| how a compact disc works (how does it play...) |
 |
If you could stretch out into a straight line all the data
stored on a single compact disc it would be over 4 miles long!
 ll Compact discs and other optical disc players read
from the inside of the CD out, just the opposite of a vinyl record.
The inner 3/4 inch radius region is for "clamping",
followed by a thin text band used to usually identify the
manufacturer. These regions do not contain data and are not affected
by scratching or other damage.
 n a music CD, the first, inner band of data, called
the "lead-in", contains the table of contents for the CD.
It tells the CD player how to navigate around the disc (where to
find the various musical tracks, etc). Scratches or other surface
damage in this area can leave the cd completely unplayable. The song
tracks begin just outside the lead-in. The longer the song, the
greater the area/width of data in the track. Damage to a disc in an
area of data outside the lead-in usually affects only the music that
is contained in that area, although with more severe damage the CD
player can sometimes "lock up" (or jump) on the damaged
area so it can't get to the later tracks.
full CD holds a maximum of 74 minutes of music. When
a disc isn't full the outer portion of the CD appears blank. Damage
in the blank region has no effect on the playability of the CD. You
can usually identify the lead-in and blank regions because they
reflect light a little bit differently, particularly when viewed at
an angle and under good light. Although it is difficult to see the
individual tracks, they can sometimes be identified by the very thin
faint circles that separate them.
 nlike a vinyl record, which is read by a needle which
lies within the grooves, a CD is read by a laser which allows it to
be read with only the laser beam "touching" the data. The
laser beam enters the CD on the play side, travels through the CD's
clear plastic layer, picks up information from the data layer, then
bounces off the reflective coating on the back side of the data
layer. The reflected laser beam then travels back through the clear
plastic layer, out of the CD, and into the CD player's "detector".
The detector then helps the CD player convert the information
carried by the laser into sound.
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