How does a compact disc player work and why are they better than record players?

Question submitted by: Patti Hermann from Williamston

Compact Disc (CD) technology has become possible because the miniaturization of computer chips has greatly improved the speed at which computers can process  information. The huge quantities of information computers are capable of handling are reduced to a computer language consisting of patterns of 0's and 1's called `bits'. In the studio, music is electronically translated into computer language and saved. The bits are transformed onto five inch silvery pieces of metal called compact discs (CD's) by burning a series of pits into the surface of the disk. The length of the pits and the spacing between them represents the computer code of 1's and 0's. these pits are much too small to be seen individually with the naked eye; however, the rainbow patterns you see is due to the reflection from the irregular surface. After the code has been burnt into the metal, the disc is encased in a layer of protective plastic.

Your CD player rotates the disc at an average of about 500 revolutions per a minute. The code lies in a spiral trail of over three miles in length and is read by a laser beam of diameter 1.7 microns - just 0.000068 inches! The diagram shows the surface of a CD as seen from the side. The higher portions of the disk (called lands) reflect the laser light differently than the burnt away portions (called pits) of the disk. The CD player compares the amount of light reflected from the disc to the amount sent to the disc and uses this information to determine if that portion of the disc contains a land or pit. A land-to-pit or pit-to land transition is read as a `1',while areas with no transition produce a string of 0's. The computer language is thus recovered and sent to the rest of your audio system to be converted back into music.

To understand why compact disc players are replacing record players, we have to understand how a record player works. In a record player, a small piece of crystal called a stylus rides along the grooves of a vinyl record. The vibrations of the stylus recover the music from the record. This procedure leaves a lot of problems. Unlike a CD, a record rotates at a constant rate, so the stylus travels further during the same time if it is on the inner part of the record than the outer part. This means that the information the stylus reads has to be more densely packed toward the inside of the record, resulting in a loss of treble (high) frequency information. Strong bass (low) frequencies can cause the needle to jump right out of the groove. Surface noise - the hisses and pops you hear due to the tiny imperfections or dust on the record - is totally absent in CDs. Finally, the vinyl record itself can be warped or melted due to heat, ruined if something is spilled on it, and worn our with repeated playing. These problems limit the quality of sound which can be produced on a record.

The main advantage of CDs over record players is that no mechanical component, such as a stylus, ever touches the surface of the disk, so the pattern remains intact. There is no wear on the disk and no degradation in the sound after repeated playing. Computer processing can be used to detect and remove imperfections and noise. Even though you can't record directly onto a CD, the duplication of the music is exact from one reproduction to the next.

Compact disks thus produce better sounding music due to the use of non-mechanical means to recover music from the disk and the resistance of the disk to damage over time.

Source: Ira Flatow: Rainbows, Curve Balls, and Other Wonders of the Natural World Explained. William Morrow and Co, New York, 1983

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