Compact disk

A CD is a plastic disk having diameter 4.72 in, on which data is permanently recorded in the digital form. Any kind of data is digitized: images, sound, and computer programs and files. In computer lingo, a CD containing files and software is called as CD-ROM.

Digital sound, recorded on surface of a compact disk, is practically devoid of hiss and crackle that have historically bedeviled recordings on the other media. This is because the information on disk is binary: a bit (binary digit) is 1 (high) or 0 (low). The distinction between these 2 states is more clear-cut than the subtle fluctuations of the analog signal. Noise is reduced further by a scattering/unscattering process which “smears” recordings throughout disk.When a CD is manufactured, sound is 1st subjected to analog-to-digital (A/D) conversion.  This changes continuously variable audio waves into logic bits. These bits are then burned into surface of disk in the form of microscopic pits. The pits are arranged in a long spiral track which measures several miles in length. When the sound is played back, bits are unscattered. Digital signal processing (DSP) can remove some noise which has been introduced by the environmental factors beyond people’s control, like microscopic particles on disk or random electronic noise in circuit hardware. Then data can be subjected to digital-to-analog (D/A) conversion. Finally, signal can be amplified and sent to speakers or the headset.

Compact-disk players recover sound from a disk without touching physically the surface on which data is recorded. A laser beam scans the disk. The beam is scattered by pits and is reflected from unpitted plastic. The result is a digitally modulated beam which is picked up by the sensor and converted into electrical currents. These currents proceed to the unscattering (or descrambling), DSP, D/A, and audio amplification circuits. The speakers or headphones convert the audio cur- rents into sound waves finally.

With a CD player, track location processes are electronic, and they can all be done rapidly. Tracks are assigned numbers which you select by pressing buttons. It is impossible to damage CD, no matter how much you skip around among songs. You can fast-forward or rewind to different points within the individual track. You can program a CD player to play those tracks only which you want, ignoring the others.