31
Servo
Control
Driver
S101
S801
Focus coil in optical assembly
lens
focus
Servo / Audio Processing Block
In order for the CD player to produce audio, the servo section must con-
trol the laser beam so it can recover the digital data from the disc. Once
the data is stable and output at a constant rate, it can be processed and
returned to analog audio.
Servo Block
A chain of events must occur for the laser to read the disc’s data. The
sequence is as follows:
CD Play Sequence
Operation Purpose Check
1. Optical block
returns to home
position
Begins reading the
CD at the TOC area
Sled moves the
block toward spindle
motor at Power ON
2. Laser turns ON To read the disc
data
Laser power meter
3. Focus search /
servo ON
Find and maintain
focus on the disc
Scope at the FOK
test point. Disc
spins after focus is
found.
4. Tracking servo
ON
Follow the disc s
spiral information
track
Scope at the TE test
point
5. Spindle servo
ON
Spin the disc so the
data is output at a
constant rate
Disc spins. Lock
test point = HIGH
when data rate
controls the speed.
6. CD Data to
analog
conversion
Music recovery
(processing)
C2PO test point =
LOW when no
recovery errors.
Audio output
When there is a break in the sequence, music cannot be recovered. Ex-
amining the waveform at test points can test each servo.
Sled Motor
When the CD Changer is turned ON (no disc chucked), the tray table
rotates to determine its position. Within the CD mechanism, the sled
motor drives the optical assembly toward the spindle motor until the limit
switch is closed. If this switch doesn’t close, the laser and focus search
will not take place and the CD will not spin.
Focus Search
The focus operation is divided into search and servo parts. Servo Control
starts focus search when it knows the sled is at home position (S101) and
the CD Mechanism enters the chucked position (cam switch S801).
Servo Control makes a ramp voltage used to drive the objective lens
through its full length of travel, moving the distant focus point along with it.
Focus Servo
When the laser focuses on the disc’s reflective information layer, light is
returned to the optical assembly detectors. The A-D detectors are output
to the RF Amplifier. The RF stage not only amplifies the level but also
matrixes the signals to produce FE and RFO signals. FE is an output
signal used to identify and maintain focus when finally applied to the fo-
cus coil of the optical assembly. RFO is the main signal that will be
processed into audio if the servos all work.
Servo Control receives the FE signal and uses it with RFO to determine
when focus has been found during focus search. Focus is found when
the FOK (test point) goes HIGH. This is when Servo Control replaces the
Servo
Control
Driver
M
5V
Limit switch S101
To Next Page
Loading...
