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PR-8210-A
5-4
FOCUS
SERVO
Due
io ihe ingenious combination
of cylindri-
cal lens and. 1-way
divider photo-detector, the laser
beam
reflected
from
the dise into the photo-detec-
tor mounted
ou the slider results in the generation
Xu
focus error signal where the variation between
output level and rhe distance between the objective
ler
describes an S curve. By utilizing the
linear
section
of this focus error signal variation
curve
13 curve),
changes in the distance
between
the object
lens and the dise сап be detected апа
employ
а focus servo mechanism.
After this
focus error signal is amplified in the
RF AM
board
and processed in the УЗОР,
it is further amplified
tcurrent amplification) in the SYPS board in order
to drive
the focus motor.
In this way
the focus
motor
{objective
lens)
is operated
to
maintain
focussing error constantly at zero, thereby ensuring
that the laser beam
is accurately
focussed on the
disc at all times.
When
starting, the objective lens is first raised
(as a result of the FOCUS
ON command
from the
CONT
board)
to a position where the focus servo
сап
be
applied.
and
once
the beam
is properly
focussed ("in focus") the focus servo loop is closed
and d
ed
i
sand
the focus-
is sll not pos-
sible to lock the focus serve mechanism, the player
is switched automatically te reject mode
Starting
operation
levels.
-[
ZERO
+ | CROSS
+
|СЕТЕСТОН
Fig. 10 Focus serve block diagram
16
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