LG CED-8042B Manual page 34

1. Optimum Power Control
1). Purpose of operating OPC
* The difference of Disc material : Sensitivity of fixed wavelength/sensitivity transition of wavelength transition.
* The variation of CD-R/RW Recorder : Configuration of laser spot in disc Layer surface and Optical quality.
* Real transition of wavelength in recording
: Depend on > type of laser > wavelength distribution of each laser > temperature of laser
2). Asymmetry and optimum recording power
* The asymmetry of recorded EFM signal is different according to recording power
: Record with several recording powers, then measure the asymmetry of HF signal at each of cases.
=> Find optimum recording power.
3). Measurement of Asymmetry
* Parameter Setup (Beta) : Using AC coupled HF signal before equalization.
Beta = (A1 + A2)/(A1 - A2)
A1
0
A2
2. Running Optimum Power Contorl
1). Variable primary factor of Optimum Power
- Power sensitivity fluctuation over the disc. (limited to 0.05 * Po)
- Wavelength shift of the laser diode due to change in operation temperature.
- Change of spot aberration due to change in disc skew, substrate thickness, defocus, etc.
- Changed conditions of disc or optics, when OPC was carried out a
long time before actual recording.
==> The purpose of the Running OPC is to continuously adjust the
writing power to the optimum power that is required.
2). Principle of Running OPC
* As the above mentioned effect, horizontal direction change of curve
occur.
* Beta = f( B-level) = constant on the Recorded Disc.
* Procedure of the Running OPC.
- During the regular OPC Procedure,the correct value for the reflected.
B-level is determined and kept in memory as a reference value.
- During recording, the actual reflected B-level is controlled in a control
loop in which the actual value is kept as close as possible to the
reference value.
- In order to cancel out the effect of reflectivity fluctuations,
it is recommended to additionally normalize the reflected B-level by
the disc reflectivity itself.
11T
11T
Sample B-level (write Power)
Sample B-level (write Power)
OPTIMUM POWER CONTROL
P << Po
Time
Incident recording pulse
Reflected recording pulse
Sample Disc reflectivity
Sample Disc reflectivity
(Read power)
(Read power)
P = Po
Time
Sampled timing B
Level B with Pwo Level B with Pwo
Pwo decided by OPC
Pwo decided by OPC
P >> Po
Time
Incident recording pulse
Sampled timing B
Reflected recording pulse
Sampled at timing B
Sampled at timing B
normalized to recording power
normalized to recording power
Sample Disc reflectivity
(Read power)
Recording Power
Recording Power
Level
17