When was the Compact Disc invented and How do they get the music onto a CD?

In March 1983 a new technology appeared on the U.S. music scene that would make the traditional LP record a mere item of nostalgia, a curio to be picked up at yard sales by baby boomers.

The digital compact disc (CD) marked a radical departure in quality and convenience from the twelve-inch long-playing record.

The LP is an analog recording, which means it is an analogy or model of the actual music. Sound is stored in continuous grooves, whose side-to-side squiggles and variations in depth correspond to the original audio signal. A stylus running along the groove converts the mechanical motion into an electrical signal, which is then amplified by your speakers.

A nice system, but not without its flaws. Records can warp and develop mysterious crackles. Small children with sticky hands need only handle them once to leave lasting reminders. And the mere action of the stylus running over the surface hundreds of times can result in wear that causes hissing and static.

The musical information on a CD, on the other hand, is engraved on a disc that is safely sandwiched between two protective layers of plastic. A laser beam, not a stylus, scans the disc to reproduce the audio signal; thus there is no mechanical damage, be it the first or thousandth time you play the disc.

The CD is a digital recording, which means that the information is broken down into discrete values. In this case, each nuance of sound is assigned a numerical value in a binary code, that is, strings of ones and zeros. Each second of music is sliced into 4.3 million discrete units and then coded. The code is engraved on the CD in the form of microscopic pits, some fifteen thousand per inch or 3 billion per disc.

The making of a CD is a complex and clean affair. Workers wearing plastic jumpsuits and visored headgear operate in a “clean room,” in which no more than one thousand particles of dust per cubic foot is permitted. A single speck of dust settling in one of those tiny pits could be disastrous.

Production starts with a circle of ultraclean optical glass, which has been coated with an adhesive chrome layer and photographic film. The music to be stored on the CD’ is now in the form of a digital tape with binary data. A computer plays the tape and assigns precise pit measurements to each element in the code.

The glass plate, meanwhile, rotates, and a laser beam marks the photographic film with millions of indentations corresponding to the pit sizes determined by the computer.

Once completed, the disc is coated with nickel. Then technicians make a metal cast of the master, with microscopic peaks corresponding to the pits. From this cast come metal stampers, which are inserted into injection-molding machines where they impress load after load of melted polycarbonate plastic to produce the actual CDs.

Finally, the impeccably clean, precise disc is covered with aluminum and a protective resin, then dried by ultraviolet light. In your hand, safely stored in millions of pits, lies seventy-five minutes of ageless sound.