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It's amazing when you think about it:you can store a movie several hours long on a shiny piece of plastic no bigger than your hand!Although compact discs (CDs) have been around for more than 30 years,they are still one of the most popular ways of storing music andcomputer data. In the mid-1990s, CDs evolved into digital video/versatile discs (DVDs), which look and work in a similar way butcan store about seven times more. And now we have Blu-ray™, whichcan store six times more than a DVD—or about 40 times morea than CD! Have you ever wondered how CDs, DVDs, and Blu-rays actually work?Let's take a closer look!

Note: Throughout this article, we'll talk about CDs. But almosteverything about CDs also holds true for DVDs and Blu-ray discs.

ContentsWhat is a CD?How CDs use optical laser technologyHow CDs are recorded and played backHow a CD player worksRecordable CDs and DVDsHow does a recordable CD (CD-R) work?How does a rewritable CD (CD-RW) work?Other types of CDsMore about those zeros and onesWho invented CDs?How does Blu-ray™ work?Find out moreWhat is a CD?

A compact disc is a thin, circular disc of metal and plastic about 12cm (just over 4.5 inches) in diameter. It's actually made ofthree layers. Most of a CD is made from a tough, brittle plastic calledpolycarbonate. Sandwiched in the middle there is a thin layer ofaluminum. Finally, on top of the aluminum,is a protective layer of plastic and lacquer. The first thing you notice about a CD is that it isshiny on one side and dull on the other. The dull side usually has a label on ittelling you what's on the CD; the shiny side is the important part.It's shiny so that a laser beam can bounceoff the disc and read the information stored on it.

Small portable CD player

Photo: A small portable compact disc player made by Technics. Gadgets like this have now largely been superseded by MP3 players such as iPods, which are much smaller and lighter and pack lots more music into the same space by compressing it digitally. Read more about this in our main article on MP3 players.

How CDs use optical laser technology

Until CDs were invented, music was typically stored on vinyl (plastic) LP(long-playing) records and cassette tapes. LPs scratched easily, whiletapes could stretch and distort and sometimes snapped or seized upentirely. Both of these ways of storing music were primitive comparedto CDs. LPs were played on turntables with a moving arm that bouncedalong a groove in the plastic, reading back the music as it went.Record players (or gramophones, as they were sometimes known) used mechanicaltechnology for recording and playing back sound: the moving arm turnedthe bumps in the plastic into sounds you could hear. Cassette tapes(used in such things as the original Sony Walkmans) worked a different way. Theystored sounds using magnetic technology.When you put a cassette into your Walkman, a small electric motor dragged the tapepast a little electromagnet. The electromagnet detected the pattern ofmagnetism on the tape and an electroniccircuit changed this back intothe sounds that fizzed and popped in your headphones.

Compact disc bronzing and rot

Photo: Great music, rotten CD! CDs were billed as virtually indestructible, but some early ones have fallen victim to a problem called disc rot,which comes in various flavors. Some rotten CDs slowly turn brown (a problem known as "bronzing"); in others, bits of the reflective surface pit or disappear, eventually making them unplayable. Often the last track, which is nearest the exposed edge of a compact disc, is affected first.

Close-up of compact disc rot on the edge of the disc

Photo: A close-up of the rot on the bronzed edge of a CD, where the metal has started to disintegrate and fall away. This disc will eventually become unplayable.

With the invention of CDs, people finally had a more reliable way ofcollecting music. CD players are neither mechanical nor magnetic but optical:they use flashing laser lights to record and read back information fromthe shiny metal discs. One of the main problems with LPs andcassettes was the physical contact between the player and the record ortape being played, which gradually wore out. In a CD player, the onlything that touches the CD is a beam of light:the laser beam bounces harmlessly off the surface of the CD, so the disc itself should (intheory) never wear out. Another advantage is that the CD player canmove its laser quickly to any part of the disc, so you can instantlyflip from track to track or from one part of a movie to another.

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How CDs are recorded and played back

Note: In the explanations that follow, I'm deliberately going to simplify howCDs store music as patterns of zeros and ones. It's much more complex than I'm going to make it seem,and it's beyond the scope of an introductory article like this, but I willbriefly describe what really happens at the very end.

