Philips CDR880 - Servo Functions and Wobble Processing

14
BASICS CDR/RW
Good spot:
The used laserdiode is a high power laserdiode, to be able to
write with enough power during the writing conditions. For the
reading condition the laser has been switched on the low
power level. To obtain in both situations a stabilized wave-
length of the light the laser output in the read mode has been
modulated with 200 MHz.
in the next section the accurate track following has been
explained.
3.2.1.3 The 3-spots push pull system
In the CD-Recording disc there is a small pregroove on this
disc. To find this pregroove the mechanism uses the satellite
spots. These spots are positioned just in the middle between
two tracks and not, like the conventional 3-spot CD system, at
the border of the track. The second deviation of the
conventional system is that the photodiodes of both satellite
spots are divided into two parts. With these modifications a
push pull signal can be constructed. On figure 20 the effect of
push pull has been shown.
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Figure 20
3 spot push pull tracking
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As already known from the conventional CD system there is a
difference between the intensity of a partly reflected light from
a pit/land cross over and the intensity of the reflected light
from a complete land spot and the intensity from a complete
pit spot. The second order reflected light from the cross over
shows a third level light intensity. By dividing the satellite
photodiodes in two pieces the intensity of both reflected parts
can be subtracted and a recognizable radia! error signal is
obtained.
3.2.1.4 shock sensitivity
The shock sensitivity should be as low as possible, because
during the writing situation there is no possibility to correct
that kind of disturbances. The prevention of a shock is so
important because if it occurs during the writing mode the
CD-Recordable disc should have been damaged irreparably.
The reading time interval between the writing pulses is 13.3
ms. If after such a bump the timetable of the ATIP (Absolute
Time In Pregroove) information does not continue, the laser is
switched off. In the CD-Recordable/ReWritable recorder there
exists a recovery protocol which can save a part of the
recordings on both disc types. See next example.
Example: Recorder is writing on a CD-R disc. After having
recorded 3 pieces of music (tracks) perfect there is a bump
against the set during the recording of the 4th track. Now the
writing current of the laser is switched off, as described above.
However before opening the tray, the recorder starts finalizing
this disc, otherwise all information in the master uP should
have been lost. This disc however, will only consist out of the
3 perfect recorded tracks. Not any track can be added
(because it is finalized) but in this way the 3 perfect recorded
tracks are saved. The reason for doing so is that it is
impossible to connect new EFM signal in a good shape to the
suddenly interrupted existing EFM pattern. That's why the last
track is decided to be lost and the CD-Recordable disc can be
used only for the already perfect recorded and completed
tracks.
In case the same accident should happen with a CD-RW disc,
then only 3 tracks are stored in PMA and when a new
recording should be added it will be inserted automatically
after the third and perfect recorded piece of music. The
already recorded part of the music from the 4th track will
always be overwritten. However the ReWritable disc can be
filled completely afterwards.
3.2.1.5 focus radius
The focus radius is the minimum distance between the
spindle of the turntable motor and the objective. This distance
deviates from the existing CD format because the PCA and
PMA areas are inside them. Now the stud for the objective is
at a radius of 22 mm from the centre the spindle (was 23 mm).
3.2.1.6 eccentricity
The eccentricity is dependant to two circumstances. First the
disc eccentricity, second the eccentricity of the spindle of the
turntable motor. To lower the eccentricity of the spindle a
selfcentring carrier has been applied. There is no need to say
that the eccentricity for a recordable disc is more important
than the eccentricity of a conventional disc.
3.2.2 The CDM board
The CDM Board contains the laser control & timing unit and
the photodiode signals received from the CDM are processed
in the AEGER IC for feedback of various servo loops.
Together with the some IC's on the mainboard it supports the
following processes like:
- Laser control (reading and writing)
- Optical Power Calibration
- Normalizing
- Turntable control (reading and writing)
- Focus servo
- Radial servo
- HF-signal input and equalizer
- EFM detector
All these processes are under control of the timing unit.
See figure 21.
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