LG BP50NB40 Service Manual page 30

3-2-1. Tracking Servo
The aim of tracking servo is to make laser beam trace the data track on disc.
Tracking Error(TE) Signal is generated from tracking error detected block in MT1959H using DPP(Differential
Push-Pull) Method and DPD (Differential Phase Detection) Method.
DPP Method uses not only main beam(A, B, C, D) but also side beam(EF1~EF4) for correcting DC offset
generated in Push-Pull Method.
The remaining procedure of TE signal processing in MT1959H is similar to Focus Servo.
The role of DSP IC is Tracking Digital Controller.
TE Signal is input to DSP IC, and after A/D Conversion, Digital Equalizer Block and D/A Conversion in
MT1959H, the output signal TRD(MT1959H 30pin) is input to Drive IC(39pin).
The drive output signal TACT+/TACT- generated according TOIN(TPIC2016G4 39pin), and drives tracking
actuator in the P/U unit.
3-2-2. Sled Servo
The working distance of tracking actuator is too short to cover whole disc radius.
Sled Servo make P/U move by little and little so that the laser beam keep tracing the data track on disc
continuously when tracking actuator reaches the working limit.
TE Signal is input to DSP IC, and after A/D Conversion, Digital Tracking Equalizer Block, Digital Sled
Compensator Block and D/A Conversion in MT1959H, the output signal SL1, SL2(MT1959H, 35, 33pin) is input
to Drive IC (TPIC2016G4 SL1:2 pin, SL2:3pin) after Low-Pass filtering.
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