LP records stored music as bumps on the surface of plastic, whilecassettes stored it using patterns of magnetism. These are calledanalog technologies, because the sound is stored as acontinuously varying pattern (of bumps in the plastic of a record or fluctuations inthe magnetism on a cassette tape). In a CD, music (or otherinformation) is stored digitally (as along string of numbers).After the music has been recorded, it is converted into numbers by aprocess called sampling. Almost50,000 times a second (44,100to be exact), a piece of electronic equipment measures the sound, turnsthe measurement into a number, and stores it in binary format (as apattern of zeros and ones). The sampling process turns a CD tracklasting several minutes into a string of millions of zeros and ones.There's another bit of processing that goes on after sampling (technically, known as modulation—I'lltalk about it very briefly, further down this page). But, in very simple terms, the sampled data is essentially the information stored on your CD. In other words, there is no (analog) music on a CD at all—just a huge long list of (digital) numbers.

An ordinary CD is a sandwich of plastic, aluminum, and polycarbonate.

Illustration: An ordinary CD is a sandwich of plastic (in which bumps have been pressed by a master disc), reflective aluminum, and protective polycarbonate plastic.

CDs are made from an original "master" disc. The master is "burned"with a laser beam that etches bumps (called pits) into itssurface. A bump represents the number zero, so every time the laser burns a bumpinto the disc, a zero is stored there. The lack of a bump (which is aflat, unburned area on the disc, called a land) represents thenumber one. Thus, the laser can store all the information sampled from theoriginal track of music by burning some areas (to represent zeros) andleaving other areas unburned (to represent ones). Although you can'tsee it, the disc holds this information in a tight, continuous spiralof about 3–5 billion pits. If you could unwrap the spiral and lay it ina straight line, it would stretch for about 6 km (roughly 3.5 miles)!Each pit occupies an area about two millionths of a millionth of asquare meter. That's pretty tiny!

Once the master disc has been made, it is used to stamp out millionsof plastic duplicates—the CDs that you buy and put into your musicplayer or computer. Once each disc is pressed, it's coated with a thin aluminum layer(so it will reflect laser light), covered with protective polycarbonate and lacquer, and the label is printed on top.

How a CD player works

So what's going on in your CD player when the disc spins around?

Artwork showing how a CD player uses a laser beam to read bumps from a compact disc and turn them back into audible sounds.

Inside your CD player, there is a miniature laser beam (calleda semiconductor diode laser)and a small photoelectric cell (an electronic light detector). When you press play,an electric motor (not shown in this diagram) makes the disc rotate at high speed(up to 500rpm). The laser beam switches on and scans along a track, with the photocell,from the center of the CD to the outside (in the opposite way to an LPrecord). The motor slows the disc down gradually as the laser/photocell scans from the center to the outside of the disc (as the track numberincreases, in other words). Otherwise, as the distance from the center increased, the actual surface of the disk would be moving faster and faster past the laser and photocell, so there would be more and more information to be read in the same amount of time.The laser (red) flashes up onto the shiny (under) side of the CD, bouncing off the pattern of pits (bumps)and lands (flat areas) on the disc. The lands reflect the laser lightstraight back, while the pits scatter the light. Every time the light reflects back, the photocell (blue) detects it, realizes it's seen a land, andsends a burst of electric current to an electronic circuit (green) thatgenerates the number one. When the light fails to reflect back, thephotocell realizes there is no land there and doesn't registeranything, so the electronic circuit generates the number zero. Thus thescanning laser and electronic circuit gradually recreates the pattern of zeros and ones (binary digits) thatwere originally stored on the disc in the factory.Another electronic circuit in the CD player(called a digital to analog converter or DAC)decodes these binary numbers and converts them back intoa changing pattern of electric currents.A loudspeaker transforms the electric currents into sounds you can hear (by changing their electrical energy into sound energy).

Laser and photocell inside a CD playerClose-up of laser diode in CD player.

Photos: 1) The diode laser and photocell move along aradial track so they can scan the entire surface of the CD as itrotates. 2) Here's the diode laser (bottom) and photocell (top) in closeup.WARNING! Don't try to fiddle withyour CD player to see the laser lit-up inside.It could damage your eyes or blind you.All CD players are designed to stop you looking at the lasers bymistake. Don't ever fool around with them!

