CD-Recordable Basics page 3
[Page 1 | Page 2 | Page 4 | Return
to Continuing Education Page
3. RECORDING PROCESS
Before the recording process is described a few special features of the CD-R disc need
to be considered. The areas of a CD-R disc previously described must be recognised by a
recorder, even when the disc is completely blank. The recorder also needs a guide to
ensure that the data track it produces follows a spiral track pattern as in a conventional
CD. The CD recorder must have a speed control and absolute time information on the disc.
A track impressed during manufacture of the CD-R disc meets these requirements.
 Figure
6: Structure of the preformed track.3.1. The preformed track. During
production of CD-R discs a track is impressed in the substrate (see Figure
2 on Page 2). This track has the same spiral path as a
conventional CD. At 0.6 µm the track width is also identical to a conventional CD as is
the track pitch of 1.6µm. In addition to the spiral pattern, a slight sinusoidal, or
wave-like, excursion of 0.03 µm is incorporated in the track path. Figure 6
illustrates the principle. Track excursion is drawn exaggerated. The basic frequency of
the sinusoidal excursion is 22.05 kHz. This is used by the recorder to control the
rotation speed. If, for example, a higher frequency is read, speed has to be reduced.
Conversely, if too low a frequency is read the speed is increased.
Further information is fed to the recorder through a frequency modulation of 1 kHz of
the 22 kHz excursion frequency. In this way the recorder receives information about the
"absolute time". |
Figure 7: Time allocation for the additional CDR areas.
The preformed track contains a continuously increasing time indication. So
that this "absolute time" corresponds to the time indication of conventional
CDs, time zero (00:00) is set at the beginning of the Program area. However, the areas of
the CD-R disc that are specific to the CD recorder lie in front of the lead-in track and
are therefore defined according to Figure 7.
3.2. Setting laser power. Depending on the materials used in the production of a
CD-R disc, laser power of between 4 and 8 mW (milliwatt) is required for recording. The
reference value is stored in the preformed track and is used by the CD recorder to set the
laser power.
Example of Setting Laser Power: The laser power is preset on
the CD-R disc at 6 mW. In the PCA several trial recordings are then performed with laser
powers of between 4 and 8 mW. This is necessary because production tolerances require a
different optimum laser power. |
 Figure
9: Writing process in a CD recorder3.3. Writing
process. To store the digital information on the CD-R disc, pits are burned (formed)
into the recording layer. Figure 9 illustrates this process.
The energy of the laser beam leads to a limited heating of the substrate (carrier)
material and of the recording layer to approximately 250º C. At this temperature the
material of the recording layer melts and thereby reduces its volume. The carrier material
swells to increase the space that has become available. By constant switching between
"writing" and "reading" power a pit pattern is produced that
corresponds to that of a conventional CD. |
Next Page
Page 4, Function Blocks of a CD Recorder: A/D D/A and other modules.
Previous Page Page 2, The
CD-R Disc: What's different from a "conventional" CD, before, during and after
recording.
Page 1: Introduction to Compact Disc
Recording.
