LG GSA-4040B Service Manual page 43

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 HD153721 using DPP(Differential
Push-Pull) Method(CD) and PP(Push-Pull) Method(DVD).
DPP Method uses not only main beam(EF1~EF4) but also side beam(E, F) for correcting DC offset generated
in Push-Pull Method.
The remaining procedure of TE signal processing in HD62604 is similar to Focus Servo.
The role of DSP IC(HD62604) is Tracking Digital Controller.
TE Signal is input to DSP IC(HD62604 33pin), and after A/D Conversion, Digital Equalizer Block and D/A
Conversion in HD62604, the output signal TRD(HD62604 41pin) is input to Drive IC( 23pin).
The drive output signal TRK+/TRK- generated according TKIN(BD7905 52pin), 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(HD62604 33pin), and after A/D Conversion, Digital Tracking Equalizer Block,
Digital Sled Compensator Block and PWM Conversion in HD62604, the output signal TVDN, TVDP(HD62604
18, 19pin) is input to SDAC(BU2507), After that to Drive IC (BD7905 28, 29pin) after Low-Pass filtering.
The PWM output signal A+, A-, B+, B- generated according to SLIN1, SLIN2, and drives the sled motor.
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