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5.4 PAL ENCODER (PAL Version Only)
PAL version uses IClOl, IC102 and
IC103 to encode the RGB signals to
the PAL signal. The majority of the
work is performed by IC103, the RGB
to PAL ENCODER chip. This chip is
designed to generate a composite
video signal from baseband red, blue,
green and composite sync input from
sync mixer Qll and Dl5 - Dl7. The chip
contains color subcarrier oscillator,
voltage controlled 90 degrees phase
shifter, two double-sideband
suppressed carrier chroma modulators,
RGB input matrix, and blanking level
clamps.
In the PAL version, an extra
voltage-controlled crystal oscillator
is needed for the PAL color burst
frequency of 4.433618 MHz. For this
purpose, the internal oscillator
circuit of the IC103 is used.
If the oscillator does not
synchronize with the master
oscillator, an apparent motion will
exist whenever
a
color transition
occurs.
This synchronization problem is
solved by slightly shifting of the
master oscillator frequency and the
addition of
a
phase-locked-loop
circuit. The master crystal
oscillator frequency of the PAL
version is 28 .• 475 MHz. This allows
the two oscillator to be divided down
to the horizontal frequency of 15.611
kHz and phase-locked at this
frequency.
IClOl is
a
programmable divider and
operates in
a
divisor of 71. The
output from the divider is connected
to the programmable divider part of
IC102 where a divisor of 4 is used to
complete the count-down to 15.611
kHz. IC102 also contains,....a phase
comparator part, and it compares this
divided 15.611 kHz with the HSYNC
signal which is counted down to
15.611 kHz from the master oscillator
in IC6. The phase comparator then
generates
a
control voltage in
proportion to the phase and frequency
difference between these 2 signals of
15.611 kHz and output it at 'pin 13.
This control voltage is passed
through
a
simple R-C low pass filter
and used to control
a
varactor diode
DlOl. The capacitance of the varactor
is changed to tune the 4.433618 MHz
oscillator by varying the control
voltage. This tuning allows the two
oscillators to be synchronized at any
time except during reset or power-on.
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