Harman Kardon FL8400 - Page 17

FL8400
2-4. Tracking error detection system
— 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.
Flg. 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 the
photodiode E, resulting in positive (+) tracking signal output. On the other hand, when the 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.
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Fig. 5
2-5. Tracking servo control operation
i When a tracking error signal is detected by photodiodes E and F, it is fed to pins 17 and 10 of IC104
respectively as shown in Fig. 6. InIC104, the signal is converted into voltage by the E I-V amplifier and F
|-V amplifier, transmitted to the tracking error amplifier and output through pin 20. While it passes
through!C103, in from pin 3 and out from pin 17, it is amplified in |C103 and sent to the tracking coil to
adjust pickup so that the amount of track shift is reduced as closely to none as possible.
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