A compact disc or CD is a laser read data storage device on which audio, video, or textual material can be stored.

Although it has been used primarily for music storage, it offers a huge potential as the medium for the storage of massive amounts of many types of information. Unlike the conventional phonograph record, the CD stores information in digital form. Two-channel sound signals are digitally sampled at a rate of 44,100 times per second per channel. Each sample is expressed as a binary number value consisting of 16 binary digits. The sampled digital values, along with error correction data, tracking codes, and cueing data are recorded on a digital tape, which is used to make compression-molded plastic discs 12 cm long in diameter, each covered by a thin, reflective metallic layer and protected by a clear plastic coating.

While the CD is playing, a low-powered laser beam reads the digital data through the reflective rear surface of the disc. The CD is an improvement over the conventional record and tape recording with its accurate frequency response, a complete absence of background noise, a wider dynamic range, and longer wear.

A CD-ROM is a text/graphics/sound/storage medium that is accessed through the CD-ROM drive in a computer. A DVD player plays movies like a VCR. Since its commercial introduction in 1982, the audio CD has almost completely replaced the phonograph disc for high-fidelity recorded music. Philips Electronics NV invented it and Sony Corporation in 1980. The CD has come a long technological way is still making its effect on technology today.

The layers of a CD, top-to-bottom: Notice that the reflective layer is stored on top of the plastic - this makes scratches on the top surface of the CD much more susceptible to permanently damage the CD, since these scratches might remove the data medium itself, whilst scratches to the side the laser reads merely disperses the pickup beam, leaving the reflective layer intact.

Popular CD sizes and shapes:

  • 5 "/12 cm - Originally held 650 MB of data or 74 minutes of music, in accordance to the Redbook standard. Nowadays it is possible to squeeze in 700 MB of data or up to 80 minutes of music on a five-inch CD.
  • 3 "/8 cm - Back when the CD was new (early-mid 80s), this size was used for singles and maxi-singles. It holds 21 minutes of music or approximately 184 MB of data. The central hole is the same size as the hole on the 5" disc, allowing these to be played in most CD-players. They also look extremely high-tech and exotic, since almost nobody makes them any longer - I guess this is due to the small difference in manufacturing cost compared to a 5" and the added costs of keeping an additional cd-duplicator running.
  • 80-86x61 mm - Rectangular, business card-sized. These hold between 4-5 minutes of music and 40-50 MB of data. Chic.
The data is, as you might know, stored as a spiral of pits in the reflective surface. The pits are 0.5 micron wide and the tracks (or spiral of pits) are 1.6 micron apart. The pits are 1/4 of the pickup laser wavelength, making the distance the beam travels 2*1/4=1/2 of the wavelength, resulting in a 180° phase shift, which is detected by the pickup. Contrary to popular belief (where land is interpreted as zero and a pit is interpreted as one), the encoding is differential - a change from pit to land or vice versa counts as a binary one, no change is interpreted as zero.

Sources! Sources! Sources! Sources make you strong, strength crushes enemies! Sources:
CD-Recordable FAQ, http://www.cdrfaq.org/faq02.html#S2-7
Audio Compact Disc - An Introduction, http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm
Scientific American - Working Knowledge, September 1998, http://www.sciam.com/1998/0998issue/0998working.html (These guys actually got the reading method wrong...)
Compact Disc Formats, http://cui.unige.ch/OSG/info/MultimediaInfo/Info/cd.html

And thanks to a scar faery for notifying me that square != rectangular...

The problem with all sound recording/playback systems prior to the advent of Compact Disc-style technology can be summed up with one word:  friction.

A phonograph needle wears minuscule amounts of the vinyl from the surface of the record it is playing. Worse, nearly any hard, sharp object can cause a record to skip. The oxide on recording tape is not only physically deteriorated by its rubbing across a tape recorder's playback head, but the signal contained as magnetic impulses on the oxide also diminishes in intensity and quality if it comes into contact with parts of the tape recorder which are magnetized.  A music-box's chimes slowly abrade the notches in the box's cam, and could conceivably wear them completely away given time.  Even a paper player piano roll is stressed and weakens after repeated playings.

A Compact Disc is read by a beam of light, shed upon the myriad dents contained in a thin film layer, by the compact disc player's high-tolerance servo mechanism and bounced off of the surface of the film layer, back to a photosensor which detects the pattern of dents and converts it into digital information.  Not only is this sublime metallic film of very high tolerance of more than sufficient capacity to hold the music from an entire record album; it is completely enclosed in a plastic not unlike LexanTM (G.E. Corp.), rendering it safe from dust, dirt, scratches, stale beer, etc. for eternity.

"Alright," you say, "so you've got some dented tinfoil that's been covered with polycarbonate.  That's all well and good, but what if one should obscure the view of the laser by scratching it, rubbing it with dirt, or spilling last night's cheap Chianti upon the coating?  How then will our laser "see" the data on the tinfoil?"

"Your eyeglasses are filthy," I respond.

"What?!" You're nonplussed.

Now let's learn from our soiled eyeglasses.  They can sure be coated with an awful lot of gook, yet one can still see relatively clearly out of them.  That's because our eyes focus on the objects we're seeing and do not focus upon the surface of the lens nor the filth thereupon.  Of course, a face-full of wedding cake, or some mud splashed up by a taxi crossing a pothole, will obscure your view completely.  These phenomena (the wedding cake, the mud) will also serve to obscure the view of the CD from the laser, too!  Simply put, the plastic on the outside of a Compact Disc has to be in pretty bad shape to be "un-playable."

Expensive kits are manufactured for cleaning CDs.  These devices slather your precious discs with a solvent and gently caress it with velvet brushes.  (The frugal audiophile, however, will have long dispensed of his/her expensive CD-cleaning potions and opt for liquid hand dish soap in tepid water, applied with a sponge, to remove even the most stubborn party-soil from discs.)

There are also just a few tid-bits to add to the already-bountiful factoids offered herein about CDs:

  • As early as 1978, the Magnavox Corporation's "Magnavision" was utilizing a laser reading a plastic-coated disc to play movies through one's television set.  The concept was an expensive flop.  (The discs were over 12" in diameter, and available movie titles were rather limited. But the system sure was a great status symbol!)

  • James T. Russell, an American physicist, was the first to patent Compact Disc technology, in 1970, on behalf of the Battelle Memorial Institute.  Russell had worked on the project for the better part of a decade prior to its practical application. It was called a "digital-to-optical recording and playback system."  The unit was received coolly by potential corporate investors.  It was finally funded by a venture capitalist, Eli Jacobs, who started the Optical Recording Corporation in 1971, hiring Russell and a number of his colleagues.  Philips' disc was an analog device, which emerged at the same time as Russell's.  In fact, Philips announced their own digital "Laserdisc" device two months after Optical Recording Corporation demonstrated its own color-capable digital laser video recording and playback system at an industry trade show.

  • Optical Recording Corporation was the winner of a patent infringement lawsuit in 1992, finally confirming that it alone could claim ownership of the patents for the Compact Disc.  Although the award was about $30 million, Russell didn't see a penny of it as he was merely an employee of Optical Recording. 

Sources:

Interview, Eli Jacobs, 1998, Greenwich, Connecticut
http://web.reed.edu/reed_magazine/Nov2000/a_the_discoverer/3.html
http://www.intelligentmd.com/about_us/directors.htm
http://web.mit.edu/invent/iow/russell.html
 

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