Compact Disk

A compact disk (often spelled “disc” in hi-fi applications), also called a CD, is a plastic disk with a diameter of 4.72 in (12.0 cm), on which data can be recorded in digital form. Any kind of data can be digitized: sound, images, and computer programs and files. This data can also be stored on other digital media of sufficient capacity, such as a computer hard drive or a backup tape drive. Digital sound, recorded on the surface of a CD, is practically devoid of the hiss and crackle that bedevil recordings on analog media. This is because the information on the disk is binary: a bit (binary digit) is either 1 (high) or 0 (low). The distinction between these two states is more clear-cut than the subtle fluctuations of an analog signal.
 
When a CD is prepared, the sound is first subjected to analog-to-digital (A/D) conversion. This changes the continuously variable audio waves into logic bits. These bits are then burned (literally) into the surface of the disk in the form of microscopic pits. The pits are arranged in a spiral track that would, if unwound, measure several kilometers in length. Digital signal processing (DSP) minimizes the noise introduced by environmental factors such as microscopic particles on the disk or random electronic impulses in circuit hardware. A scrambling process “smears” recordings throughout the disk, rather than burning the pits in a direct linear sequence. This further improves the signal-to-noise (S/N) ratio.
 
Compact-disk players recover the sound from a disk without any hardware physically touching the surface. A laser beam scans the disk. The beam is scattered by the pits and is reflected from the unpitted plastic. The result is a digitally modulated beam that is picked up by a sensor and converted into electrical currents. These currents proceed to a descrambling circuit, a digital-to-analog (D/A) converter, a DSP circuit, and audio amplifiers. Speakers or headphones convert the audio currents into sound waves.
 
With a CD player, the track location processes are entirely electronic, and they can all be done quickly. Tracks are assigned numbers that you select by pressing buttons. It is impossible to damage the CD, no matter how much you skip around among the songs. You can move instantly to any desired point within an individual track. You can program the system to play only those tracks you want, ignoring the others.