Philips CDR880 - Normalizing Function Details

BASICS
CDR/RW
15
CoM
SIGNAL
PROCESSING
First
look
at
the
Beta
reference
measuring
which
has
been
=
wo
|
fee
enax
checked
at
the
moment
the
first
Optical
Power
Calibration
has
ane
iene
oe
[oe
[a
been
carried
out.
If
the
recorder
reads
an
unknown
disc the
Figure 21
Blockdiagram of the CDM Board with CDM mechanism
3.2.2.1 Laser Control
The laser output level has to be stabilized by the forward
sense diode. For that reason the forward sense has three
reference currents to compare with.
- The read current.
- The write low current.
- The write high current.
The timing unit switches between the read and write power
stages and the output signal has been controlled according to
the EFM signal. See figure 22.
LASER_CONTROL
Figure 22
Laser control
The read current is the lowest current in these three situations.
To get a proper working of the laser with such a low current
the DC current has been modulated with a frequency of about
200 MHz. The levels of the laser write power have been
determined by the Optical Power Calibration.
3.222 Optical Power Calibration
In the read mode the power of the laser has been determined
by the fixed value of the read current. During writing the write
current
has
to
meet
with
the
adjusted
value
of
the
Optical
Power Calibration recorded on the used disc. The signals
which control these levels are from the so called Beta circuit.
To optimize the recording this Beta value has to be checked
during the writing operation. In case there is a black dot,
fingerprint, tilting or dust this results in a lower power intensity
on the disc. That's why the write current has to be adapted to
an optimum recording due to the variable conditions of the
disc. These adaptations have been carried out by the Alfa
circuit.
first thing he does is to make a testrecording on that disc.
With the Power Calibration Procedure the Light Device Unit
has been moved to the Program Calibration Area where the
information has been stored of the position of the first free
Power Calibration part. During 15 consecutive ATIP frames a
random EFM signal has been written with an increasing
power (FSO) at every frame. The power range of those 15
steps is 7 below and 7 above the advised OPC value in the
ATIP lead-in. After the writing the first position is being looked
for and the next 15 ATIP frames are read to find out which
frame satisfies the critical condition of the writing power. See
figure 23.
Sh tl tal
A2
P< Py P. Po P> Py
Figure 23
The power ok decision
The values from that frame such as the FSO (Forward Sense
Offset) and Alfa (Powerabsorption) are registered as setpoints
to FSO and Alfa0. In recordings later on these values take
care for adjusting the output level current at the OPC level.
On top of that there is an additional power regulation to obtain
a constant absorption in the disc. It is the intensity of the
absorption which results in the deforming of the disc. This
depends of the local quality of the disc, the spot, tilting, dust
and fingerprints. This function has to be controlled during the
writing by the ALFA circuit. The principle of sample and hold
has been used to measure the intensity of absorption. At the
beginning of a write pulse the intensity is a high write current.
This is always the same pulse (current and time value). After
that pulse, the contents of the EFM signal together with the
timing unit, decide how long the low write current continues.
At the same time that the high pulse has been transmitted,
the reflected light is also measured (reading). The difference
between send pulse and reflected pulse is a gradiation of the
absorption. See figure 24.
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