Recordable CDs and DVDs

When CDs first became popular in the 1980s,they were sold purely as read-only audio compact discs (CD-DA,ones you could play music from but not record onto). It wasn't longbefore computer companies realized they could use CDs to distributesoftware (programs) very cheaply, and ordinary computer userssoon saw that CDs would be even better if you could write music anddata on them as well as just read from them.That's how recordable CDs (CD-Rs) came to be developed, but the snag was that theycould only be written on once; you couldn't erase and reuse them.Soon enough, though, the computer whizzkids developed rewritable CDs (CD-RWs) that you could erase and rewrite any number oftimes.

The read/write laser head from a typical CD writer/burner.

Photo: A CD/DVD writer/rewriter has a much more sophisticated laser read/write head than an ordinary CD/DVD player. Depending on the type of player, the read/write head needs to be able to read ordinary CDs and DVDs, recordable discs, and rewritable discs—so it really needs to be capable of several quite different reading and writing operations.

How does a recordable CD (CD-R) work?

In theory, if you wanted to make ordinary CDs in your own home,you'd need to install a huge and expensive CD-pressing machine.Fortunately, you don't need to do this—and that's because recordableCDs (CD-Rs) work in a completely different way. This time, there are nopits and lands imprinted on plastic. Instead, in between the protective polycarbonate and the reflective aluminum, there's a layer of dye. Normally the dye is translucent: laser light zooming into the disk from a CDplayer will pass straight through it, hit the reflective aluminum, andbounce straight back down again.

So far so good, but how do we store information on a compact disc like this? A CD-Rwriter has a higher-powered laser than normal, which generates heat whenit strikes the disc, "burning" the dye and making a tiny black spot.Later, when a CD reader aims its laser at that spot, the light is completelyabsorbed and doesn't reflect back. This indicates thata zero ("0") is stored on the disc at that point. In places where thedye is unburned, the laser light reflects straight back again, indicating that a "1" is storedon the disc. See where this is going? Bycreating areas of "burned" dots, and other places where the dye is leftalone, a CD-R writer creates a pattern of binary zeros and ones thatcan be used to store information. Unfortunately, once the dye is "burned" it's permanently transformed: you can't change it back again. And that's why you can only write a CD-R disc once.Just in passing, we should note that, although CD writers are widely referredto as CD burners, they do not actually burn things (combust them with oxygen): they simply use a laser to change the light-sensitive dye.

How a CD-R stores data with areas of burned and unburned dye.

Illustration: With a CD-R, binary information is stored as "burned" areas (0) and unburned areas (1) in the dye layer sandwiched between the protective polycarbonate and the reflective aluminum.

How does a rewritable CD (CD-RW) work?

Let's say you're charged with the task of developing a type of compact discthat can be written to or erased over and over again. Clearly youcan't use either of the methods we've discussed so far (the pits andlands method from read-only audio CDs or the "burned"-dye method used inCD-Rs). What you really need is a CD made from a substance that caneasily be converted back and forth between two different forms, so itcan be used to store a pattern of zeros and ones, then erased and usedto store a different pattern later on if necessary.

Most of us learned in school that the atoms (or molecules) insolids, liquids, and gases arrange themselves in different positions,with atoms in solids tightly locked together. Some solid materials are more complex than this: theiratoms (or molecules) can be arranged in two or more different wayscalled solid phases. (Solid carbon, for example, can exist invery different phases that include graphite and diamond.) That's just what we need to make a CD-RW disc.

Instead of having a layer of dye, a CD-RW has a layer of metallic alloythat can exist in two different solid forms and change back andforth between them. It's called a phase-change or phase-shiftmaterial. Sometimes it's crystalline, with its atoms/molecules arrangedin orderly ways, so it's translucent and light can pass straight through it;other times, its atoms/molecules are jumbled up in a much more random and disorderly form called anamorphous solid, which is opaque and blocks light. When a CD-RW laserhits this material, it changes tiny little areas of it backand forth between the crystalline and amorphous forms. When itcreates a crystalline area, it's making part of the CD reflective andeffectively writing a one ("1"); when it makes an amorphous area,it's making the CD non-reflective and writing a zero ("0").Because this process can be repeated any number of times, you canwrite and rewrite a CD-RW pretty much as many times as you like!

