Philips CDR880 - CD-RW Disc Operations; CD-RW Disc Erasing Procedure; CD-RW Overwriting Strategy; Disc Groove Wobble Phenomenon

BASICS CDR/RW
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2.2.2.2 Erasing
Erasing of a CD-RW disc is performed by returning the
material in the recording layer which has been changed to the
amorphous state back to the crystalline state. This is done by
an annealing process, consisting of heating the layer to a
temperature of about 200°C (= less than melting point) and
maintaining that temperature for an extended period. The disc
is than returned to its original, completely unrecorded state.
This function is called DC-Erase and takes about 37 minutes
for acomplete disc. See figure 8.
A much faster ‘on the fly’ erasing facility is also available,
allowing the last recorded track to be erased simply by
erasing the subcode reference to that track while leaving the
recorded data in place in the recording layer.
T melt +/- 600° C
| temp. behaviour
——» temperature
> X\
amorphous crystalline
RS
taryst a t awe spot
Figure 8
CD-RW erasing
2.2.2.3 Overwriting
A direct overwrite strategy is obtained by combining the write
and erase technologies. In this case, new pits are written in
the recorder layer using the same pulsed laser beam energy
as inthe recording strategy. However, in the areas between
the newly recorded pits, a lower-energy, non-pulsed
laserbeam is switched to the lower-energy erase level
between the new pits, resulting in complete erasure of the
audio data that was formerly contained on these areas.
As inthe writing of a CD-R disc, a higher energy level is used
initially to create the temperature increase required to melt the
recording layer. Between the pits, the temperature is reduced
to the erase level. This provides a higher starting temperature,
so less energy is subsequently needed each time the melting
temperature has to be reached. See figure 9.
2.3 The groove in the CD-Recording discs
2.3.1 Dimensions
In aconventional CD system the spiral shaped track with pits
is used for radial tracking, rotational speed control and time
indication. As there are no pits in the blank CD-R disc, the
groove in the substrate is used for the same purposes.
The typical dimensions of such a groove are:
- width +: 600nm
- depth : 100 nm
- profile U-shape
A groove with these dimensions can be used for radial
tracking of the laser spot.
B PH I LI PS Service disc systems
temperature
Figure 9
CD-RW overwriting
2.3.2 The wobble of the groove
The groove is not a perfect spiral. It deviates from the ideal
spiral shape with a sine wave. The amplitude of this
sinusoidal radial deviation is typically 30 nm, and the spatial
period is about 60 um. This wobbling of the groove is visible
in the reflected read out light beam (more specific: in the radial
tracking error signal derived from it). By locking the wobbling
read out signal to a specified frequency of 22.05 kHz, the disc
will rotate at the correct rotational velocity.
2.3.3 The wobble modulation
The groove is not wobbled with a constant period of the
sinewave. Around the average frequency of 22.05 kHz it is
additionally modulated in frequency with a deviation of + 1
kHz. By adding this modulation to the groove wobble,
additional information is stored in the disc. The most
important information is the absolute time, which increases
monotone from inside to outside the diameter of the disc.
This is called ATIP: Absolute Time In Pregroove.
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t ot ty ty ts
of_disc Program_Area
PCA | PMA | Lead-in
Ava
Information_Area.
Figure 10
ATIP versus disc diameters
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