BBK DV718SI, Руководство по обслуживанию, Страница: 21 / 120

Note: 1. When reading DVD, there are only A, B, C, D signals.
2. When reading CD, there are A, B, C, D, E, F signals.
3. RFO=A+B+C+D.
4. Focus error signal=(A+C)-(B+D) Trace error signal=E-F.
2. Working principle
(1) Laser tube: wave length of loader DVD laser diode is 650nm, wave length of CD laser diode is
790nm, the wave length which is within 370nm and 750nm is visible light, the laser in the course of
reading DVD disc is visible light, and that when reading CD disc is infrared light.
(2) Principle about laser head picks up signal: laser beam projects onto disc, when laser beam
focus projects onto disc vertically, laser beam will produce reflection, reach on light sensor through
reflection loop and converse into electronic signal through photoelectric cell. For the reflection loop
produced in non pit information area and pit information area in disc has difference and reflects into
different position of light sensor, photoelectric diode in different positions on light sensor will produce
different signals to process all signals on light sensor and then produce digital signals.
(3) Focus, trace coil: when laser head is reading signals normally, information side should be in the
focus of laser beam, because of factors of disc error, high speed rotation and machine error, it is
unavoidable that laser beam focus deviates from information face to produce phenomena of orbit boas
and refocusing. Focus , trace coil is added on loader to adjust laser beam to make it correctly focus in
information area.
(4) Formation of R F signal: when disc reading is normal, light sensor will have 160MV, vague and
eye pattern waveform which is added on A, B, C, D respectively, and output R F signal from FRO pin
after being overlapped by adder inside light sensor, the frequency when reading DVD disc is much
higher than that when reading CD disc, output amplitude is about 1.4V.
1. Servo system of this player adopts SANYO 65 loader and MTK decode solution
(MFLASH (16M) +SDRAM (64M)), and its servo circuit is mainly composed of front end signal
processing, digital signal processing and digital servo processing, digital signal processing IC Mt1389
and drive circuit Am58888, in which Mt1389 is also the main part of decode circuit. Servo circuit block
diagram is shown in the figure
3.2.2 Servo circuit
3.2.2.1:
22 Blanking unused 0.01 0.1 0
23 LD-DVD Input loader 2.21 0.1 0 DVD laser power control signal
24 GND unused 0.01 0.01 0 Grounding
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