How a CD-RW stores data with areas of amorphous and crystalline metal alloy.

Illustration: With a CD-RW, binary information is stored as areas of metal alloy that are either crystalline or amorphous. Crystalline areas have a regular structure that lets light pass through to the aluminum area and reflect back down again, thus storing ones. Amorphous areas have a random structure that scatters incoming laser light, so it can't reflect back, thus storing zeros. A CD-rewriter can change the metal alloy on the CD from one form to the other and back again, which is why this kind of disc can be erased and rewritten many times over.

Other types of CDs

CDs were originally used just for storing music. Each disc couldstore 74 minutes of stereo sound—more than enough for a typical LPrecord. During the 1990s, CD technology also became popular for storingcomputer programs, games, and other information. Kodak's PhotoCD system (a way ofstoring up to 100 photos on a compact disc), was also launched in the1990s.

The original form of computer CD was called CD-ROM (CD-Read Only Memory), because mostcomputers could only read information from them (and not store anyinformation on them). In those days, you needed a separate piece ofequipment called a "burner" to write your own CDs, which were oftencalled WORMs (Write Once Read Many). It's now more common for computersto have CD-R or CD/RW drives for burning their own CDs, although most new computers now have DVD drivesinstead.

Pile of about a dozen compact discs being held in someone's hand

The difference between CDs and DVDs is the amount of informationthey can store. A CD can hold 650 megabytes (million characters) ofdata, whereas a DVD can cram in at least 4.7 gigabytes (thousandmegabytes)—which is roughly seven times more. Because DVDs are the samesize as CDs, and are storing seven times more information, the zerosand ones (or pits and lands) on a DVD have to be correspondinglysmaller than those on a CD. The latest optical discs use a technology called Blu-rayto store six times more data than DVDs or 40 times more than CDs (see the boxat the bottom for a full explanation).

Photo: CDs introduced us to digital music, but they're now being superseded by MP3 players and digital downloads. Why? Look how hard it is to hold just a dozen CDs in your hand. Even a 20GB Apple iPod MP3 player can hold something like 400-500 CDs worth of music without even blinking—and it fits in your shirt pocket! Having said that, a music track on CD will always sound better than than the equivalent MP3, for reasons we explain in our article on MP3 players and digital music.

More about those zeros and ones

It's nice and easy to explain CDs by saying that pits correspond to zeros and lands to ones, but it's not really true.The information on a CD is encoded in a much more subtle way that uses complex and clever data encoding techniques, including eight-to-fourteen modulation (EFM) and non-return to zero inverted (NRZI) coding. That sounds extremely technical, but it's nottoo hard to understand. EFM essentially just means converting short patterns of data into longer ones (paradoxically) to store them more efficiently with less risk of error. NRZI means that instead of reading individual lands and pits, the laser is looking out for changes between a pit and a land, or long strings of pits and lands, and converting those into ones and zeros instead. So, for example, if it reads a long pit and suddenly comes across a land, that is interpreted as a one. If it reads a land and suddenly comes across a pit, that's also interpreted as a one. On the other hand, unchanging areas of land or pit are both interpreted as zeros.

How pits and lands encode information on a CD surface using NRZI encoding

Artwork: How pits and lands encode zeros and ones on a CD's surface. The transition from pit to land, or land to pit, encodes a one; a length of uninterrupted pit or land encodes a zero.

Why use these sorts of techniques instead of the simple "pit equals zero, land equals one" method I described above? It uses the disc spacemore efficiently (so we can pack more data on a disc), avoids the need for very short or long pits or lands, and minimizes the importance of bits that get lost due to scratches or dirt (so it helps correct against errors). Unless you're building your own CD player or monkeying around with data communication, you really don't need to know precisely how your data is stored on a CD or DVD, so if you want to think of pits being zeros and lands one, that's a perfectly good approximation to what's happening—and all most of us care to know. (For much more detail, check out the section on Data Encoding in The Compact Disc Handbook by Ken C. Pohlmann, from page 74 onward.)

Who invented CDs?

