Phase
shifter (see Fig. 1
3-48)
The
purpose of
the phase
shifter
is
to
combine
(in
C108
and
VR101)
the
same and
opposite phase
components
of
the
inverter
amplifier
Q106,
and
to
shift
the phase of the
color
synchronizing
signal
(color burst)
in
the external video
signal
applied to the color subcarrier
reproduction
circuit
IC101, thereby matching the
hue
of the
computer
picture
formed
at
the
VIDEO
OUT
terminal with
the
hue
of the
external
video
signal.
The
adjustment
range
of the
hue
control
(VR101)
is
shown
in
Fig.
13-49
•
Color
subcarrier
reproduction
circuit
(see Fig.
13-48)
In addition to the color subcarrier (with
90°
phase
difference)
required
by
the
carrier
color
signal
modulator, the
color
subcarrier
reproduction
circuit
also
generates a reference clock
in
the
synchronizing pulse generator
circuit
IC102.
Since
Q103
is
turned
off
when
VOVLY
is
changed
to
L
level in
external video
and
superimpose
modes, the
chroma component
of the
external
video
signal
is
passed
via
the phase
shifter
to pin
1
of IC101.
The
level
of
this
chroma
signal
is
kept constant
by
the
ACC
amplifier (amplification to
match
the
input
color
bar
level)
by
a control
voltage
from
the
ACC
detector.
The
output
signal
appears
at
pin
4
via
a
drive
circuit (see
Photo. 13-3
waveform
(a)
).
Then
following a
DC
level
shift
at
D137
to
ensure
that
the
Q107
gate potential
does not
exceed
5V
(IC102
gate
array
output
voltage
condition),
the
IC102
burst gate pulse
(BGP2)
(see
Photo. 13-4
waveform©)
is
applied to the gate
of
Q107,
thereby applying only the
color burst
signal
from
the
chroma
signal
to the pin
8
input
(see
Photo. 13-3
waveform
(b)).
This color burst
signal
applied to pin
8
is
gated
again
in
the burst
removing
circuit
by
the
burst
gate pulse
(BGP1)
(see
Photo. 13-4
waveform
(3))
on
pin
15.
The
phase of
this
gated color
burst
signal
is
then
compared
in
a
phase
comparator with
the phase
of the color
subcarrier
oscillator
output
from
the
4.43MHz
self-running
oscillator,
the
error
voltage
being passed
via
an
APC
filter
(Cl
17,
R124, C120, and C118)
to control
the
carrier
oscillation
phase
in
generating a color subcarrier
synchronized with the
color burst of the external
video
signal.
The
carrier
vector
in
the color
subcarrier
circuit
is
separated
into
components which
are
mutually
out of phase
by
90°
(orthogonal).
One
of the
com-
ponents
serves as
the color
subcarrier
output
of
the constant phase
B-Y
axis
component,
while
the other
serves as
the color
subcarrier
output
of
the
R-Y
axis
component
where
the
phase
is
inverted
at
each
successive
line.
The B-Y component
output
is
applied to
the
B-Y
demodulator,
appearing
unchanged
at
pin 11
due
to
demodulator
balance
being upset
by
R125
and R126.
And
the
R-Y
component
reflected
output
is
synchronized with
the
polarity
of the
external color burst
by
control
pulse
from
the
PAL
switch
line identifier
circuit,
resulting in
switching of
reflected
outputs
at
each
line
before being applied to the
R-Y
demodulator
input.
The
final
output
is
obtained
from
the
same
pin
10
as
the
B-Y
demodulator output
(see
Photo.
13-5 thru Photo.
13-7).
1D4
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