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Data Encoding and Decoding
Like hard disks and floppy disks, the compact disk is a digital storage medium. At the very lowest level, only two different values can be recorded on a disk: a one, or a zero. Magnetic disks record data using tiny magnetic fields, and the flux reversals that are detected by the read head as the disk moves from one type of field to another. Compact disks use a physical recording technique instead of a magnetic one.
The disk starts out totally flat. At each data-holding position on the disk, the CD is either left flat (these areas are called "lands") or is imprinted with a "pit", which is burned by a laser into the CD master, and then stamped into production CDs using a metal stamp made from the master. So as the disk spins, the laser traverses from lands to pits, many thousands per second. When the laser hits a land, it reflects cleanly off the aluminum coating, but when it hits a pit much of the light is diffused. The photodetector in the read head senses the difference and this is how it knows if the bit was a one or a zero.
While CDs are often referred to as having "tracks", this is actually imprecise. In fact, the entire CD is one very long, tightly-packed spiral. This is just like the single track on a phonograph record in concept, but there is a huge difference in scale. A standard CD has a spiral comprised of about 20,000 "tracks", so the spiral is in fact about three miles long! The tracks of the spiral are spaced about 1.6 microns apart. This is equivalent to a track density of about 16,000 tracks per inch, which exceeds that of even high-end hard disks today.