Harman Kardon HD7725 - Page 21

2-3. Focus servo control operation Fig. 4
The focus error signal, after being ic101
converted to voltage by the RF 1-V
amplifier, is transmitted to the operation
amplifier in the IC and output from pin
19. When the disc to objective lens
distance
is
in
just
focus,
the
beam
forms
.
:
COIL
a true round. In this state, the beams
applied to four elements in four division
photodiodes become equal and thus the
output provided then is 0 (zero). When
the disc to objective lens distance is too
close (near focus), the beam is reflected
divergently to form an oval in crosswise
direction. In this state the outputs provided from photodiodes A and C are higher than those from B and D, resulting in negative (-)
output voltage. On the other hand, when the distance is too far (far focus), the beam is reflected convergently to form an oval in
longitudinal direction. Then the outputs from photodiodes B and D are higher, resulting in positive (+) output.
16102 16103
RF |- AMP, (1)
RF |- AMP. (2)
— The output voltage (focus error signal) from pin 19 of IC-101 passes through IC-102, in from pin 39 and out from pin 37, and !C103, in from pin
7 out from pin 9 as shown in Fig. 4. It is amplified in each IC and fed to the focus coil which then drives the objective lens of the pickup.
2-4. Tracking error detection
Fig. 5 shows the principle of the tracking error detection system which employs the three beam system.
The laser beam is divided into the main beam and two sub-beams by diffraction grating and they are arranged on one line, The center
line connecting these three beams has a slight offset angle against the main beam. The main beam is received by photodiodes A, B.
C and D and two sub-beams by E and F respectively.
Fig. 5-A shows the on-track state. As both auxiliary beams 1 and 2 are slightly on the track in this state, the outputs of photodiodes E
and F are equal and the tracking signal is 0 (zero). When the track is shifted to the left (Fig. 5-B), the auxiliary beam 1 is off the pit.
This allows more light to be received by photodiode E, resulting in positive (+) tracking signal output. On the other hand, when the
i track is shifted to the right (Fig. 5-C), the amount of light received by the photodiode F increases, resulting in negative (-) tracking
signal output. And these extreme signals are detected as tracking error signals.
SPIT:
oo oo oooM SS
iy p1 PHOTO ek -1 PHOTO
1 et _ , DETECTOR iv Baw! DETECTOR
NEGATIVE
Overt CORRECTION
(A) TRACKING CORRECT (B) TRACKING ERROR
TRACKING ERROR
oog&{o00
lc PHOTO
a DETECTOR
POSITIVE
CORRECTION
(C) TRACKING ERROR
Fig. 5
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