Philips CDR880 - Recording Principles Explained; CD-R Writing Process; CD-RW Writing Process

BASICS CDR/RW
2.2 The principle of recording
2.2.1 The writing process CD-R
In figure 4 is shown the cross-section of the recordable disc,
which consists of protective layer, AU reflection layer, Dye
recording layer and PC-substrate.
The proper working of the pre-recorded CD-Audio system has
been realised by the difference of the intensity of the reflected
light between pits and lands. Due to the diffraction caused by
the pits the light-intensity of the reflection deviates.
If an unrecorded CD-R disc has been read the intensity of the
laser beam is so low that nothing changes on the disc and
almost all laser light will be reflected. With the reflected
information the position of the laser-spot with respect to the
pregroove will be detected and the pregroove track will be
followed.
In the recording mode the intensity of the laserlight is at the
same intensity to read and follow the pregroove information.
On that location where a pit should be written the intensity of
the laserlight will be increased to a high level. Now all
laserbeams will be reflected by the AU-reflection layer. The
energy of the laserlight (in the range 4 - 11mW) causes
limited heating of the Dye recording layer and the
PC-substrate to approximately 250°C. At this temperature the
recording layer melts reducing its volume, while the substrate
expands into the space that becomes available. By constant
switching between writing and reading power, a pit pattern
corresponding to that of a pre-recorded CD is produced.
The write pulse initially has a higher power to produce the
required heating of the dye. Subsequently, the power is
reduced to a level that is sufficient to maintain the dye
temperature at the desired level. (See also detailed
information at paragraph 3.2.2.2. Optical Power Calibration)
reflectivity
—— >
-1 Oo 1 2 3 4 5 6 7 8
Position_in_pregroove_(+m)
PROTECTIVE COAT
REFLECTION_LAYER
=== DYE LAYER &
Figure 5
The principle of recording on CD-R disc
The write pulse initially has a higher power to produce the
required heating of the dye. Subsequently, the power is
reduced to alevel that is sufficient to maintain the dye
temperature at the desired level. (See also detailed
information at paragraph 3.2.2.2. Optical Power Calibration)
See figure 6.
temperature
——>
|
Figure 6
The CD-Recordable write pulse
2.2.2 The Writing process CD-RW
2.2.2.1 Recording
In the CD-RW disc the recording layer is made of an alloy of
silver, indium, antimony and tellurium. In its original state, this
layer has a polycrystalline structure. During the recording
process, the laser selectively heats tiny areas of the recording
track to a temperature above the layer's melting point (500 -
700°C). For CD-RW writing, the laser power used is in the
range 8 to 14mW.
The pulsed energy delivered by the laser beam melts the
crystals in the heated areas into a non-crystalline amorphous
phase (‘pits’), which has a much lower reflectance than the
remaining crystalline areas (‘lands’). This difference in
reflectance allows the recorded data to be read-out,
producing a signal similar to that obtained from a standard
CD. The physical characteristics of the amorphous phase are
‘frozen-in’ during cooling, making the recording just as
permanent as any standard CD. See figure 7.
temp.
behaviour
T
melt
+/-
600°C
_Tayst — 4.-2007¢
—~-» temperature
——-» time
CO @8 ©
O
crystalline molten amorphous
tet < toryst.
Figure 7
CD-RW writing
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