The layers of a CD, top-to-bottom:
Notice that the reflective layer is stored
on top of the plastic - this makes scratches on the top
surface of the CD much more susceptible to permanently damage the CD, since these scratches might remove the data
medium itself, whilst scratches to the side the laser reads merely disperses the
pickup beam, leaving the reflective layer
intact.
Popular CD sizes and shapes:
-
5 "/12 cm - Originally held 650 MB of data or 74 minutes of music, in accordance to the Redbook standard. Nowadays it is possible to squeeze in 700 MB of data or up to 80 minutes of music on a five-inch CD.
-
3 "/8 cm - Back when the CD was new (early-mid 80s), this size was used for singles and maxi-singles. It holds 21 minutes of music or approximately 184 MB of data. The central hole is the same size as the hole on the 5" disc, allowing these to be played in most CD-players. They also look extremely high-tech and exotic, since almost nobody makes them any longer - I guess this is due to the small difference in manufacturing cost compared to a 5" and the added costs of keeping an additional cd-duplicator running.
-
80-86x61 mm - Rectangular, business card-sized. These hold between 4-5 minutes of music and 40-50 MB of data. Chic.
The data is, as you might know, stored as a spiral of pits in the reflective surface. The pits are 0.5
micron wide and the tracks (or spiral of pits) are 1.6 micron apart. The pits are 1/4 of the
pickup laser
wavelength, making the distance the beam travels 2*1/4=1/2 of the wavelength, resulting in a 180
° phase shift, which is detected by the pickup.
Contrary to popular belief (where land is interpreted as zero and a pit is interpreted as one), the encoding is
differential - a change from pit to land or vice versa counts as a
binary one, no change is interpreted as zero.
Sources! Sources! Sources! Sources make you strong, strength crushes enemies! Sources:
CD-Recordable FAQ, http://www.cdrfaq.org/faq02.html#S2-7
Audio Compact Disc - An Introduction, http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm
Scientific American - Working Knowledge, September 1998, http://www.sciam.com/1998/0998issue/0998working.html (These guys actually got the reading method wrong...)
Compact Disc Formats, http://cui.unige.ch/OSG/info/MultimediaInfo/Info/cd.html
And thanks to a scar faery for notifying me that square != rectangular...