The technology behind CDs was invented in the late 1960s by James T. Russell (1931–). An avid music fan, helonged for a sound-recordingsystem that would reproduce music more exactly than LP records andcassette tapes. He patented the first ever optical sound recordingsystem in 1970, refining it over the years that followed. Audio CDsfinally made their commercial debut in Europe in 1982, launched by theSony and Philips electronics corporations, and appeared in the UnitedStates the following year. CD-ROMs became popular in the 1990s, whenpublishers such as Encyclopedia Britannica, Broderbund, and DorlingKindersley released popular "multimedia" encyclopedias containingwritten text, sound, pictures, animations, and videos. CD-ROMs are lesspopular today, thanks to the World Wide Web (WWW), which makes iteasier to publish and update information instantly and linktogether pages from lots of different sources.

How does Blu-ray™ work?

Red and blue laser beams in a science experiment

People forget things all the time, but that doesn't really matterbecause we have books, computers,CDs, DVDs, and all kinds of other technologies to help us remember. Youcan store 10,000 thick books on a DVD—which is about seven times morethan you can fit on a CD. Imagine that: 10,000 books is about 200shelves or 6–7 bookcases worth or knowledge. But there's no such thingas too much information. DVDs may be amazing, but sometimes you need tostore even more information than they can cope with. So thank goodnessfor a new kind of disc called Blu-ray, which can store six times moredata (digital information) than even the best DVDs—that's a whopping 50gigabytes worth!

Photo: A blue laser (left) and a red laser (right).Photo by National Energy Technology Laboratory, Morgantown courtesy of US Department of Energy.

Why Blu-ray can store more information

Blu-ray discs are exactly the same size as DVDs, which arethemselves the same size as CDs. How do Blu-rays store more than DVDs?How do DVDs store more than CDs? The answer is simple. If you've everhad to squeeze a certain amount of text on a single sheet of paper(maybe to make a poster) and found it difficult to get everything on,you'll know there's a simple solution: you just make your words a bitsmaller (lower the font size). The same idea works when you're writingcomputer data on discs with laser beams. Youcan store more on a DVD than a CD by using a laser beam that "writessmaller". And to read or write a Blu-ray disc, you use a laser to writeeven smaller still.

A DVD uses a red laser beam that makes light waves with a wavelengthof 650 nanometers (0.00000065 meters, or less than one hundredththe width of a human hair). That's considerably shorter than thewavelength of invisible, infrared light that a CD player uses(780 nanometers), which is why DVDs can store more than CDs. A Blu-rayplayer uses an even more precise laser than a DVD player, with a beamof blue light shooting out of it instead of red or infared. Blue lighthas a much shorter wavelength (about 450 nanometers) than red light soa blue laser can write things that are far smaller. That means Blu-raydiscs can store movies in a much higher quality format known as HighDefinition (HD), store much longer movies on a single disc, or juststore more altogether. If you can fit four, half-hour episodes of Friends on a DVD, you can fit 24 episodes (awhole series) on a Blu-ray disc.

Artwork showing why you can fit more data on a Blu-ray disc

With a DVD, you use a red laser beam to read and write the information.The information you write onto the disk can't be smaller than the size of the beam.By using a much finer blue laser beam, Blu-ray can write smaller and store more informationin the same space.

Is Blu-ray becoming more popular?

Despite a slow start, Blu-ray discs are beginning to gain inpopularity—especially since a rival type of disc, called HD-DVD(High-definition DVD), fell by the wayside in early 2008. Blu-rayplayers are widely available and powerful games machines like the SonyPlayStation have built in Blu-ray drives. A few years ago, there werejust few hundred Blu-ray discs on the market; today, there are tens of thousands—and many more will followin the next few years.

Blu-ray isn't the end of the story, by any means. It's only a matterof time before cunning engineers develop lasers that can pack even moredata on a disc. But whether we'll actually be using discs at all in thefuture is another matter. Most people are already using their broadband Internet connections todownload or stream MP3 music tracks, movies, and TV programs instead and it may just be a matter of time before disc players disappear altogether.Then again, people said the same thing about vinyl records when CDs came along—andthat prediction proved untrue.

