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How CD Drives Work

Optical StorageThe optical storage device that most of us are familiar with is the compact disc(CD). A CDcan store huge amounts of digital information (7! "#) on a ver$ small surface that is

incredi%l$ ine&pensive to manufacture. The design that makes this possi%le is a simple one'The CD surface is a mirror covered with %illions of tin$ %umps that are arranged in a longtightl$ wound spiral. The CD pla$er reads the %umps with a precise laser and interprets theinformation as %its of data.

The spiral of %umps on a CD starts in the center. CD tracks are so small that the$ have to %emeasured in microns(millionths of a meter). The CD track is appro&imatel$ .* micronswide with +., microns separating one track from the ne&t. The elongated %umps are each.* microns wide a minimum of .! microns long and +-* nanometers(%illionths of ameter) high.

"ost of the mass of a CD is an inection/molded piece of clear pol$car%onate plastic that isa%out +.- millimeters thick. During manufacturing this plastic is impressed with themicroscopic %umps that make up the long spiral track. A thin reflective aluminum la$er isthen coated on the top of the disc covering the %umps. The trick$ part of CD technolog$ isreading all the tin$ %umps correctl$ in the right order and at the right speed. To do all of thisthe CD pla$er has to %e e&ceptionall$ precise when it focuses the laser on the track of%umps.

When $ou pla$ a CD the laser %eam passes through the CD0s pol$car%onate la$er reflectsoff the aluminum la$er and hits an optoelectronic device that detects changes in light. The%umps reflect light differentl$ than the flat parts of the aluminum la$er which are calledlands. The optoelectronic sensor detects these changes in reflectivit$ and the electronics inthe CD/pla$er drive interpret the changes as data %its.

The basic parts of a compact-disc player

Optical: CD-R/CD-RWThat is how a normal CD works which is great for prepackaged software %ut no help at allas remova%le storage for $our own files. That0s where CD-recordable(CD/1) and CD-rewritable(CD/1W) come in.

CD/1 works %$ replacing the aluminum la$er in a normal CD with an organic d$e compound.This compound is normall$ reflective %ut when the laser focuses on a spot and heats it to a

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certain temperature it 2%urns2 the d$e causing it to darken. When $ou want to retrieve thedata $ou wrote to the CD/1 the laser moves %ack over the disc and thinks that each %urntspot is a %ump. The pro%lem with this approach is that $ou can onl$ write data to a CD/1once. After the d$e has %een %urned in a spot it cannot %e changed %ack.

CD/1W fi&es this pro%lem %$ using phase change which relies on a ver$ special mi&ture ofantimon$ indium silver and tellurium. This particular compound has an ama3ing propert$'When heated to one temperature it cr$stalli3es as it cools and %ecomes ver$ reflective4when heated to another higher temperature the compound does not cr$stalli3e when itcools and so %ecomes dull in appearance.

5hoto courtes$ 6omega Corporation

The Predator is a fast CD-RW drie from !omega"

CD/1W driveshave three laser settings to make use of this propert$'

Read/ The normal setting that reflects light to the optoelectronic sensor

#rase/ The laser set to the temperature needed to cr$stalli3e the compound

Write/ The laser set to the temperature needed to de/cr$stalli3e the compound

ther optical devices that deviate from the CD standard such as D$D emplo$ approachescompara%le to CD/1 and CD/1W. An older h$%rid technolog$ called magneto-optical(")is seldom used an$more. " uses a laser to heat the surface of the media. nce thesurface reaches a particular temperature a magnetic head moves across the mediachanging the polarit$ of the particles as needed.

How CD #urners Work6n - one of the %iggest news stories was the rise of 8apsterand similarfile/sharingprograms.With these programs $ou could get an "5! versionof ust a%out an$ song $ouwant without shelling out a dime. The record companies were fairl$ upset over this turn ofevents and understanda%l$ so' The$ weren0t making an$ mone$ off the distri%ution of theirproduct to millions of people.

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%n e&ternal writable CD drie' also called a CD b(rner: With

this type of drie' yo( can ta)e m(sic or data files from yo(rcomp(ter and ma)e yo(r own CDs"

#ut there was mone$ to %e made on the 28apster revolution2 as electronics manufacturersand retailers soon discovered. 6n +999 - and earl$ -+ sales of CD %urners and %lankCD/1ecorda%le discs sk$rocketed. :uddenl$ it was feasi%le for the average person to gathersongs and make their own CDs and music/mi& makers ever$where wanted to get theirhands on the means of production. Toda$ writa%le CD drives (CD %urners) are standarde;uipment in new 5Cs and more and more audio enthusiasts are adding separate CD%urners to their stereo s$stems. 6n less than five $ears CDs have eclipsed cassette tapesasthe mi& medium of choice.

6n this article $ou0ll find out how CD %urners encode songs and other information onto %lankdiscs. We0ll also look at CD re/writa%le technolog$ see how the data files are put togetherand find out how $ou can make $our own music mi&es with a CD %urner.

CD #asics' The #umps

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6f $ou0ve read How CDs Work$ou understand the %asicidea of CD technolog$. CDs store music and other files indigitalform // that is the information on the disc isrepresented %$ a series of +s and s (see How Analog andDigital 1ecording Worksfor more information). 6nconventional CDs these +s and s are represented %$

millions of tin$ %umps and flat areas on the disc0s reflectivesurface. The %umps and flats are arranged in a continuoustrack that measures a%out .* microns (millionths of ameter) across and *"+ miles(* km) long.

