Compact Disc (CD)

Compact Discs are a digitial data storage format, and is a form of optical media.

Optical data storage makes use of a precision laser beam to create and read pits and lands (or flat sections) which represent binary 0 and 1 respectively. The reflectiveness of the lands, and the non-reflectiveness of the pits are then interpreted by the optical drive as binary code. In this way, any form of digital data can be written to a disc and interpreted by any other machine with an optical drive. The layers used in a CD's construction can be seen in Fig. 1 - the pits and lands constituting the data are written to the data layer, which is most often made of aluminium. Hard-drives also work on similar operating principles.

Fig. 2 illustrates the reading/writing process which takes place in an optical drive.

CD was developed as a format by the electronics companies Philips and Sony, eventually creating a commercially viable unified format in released to the public in 1982.

Uses
Originally CDs were designed for audio only, for storing and reproducing sound, using the format CD-DA (Compact Disc-Digital Audio). The format CD-ROM (CD Read-Only Memory) developed later allows storage of virtually any set of digital data. CD-ROMs were used for transfer of data including software installations and physical exchange of other information for a number of years. Because of their equivalent flexibility and expanded storage space, DVDs have in many cases (especially in video games) replaced CD usage for data transmission. Even more so, where material cannot be transmitted over the internet, USB flash memory drives are used for similar purposes more recently because of their re-usability, low cost, and simplicity.

Advantages and Disadvantages
CD works on similar principles to its spiritual predecessor - vinyl/gramophone records. The concept of storing information on a rotating disc by inscribing patterns on its surface is a proven formula for data transmission and distribution. The consistency across the format is one of its strengths - CDs can be read at (virtually) any speed, the upper limit being the speed at which the computer can interpret its output. Vinyl playback systems however had to be calibrated to operate at the same speed as the original writing lathe for the sound to be reproduced accurately, but therein lies one of the differences between analogue and digital systems.

CD has a substantial longevity as a storage format. The distinct advantage to the digital storage medium (also adopted by MiniDisc and indeed DAT tape from 1987) is the lack of recording artefacts and the consistency of the playback. Formats like vinyl acetate, due to their mechanical nature, degrade slightly with every single use. CDs are still susceptible to accidental damage, as the various surfaces can be scratched creating read errors and they can also degrade in severe atmospheric conditions, including extremely humid and corrosive environments.

CD-DA is known as Red Book format. The Rainbow Books specifications detail the physical parameters of different CD formats, including the acceptable (correctable) errors. Other features include the stylus features (diameter, material etc) and the data format (PCM encoding, 16-bit Bit Depth, 44.1kHz Sample Rate).

Popularity
CD was a replacement for (then rapidly obsolete) vinyl acetate records and magnetic tape cassettes. These formats were quickly replaced in consumer music devices including home hi-fi systems, car stereos and also in professional recording and production environments. However, with the rise of other digital audio formats like mp3 and portable music players, CD sales began to decline. [Ref 1] This trend has continued to the point where most newly-factored laptops do not include an optical drive. Likewise, the majority of new vehicles which are equipped with a music system include an auxiliary mini-jack input for use with portable music players which have become the modern standard for music storage and listening.