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Find out moreOn this websiteHistory of communicationLP record playersMP3 playersOn other sitesJames T. Russell: A short biography from the Lemelson-MIT inventor program. (Archived page via the Wayback Machine.)There's also a brief biography on Wikipedia.How the CD was developed: This BBC News article from 2007 celebrates the 25th birthday of CD technology with a timeline explaining how the invention progressed.BooksTechnicalDigital Audio Technology: A Guide to CD, MiniDisc, SACD, DVD(A), MP3 and DAT by Jan Maes, Marc Vercammen. Taylor & Francis, 2017. An up-to-date technical guide that covers some of the common principles of digital music, including sampling, compression, error correction, and so on.Blu-ray Disc Demystified byJim Taylor et al. McGraw-Hill, 2009. A comprehensive introduction that covers the history of Blu-ray, the technology behind it, disc formats, and practical applications.The Book of Nero 7: CD and DVD Burning Made Easy by Wally Wang. No Starch Press, 2006. This covers everything you'd possibly want to know about CD and DVD burning, from storing files to designing attractive labels.The Compact Disc Handbook by Ken C. Pohlmann. A-R Publications, 1989/1992. A technical introduction to CD technology that explains the difference between analog and digital, the basics of digital audio, how CD players work (in much more detail than we go into here), the various CD formats, and how CDs are manufactured. It might seem a dated book now, but it's still very relevant and useful.The CD-ROM Handbook by Chris Sherman. Intertext Publications, 1994. Another old but very useful resource for CD-ROM developers.General/historicalHow Music Got Free: The End of an Industry, the Turn of the Century, and the Patient Zero of Piracy by Stephen Witt. Penguin, 2015. How digital downloads spelled the end of the CD.America on Record: A History of Recorded Sound by Andre Millard. Cambridge University Press, 2005. Compares and contrasts the three eras of music: acoustical, electrical, and digital.ArticlesPractical tipsDigitize Your CDs and Reclaim Your Closet by J. D. Biersdorfer. The New York Times, 9 February 2017. I'm not sure I necessarily agree with the advice here—CDs are better quality than MP3s—but it's worth considering if you're short of space.DVD Burning Tips: How to Avoid the Top Five Disc-Burning Mistakes by Jon L. Jacobi. PC World. 22 November 2006. Some simple but good advice that includes using the right disk, verifying a burn, and choosing the correct burn speed.Disc Goal: Pain-Free Burning by Wilson Rothman. The New York Times, 24 July 2003. A comparison of different burner software.Basics: Burn-Your-Own DVDs: First, Mind the Format by Wilson Rothman. The New York Times, 5 September 2002. A simple introduction to DVD burning.Sound: Brush aside the idea of painting CDs by Hans Fantel. The New York Times, 3 June 1990. An interesting article from the archive exploring the myth that painting CDs makes them last longer.NewsOh for the Days of the Making-Of Featurette—Seriously by Fabrice Robinet. The New York Times, April 6, 2018. Streaming movies have killed off DVD extras that helped film-makers learn their craft.How the compact disc lost its shine by Dorian Lynskey. The Guardian, May 28, 2015. A fascinating look at the rise and fall of the compact disc.Goodbye, DVD. Hello, Future. by Dave Kehr. The New York Times, March 4, 2011. Will services like Netflix and Amazon Video spell the end of DVDs?Blu-ray's Fuzzy Future by Matt Richtel and Brad Stone. The New York Times, January 5, 2009. What does the future hold for Blu-Ray?Compact disc hits 25th birthday: BBC News, 17 August 2007. Looking back on the first quarter century of the CD.Record Biz Has Burning Question by Brad King. Wired, 14 June 2002. Does it make more sense to allow some music sharing?At Last, CD Players That D-D-D-Don't Skip by Michael Marriott. The New York Times, 15 March 2000. Sony introduces anti-jump features, called "quick focus search" and "fine access," which stop CD players from jumping when they're bumped or jolted.Patents

For much more technical detail, try:

US Patent 4,512,006: Optical type information read/write apparatus by Teruo Murakami et al, Tokyo Shibaura Denki Kabushiki Kaisha. 16 April 1985.US Patent 5,161,150: Optical Recording Medium by Kenryo Namba, TDK Corporation. 3 November 1992.US Patent 5,060,221: Digital data recording/reproduction apparatus by Yoichiro Sako et al, Sony Corporation. 22 October 1991.

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