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188.8.131.52 CD-R (Compact Disc Recordable)
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Recordable (CD-R, or Compact Disc
Recordable) and Rewritable (CD-RW, or Compact Disk-Rewritable) CD-ROM
technology has become popular for moderate-sized backups, due to the
modest cost of drives and media. Other interesting uses have appeared
Figure 31: If you look carefully you can see that the CD-RW (left) has less reflection than the CD-R (Right). This can cause some CD units to not see a CD-RW.
CD-R allows for an area
of the disc to be written once, although the entire disk does not have
to be written all at the same time; CD-RW allows any area of the disc
to be rewritten multiple times. Unlike tape and hard discs, CDR is
based on optical technology with a laser used to create pits
and lands in the media to store data.
How do you choose between the different
types of CDR media? CD-R media tends to be less expensive, but keep
in mind that it cannot be re-used. Once youve filled up the disk,
you cant go back and delete data off it to free up space, and
continue writing. However, this is not necessarily a bad thing from
the point of view of archiving. You may specifically WANT to write
data onto media permanently, so that it cannot be altered or deleted
once it is written. With CD-RW on the other hand, you can use it almost
like a hard disk, adding and deleting files as desired.
CD Replacing Tape
CD has largely replaced both tape and diskette as the software distribution media of choice, because of the Orange Book physical standards which help ensure compatibility, capacity (usually 650meg to 700meg, per disk), and adoption of standardized CD file system formats such as ISO and El Torito by vendors and the wide availability of CD-ROM drives. CD-Rs and CD-RWs are usually the size of a typical audio CD, however, some manufacturers make them in interesting formats such as business-card-sized CDs, and cd single sized CDs which have a lower capacity (150MB) than full-sized CDs.
A note to those buying media: CD-Rs and CD-RWs are not quite created equal. There are fast discs (supporting fast write speeds like 24x) and slow discs (supporting slower write speeds like 4x), and varying levels of media quality. You generally get what you pay for here. Those bargain CD-Rs may only be burnable at low speed (which increases the time it takes to write the info to the disc), or may have a very short life. For one-off temporary-use copies, bargain media may be sufficient. For backups, it almost certainly is not.
The composition of the media varies, depending on brand and quality. It consists of a reflective layer (either a silvery alloy, or 24K gold) and can include:
1. Cyanine dye (blue)
2. Phthalocyanine dye (aqua)
3. Metallized azo dye (dark blue)
4. Formazan dye (light green)
The type of reflective layer and dye may have a bearing on how long the media lasts (for example, phthalocyanine dye based CDs are less sensitive to sunlight and UV radiation, while cyanine dye based CDs are more forgiving of read/write power variations between drives). Additionally, some CDs have an extra coating to help the CD resist scratches. Since scratching is one of the easiest ways to damage a CD-R or CD-RW, it is recommended that full-disc labels be used, to cover the top of the disc, to protect it from scratches and prolong its life.
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