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2-1] How is the information physically stored?




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

2-1] How is the information physically stored?

(2004/02/20)

From _The Compact Disc Handbook, 2nd edition_ by Ken Pohlmann, 1992 (ISBN
0-89579-300-8):

"Write-once media is manufactured similarly to conventional playback-only
discs. As with regular CDs, they employ a polycarbonate substrate, a
reflective layer, and a protective top layer. Sandwiched between the
substrate and reflective layer, however, is a recording layer composed of
an organic dye. .... Unlike regular CDs, a pre-grooved spiral track is
used to guide the recording laser along the spiral track; this greatly
simplifies recorder hardware design and ensures disc compatibility."

Your basic CD-R is layered like this, from top to bottom:

[optional] label
[optional] scratch-resistant and/or printable coating
UV-cured lacquer
Reflective layer (24K gold or a silver alloy)
Organic polymer dye
Polycarbonate substrate (the clear plastic part)

Yes, it's real gold in "green" and "gold" CDs, but if you hold a CD-R up to
a light source you'll notice that it's thin enough to see through (the gold
layer is between 50 and 100nm thick). Something to bear in mind is that
the data is closest to the label side of the CD, not the clear plastic side
that the data is read from. If the CD-R doesn't have a hard top coating
such as Kodak's "Infoguard", it's fairly easy to scratch the top surface
and render the CD-R unusable.


A pressed CD has raised and lowered areas, referred to as "lands" and
"pits", respectively. A laser in the CD recorder creates marks in the
disc's dye layer that have the same reflective properties. The pattern
of pits and lands on the disc encodes the information and allows it to be
retrieved on an audio or computer CD player. See section (2-43) for
specifics.

Discs are written from the inside of the disc outward. On a CD-R you can
verify this by looking at the disc after you've written to it. The spiral
track on a 74-minute disc makes 22,188 revolutions around the CD, with
roughly 600 track revolutions per millimeter as you move outward from the
lead-in (23mm from the center) to the outer edge at 58mm. If you "unwound"
the spiral, it would be about 5700 meters (3.5 miles) long.

The construction of a CD-RW is different:

[optional] label
[optional] scratch-resistant and/or printable coating
UV-cured lacquer
Reflective layer (aluminum)
Upper dielectric layer
Recording layer (phase change film, i.e. the part that changes form)
Lower dielectric layer
Polycarbonate substrate (the clear plastic part)

See the net references section for pointers to more data (especially
http://www.cd-info.com/). You can find some nice drawings at
http://www.pctechguide.com/09cdr-rw.htm. The various pages connected
to http://www.chipchapin.com/CDMedia/cdda5.php3 have some computations on
disc parameters.

The Philips document "Principles of Phase Change Recordings" at
http://www.licensing.philips.com/information/cd/rec/ has some nice drawings
and a very detailed explanation of how CD-RW works.


 

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