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The construction of the tracking mirror is same as that of galva-
nometer in the voltmeter. The mirror has a permanent magnet
attached to it. Surrounding the mirror, but not attached to it is a
coil of wire. Angle of the mirror can be controlled by the current
flowing through the coil, moving around the pivot. The mirror
alone cannot follow all the tracking from its beginning to end of the
videodisc because the angle which the tracking mirror can follow is
limited.
As the de component of the tracking error signal is increased with
the advancement of the spiral track, the de component is supplied
to a motor drive control circuit for the sled feeding motor so that
the overall optical sled is slided in the radial direction of the disc.
Therefore the tracking angle of the mirror is refreshed.
2-3-4.
Spindle Servo and Tangential Servo
Both spindle motor and TBC unit are used mutually for remov-
ing a time base variation of the playback signal and stabilizing an
output signal. The main factor of the time base variation are varia-
tion in the rotation of the spindle motor, eccentricity of the center
hole on the disc, and about 30 Hz inherent error generated by the
fact that the track · is not a perfect circle and actually it is spiral.
Since the revolution of the spindle motor are different in the inner
and the outer circumference plays of the CLV disc, the spindle
motor speed must be controlled in accordance with the difference.
The spindle motor speed is controlled by comparing the fre-
quency of the reference sync signal with the one of the PB hori-
zontal sync signal. But about 30 Hz speed error caused by the
eccentricity of the track pattern cannot be removed. The TBC unit
using a CCD (Charge Coupled Device) is provided for coping with
about 30Hz error and the momentary time base variation.
•
TBC
unit using
CCD
-. CCD is a shift register for
0
analogue signal and its delay time is
determined by the frequency of a transfer clock.
·
1
.
T
Number ofCCD's bits
(N.)
De ay time ( )
= - - - - - - - - - -
Clock frequency ( f
cL )
The CCD can be used as a variable delay line when the clock fre-
quency is made variable, and can be used as an analogue memory if
the clock is stopped temporarily. The TBC unit utilizes the CCD as
the variable delay line.
Phases of H-sync extracted from Video signal at the CCD-OUT,
that of color burst signal, reference H-sync, and reference· SC are
compared with and a time base error signal is detected. The CCD
transfer clock is frequency-modulated by a control signal. The con-
trol signal is the sum of the time base error signal and the 30 Hz
component signal fed from the spindle servo. A delay time corre-
sponding to the error is obtained and a closed loop is formed. The
loop provides outputs of composite video signal without the time
base variation at the CCD-OUT.
•
Spindle Servo
Spindle servo is c,omposed• of three servo loops. The first servo
loop is an H-SPEED system that is sampling the "period" of the PB
H-syncs separated from the PB video signal after period is converted
to de voltage. The second servo loop is an
H-</>
system comparing
the phase of the PB H-sync with the that of the REF H-sync in
order to detect an phase error. The third is a V-</>2 system that is
working to maintain the phase relationship between the PB V-sync
and the REF V-sync to constant.
17
When the PB video signal cannot be obtained due to focu~ error
caused by a scratch or a dropout on the disc, or when the spindle
motor is in starting up period, the servo loop is switched to the FG
mode. In the FG mode, the voltage obtained by a voltage conver-
sion of the spindle FG period is compared with the reference volt-
age and the revolution of the spindle motor is controlled. The servo
loop switch is set to its original position as soon as the motor speed
enters the rated servo lock range. The reference voltage for the FG
mode is constarit in the CAV mode but not in the CLV mode. Since
the. revolution speed of the spindle motor for the inner circumfer-
ence play differs from the one for tlie outer circumference play on
the CLV disc, a voltage corresponding to the optical sled position
detected by a variable resistor is generated as the reference voltage.
2-3-5.
Trick Play
The trick play, one of the features of the optical videodisc
player, will be described below.
The trick plays are made possible because the beam can jump
from track to track in FWD or REV modes by controlling the track-
ing mirror. To prevent the jump from being visible on the
TV
screen, all track jumping takes place only during the vertical sync
interval.
·
Since two vertical sync intervals occur for each revolution of
the CA
V
disc, a maximum of two jumps per revolution can take
place. Fig. 2-18 shows the locus of the beams in the trick play.
0
_,
UJ
G::
Fig. 2· 11. Track pattern of CA V disc
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