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2-12] How does CD-RW compare to CD-R?




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This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

2-12] How does CD-RW compare to CD-R?

(2001/07/06)

CD-RW is short for CD-Rewritable. It used to be called CD-Erasable (CD-E),
but some marketing folks changed it so it wouldn't sound like your
important data gets erased on a whim. The difference between CD-RW and
CD-R is that CD-RW discs can be erased and rewritten, while CD-R discs are
write-once. Other than that, they are used just like CD-R discs.

Let me emphasize that: they are used just like CD-R discs. You can use
packet writing on both CD-R and CD-RW, and you can use disc-at-once audio
recording on both CD-R and CD-RW. Some software may handle CD-RW in a
slightly different way, because you can do things like erase individual
files, but the recorder technology is nearly identical.

CD-RW drives use phase-change technology. Instead of creating "bubbles"
and deformations in the recording dye layer, the state of material in the
recording layer changes from crystalline to amorphous form. The different
states have different refractive indices, and so can be optically
distinguished.

These discs are not writable by standard CD-R drives, nor readable by most
older CD readers (the reflectivity of CD-RW is far below CD and CD-R, so an
Automatic Gain Control circuit is needed to compensate). Most new CD-ROM
drives do support CD-RW media, but not all them will read CD-RW discs at
full speed.

A few older audio CD players and many new ones can handle CD-RW discs, but
many can't. If you want to create audio CDs on CD-RW media, make sure that
your player can handle them.

All CD-RW recorders can write to CD-R media, so the only reason not to buy
a CD-RW recorder is price. Some Internet sites like to put the devices in
completely separate categories, calling them "CD recorders" and "CD
ReWriters", but the differences between them don't really merit such a
distinction. Think of a "CD ReWriter" as a CD recorder that can also make
use of CD-RW media.

Oddly enough, it may be easier for a DVD drive to read CD-RW discs than
CD-R discs, because of the way the media is constructed.

CD-RW media is more expensive than CD-R, but recent price reductions have
narrowed the gap considerably. There is a limit to the number of times an
area of the disc can be rewritten, but that number is relatively high (the
Orange Book requires 1000, but some manufacturers have claimed as much as
100,000). Of course, this is under laboratory conditions. If you don't
handle the disc carefully, you will add scratches, dirt, fingerprints,
and other obstacles that make the disc harder for the drive to read.

It appears that CD-RW discs have speed ratings encoded on them, so discs
that are only certified for 2x recording can't be written to at 4x (or,
for that matter, 1x). To make things more complicated, different media
is required for high-speed CD-RW drives (those that exceed 4x). See
http://www.emediapro.com/EM2000/writer11.html for an explanation.

If you're trying to decide if you want a drive that supports CD-RW, see
section (5-16).


 

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