Compact Disc Media
Like other computer storage media, such as hard disks and floppy disks, a CD stores data
in digital form, which means that it actually stores only two values: 1s and 0s. However,
where the hard disk and diskette store data in magnetic form, data on a CD is recorded in
a physical recording technique. A CD starts out as a round piece of polycarbonate sub-
strateabout4.75inchesindiameterand1.2millimeters(approximately1/20thofaninch)
thick. A metal stamp, made from a master of a finished disc, is then used to stamp inden-
tations into the substrate, a process calledmastering. A CD-ROM produced this way is
said to have beenmastered. The indentations are referred to aspits,and the flat, unpitted
surfaces are referred to aslands, as shown in Figure 10-2. The substrate surface and its pits
are then covered with a shiny, reflective silver or aluminum coating, which plays a very
important part in reading and playing the contents of the CD. A clear plastic cover is then
placed over the silver coating on which a paper or silk-screened label is applied. A disc
manufactured in this way is called asingle-sessiondisc.Reading the CD
When a CD is loaded into a player or CD-ROM drive, it spins and a laser moves over the
lands and pits, sensing thousands of them per second. When the laser hits a land, its light
reflects off the metallic coating to a sensor. A pit on the CD surface does not reflect the
laser back to the sensor. When it hits a pit, the light does not reflect back to the sensor.
Whether the sensor sees a reflection or not is how it knows if the bit on the CD isa1ora0.
Unlike a floppy or hard disk, a CD is recorded on a single long (about three miles long, in
fact) spiral, rather than in discrete tracks. The spiral is wound onto the disc in a pattern
that is the equivalent of about 16,000 tracks per inch on a hard disk drive.
The to pof the CD is its data surface, and the data is placed on substrate core directly
beneath the CD’s label. The laser is focused on the bottom of the CD directly through the
CD’s substrate, which is about 1 millimeter thick. Scratches on a CD shouldn’t interfere with
the CD’s ability to be read because the laser shines through them. As long as the substrate
remains intact and undamaged, the disc should be readable. However, should the scratches
be dee penough to remove any of the reflective coating, the disc would be unreadable.Chapter 10: CD-ROMs and DVDs^207
Figure 10-2. The layers of a CD