OT: Cd-RW info

[Sean]

I found this article, and i thought some of you might be interested in it. 

Before You Fire Up That Laser...

by Bob Starrett <http://www.cdpage.com>

A lot more goes on in CD recording than spinning up the disc and turning
on the laser. Of course, your recording software performs many tasks,
but even before the first file is written to the disc, a lot of things
have to happen to ensure that you get a quality disc.

Blank recordable discs contain special areas that are not present on a
pressed (factory-made) CD. These are the Program Memory Area (PMA) and
Power Calibration Area (PCA). These areas are located near the hub of
the disc, _before_ the lead-in. CD-ROM drives and CD players cannot see
or access them, because the beginning of the lead-in is addressed as
time zero (00:00) -- the first address on a disc which a CD-ROM drive or
CD player can go to. PCA and PMA, because they exist before the lead-in
area, have negative time addresses.

The PCA starts at -35 (-00:35) seconds. It is used for a test recording
to determine the proper laser power for each recording session. The PMA
starts at -13 (-00:13) seconds. It is used as a temporary table of
contents for partially recorded discs.

Recording a CD-R disc is a complicated process. In order for everything
to work properly, the microscopic marks (pits) created by the recording
laser must be of the correct length, and be the correct distance from
each other. This is harder to achieve than you might think, because
there are so many variables involved. The characteristics of the dye on
a disc may change with variations in ambient temperature and humidity;
there are also variations in different dye batches, even from the same
disc manufacturer. The sensitivity of the recording laser will vary over
the surface of the disc and will also change as the disc ages. The laser
power needed to record a session last year may not be the same needed to
add a session to the disc this year.

Other factors, too, can affect the recording layer. Recording speed,
operating temperature, and the stability of the recording laser can all
require changes in the laser power needed to make a good disc. Because
of this, the laser power and the wavelength are defined as a range of
values, rather than a fixed number. Laser power can range between 3.6
and 8.8 milliwatts. Laser wavelength can vary between 775 and 795
nanometers.

One of the reasons that it works so well is Optimum Power Calibration,
or OPC. This operation is performed prior to each writing session on the
disc (you will see OPC also defined as Optimal Power Control - neither
is wrong; the Orange Book actually uses both terms - and you thought a
spec like this couldn't be a little sloppy!).

Optimum Power Calibration (OPC) is implemented in recorders to adjust
the power of the recording laser to the optimum level for the existing
conditions when a write is about to begin. For a starting point, each
disc manufactured has a reference value for the appropriate laser power
on the "blank" disc. The recorder reads the value and then uses it to
calculate the best laser power for the recording session it is about to
perform.

Now, this gets a little complicated, but interesting. Here's how it
works: If the laser reads the optimum power from the disc as 6
milliwatts, it will then make several short test recordings in the Test
area using a power range between 4 milliwatts and 8 milliwatts. First,
the recorder will scan the Count area of the PCA. The Count area has 100
(tiny) numbered partitions. Each partition is one frame long. Each frame
corresponds to a partition in the Test area. After the power calibration
is performed in the Test area, one of these Count area frames is
recorded with random data in sequence. The recorder keeps count of the
number of frames that are not written so it knows where to perform the
next test operation.

The PCA's Test area has 100 numbered partitions, each 15 frames long.
These frames can be recorded with uniform samples of equal numbers of
ones and zeroes, in fixed-time intervals with each frame recorded at a
different laser power. For a recommended optimum power of 5.9mW (read
from the blank disc), the 15 test recordings would be at 4.1, 4.4, 4.6,
4.9, 5.1, 5.4, 5.6, 5.9, 6.2, 6.4, 6.7, 6.9, 7.2, 7.4, and 7.7mW. The
resulting trial recordings are read back by the lower-power reading
laser at about 0.5milliwatts and are then compared for reflectivity. If
the laser power is too low, the marks on the disc will be too small, and
the reflected light will be of a high intensity. If the laser power is
too high, the marks on the disc will be too big, and the reflected light
will be of a low intensity. If the laser power is just right, the
recorded marks and the areas between them will be of equal length; the
light intensity will be correct. This power level is used to write one
frame of random data in the next available partition of the Count area.

CD recorders can "memorize" the signature of an optimally written mark.
The recorder can then write using the optimum recording power while
constantly monitoring the written signature and adjusting the write
power to maintain the optimum signature. This feature is referred to as
"Running OPC" or "Dynamic OPC". See your recorder's specifications or
documentation to determine whether it supports this function.

(I get these articles from Adaptec.  To sign up, go to
http://www.adaptec.com/adaptec/registration/cdrnews.html)