Optical Storage - Upgrading and Repairing PCs - page 559

less reflective than the remaining crystalline portions. The signal is read as the difference of the laser
reflection rate between the crystalline and amorphous states. The modulation and error-correction
codes are the same as for DVD-Video and DVD-ROM, ensuring compatibility with other DVD
formats. For rewriting, a lower-powered laser reheats the spot to a lower temperature, where it
recrystallizes.
Disc cartridges or caddies originally were required for both single- and double-sided discs but have
now been made optional for single-sided discs and are seldom used today. Double-sided discs must
remain inside the caddy at all times for protection; however, single-sided discs can be taken out of
the cartridge if necessary.
DVD-RAM specifications are shown in Table 11.13 .
Table 11.13. DVD-RAM Specifications
In the past, I have been opposed to DVD-RAM because of a lack of compatibility with other drive
types. However, if you use drives supporting the DVD Super Multi standard, you can read and write
DVD-RAM as well as other rewritable DVD formats. With the ability to read, write, and erase data
without the need to use UDF packet-writing software, DVD-RAM can be a useful alternative to other
types of rewritable DVD—assuming all of your drives can use it.
DVD-R
DVD-R is a write-once medium similar to CD-R, which was originally created by Pioneer and
released by the DVD Forum in July 1997. You can play DVD-R discs on standard DVD-ROM drives.
Some DVD-RAM drives can also write to DVD-R media.
DVD-R has a single-sided storage capacity of 4.7GB—about seven times that of a CD-R-and double
that for a double-sided disc. These discs use an organic dye recording layer that allows for a low
material cost, similar to CD-R.
To enable positioning accuracy, DVD-R uses a wobbled groove recording, in which special grooved
tracks are preengraved on the disc during the manufacturing process. Data is recorded within the
grooves only. The grooved tracks wobble slightly right and left, and the frequency of the wobble
contains clock data for the drive to read, as well as clock data for the drive. The grooves are spaced
more closely together than with DVD-RAM, but data is recorded only in the grooves and not on the
lands (see Figure 11.14 ).