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  how a compact disc works (how does it play...) return to collectors guide

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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