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How Recordable CD & DVD's (CDr DVDr + RW) are constructed;
Pressed CD DVD (Factory produced, non-recordable);
A factory produced CD is referred to as a pressed CD. Data is stored
in the form of zeros and ones in “pits” and “lands”. The surface of
a pressed CD has a number of microscopic dents called “pits” which
correspond to a zero and flat areas called “lands” which correspond
to a one.
A laser beam inside the CD drive scans the surface of the CD for
these pits and lands. When the laser hits a land it reflects back
directly to a photocell, present inside the CD player, which detects
it and reads it as one. When the laser hits a pit it gets scattered
everywhere with no or very few light returning back to the
photocell. This is read as zero. Since these pits and lands are
physically moulded into the surface of the disc, pressed discs can
last hundreds of years provided you do not damage or scratch the
recorded surface. In theory the disc itself should never wear out.
are different from pressed discs. CD-Rs does not contain pits and
lands, instead its covered with a layer of organic dye. This dye has
a special characteristic. When the disc is written, a high powered
laser causes spots on the dye to turn black (hence the term
“burning”). When such a recorded CD is played on a CD player, the
burned and the unburned areas which have differently reflectivity is
seen by the photocell inside the CD player as pits and lands. When
the laser beam encounters a dark spot, the light gets absorbed by
the spot which is the same as the light being scattered by a pit.
Hence this is read as zero. The unburned areas behave the same way
as a land and are read as one.