To read this information the CD pla$er passes a laser %eamover the track. When the laserpasses over a flatarea in the track the %eam is reflected directl$ to an optical sensoronthe laser assem%l$. The CD pla$er interprets this as a ,. When the %eam passes over ab(mp the light is %ounced awa$ from the optical sensor. The CD pla$er recogni3es this as a.

% CD player g(ides a small laser along the CD.s data trac)"!n conentional CDs' the flat areas' or lands' reflect the light bac) to the laser

assembly the b(mps deflect the light so it does not bo(nce bac)"

CD #asics' The 5athThe %umps are arranged in a spiral path starting at the center of the disc. The CD pla$er

% CD has a long' spiraleddata trac)" !f yo( were to

(nwind this trac)' it wo(lde&tend o(t *"+ miles 0+ )m1"

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spins the disc while the laser assem%l$ moves o(twardfrom the center of the CD. At astead$ speed the %umps move past an$ point at the outer edge of the CD more rapidl$ thanthe$ move past an$ point nearer the CD0s center. 6n order to keep the %umps moving pastthe laser at a constant rate the pla$er must slow the spinning speed of the disc as the laserassem%l$ moves outward.

The CD player spins the disc while moing the laser assembly o(tward from themiddle" To )eep the laser scanning the data trac) at a constant speed' the player m(st

slow the disc as the assembly moes o(tward"

At its heart this is all there is to a CD pla$er. The e&ecution of this idea is fairl$ complicated%ecause the pattern of the spiral must %e encoded and read with incredi%le precision %ut the%asic process is prett$ simple.

6n the ne&t section $ou0ll find out how data is recorded on CDs %oth %$ professionale;uipment and the home CD %urner.

1eading CDs6n the last section we saw that conventional CDs store digital data as a pattern of %umpsand flat areas arranged in a long spiral track. The CD fa%rication machine uses a high/powered laserto etch the %ump pattern into photoresistmaterial coated onto a glass plate.Through an ela%orate imprinting process this pattern is pressed onto acr$lic discs. The discsare then coated with al(min(m(or another metal) to create the readablereflective surface.

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The different layers of a conentional CD

As $ou can see this is a fairl$ comple& delicate operation involving man$ steps and severaldifferent materials. =ike most comple& manufacturing processes (from newspaper printingtotelevisionassem%l$) conventional CD manufacturing isn0t practical for home use. 6t0s onl$feasi%le for manufacturers who produce hundreds thousands or millions of CD copies.

Conse;uentl$ conventional CDs have remained a 2read only2 storage medium for theaverage consumer like =5s or conventional D>Ds. To audiophiles accustomed to recorda%le

cassettesas well as computer users who were fed up with the limited memor$capacit$ offlopp$ disks this limitation seemed like a maor draw%ack of CD technolog$. 6n the earl$09s more and more consumers and professionals were looking for a wa$ to make their ownCD/;ualit$ digital recordings.

Writing CDs6n response to this demand electronics manufacturers introduced an alternative sort of CDthat could %e encoded in a few eas$ steps. CD-recordable discs or CD-Rs don0t have an$%umps or flat areas at all. 6nstead the$ have a smoothreflective metal la$er which rests ontop of a la$er of photosensitie dye.

When the disc is %lank the d$e is transl(cent' =ight can shine through and reflect off themetal surface. #ut when $ou heatthe d$e la$er with concentrated lightof a particularfre;uenc$ and intensit$ the d$e turns opa2(e' 6t darkens to the point that light can0t passthrough.

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% CD-R doesn.t hae the same b(mps and lands as a conentional CD" !nstead' thedisc has a dye layer (nderneath a smooth' reflectie s(rface" On a blan) CD-R disc'the dye layer is completely transl(cent' so all light reflects" The write laser dar)ensthe spots where the b(mps wo(ld be in a conentional CD' forming non-reflecting

areas"

#$ selectivel$ darkening particular points along the CD track and leaving other areas of d$etranslucent $ou can create a digital pattern that a standard CD pla$er can read. The lightfrom the pla$er0s laser %eam will onl$ %ounce %ack to the sensor when the d$e is lefttranslucent in the same wa$ that it will onl$ %ounce %ack from the flat areas of aconventional CD. :o even though the CD/1 disc doesn0t have an$ %umps pressed into it at

all it %ehaves ust like a standard disc.

A CD %urner0s o% of course is to 2%urn2 the digital pattern onto a %lank CD. 6n the ne&tsection we0ll look inside a %urner to see how it accomplishes this task.

#urning CDs' =aser Assem%l$6n the last section we saw that CD %urners darken microscopic areas of CD/1 discs torecord a digital pattern of reflective and non/reflective areas that can %e read %$ a standardCD pla$er. :ince the data must %e accuratel$ encoded on such a small scale the %urnings$stem must %e e&tremel$ precise. :till the %asic process at work is ;uite simple.

The CD %urner has a moving laser assem%l$ ust like an ordinar$ CD pla$er. #ut in additionto the standard 2read laser2 it has a 2write laser.2 The write laseris more powerful than theread laser so it interacts with the disc differentl$' 6t alters the s(rfaceinstead of ust%ouncing light off it. 1ead lasers are not intense enough to darken the d$e material sosimpl$ pla$ing a CD/1 in a CD drive will not destro$ an$ encoded information.

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The laser assembly inside a CD b(rner

n the ne&t page $ou0ll find out how this write laser operates.

#urning CDs' Write =aserThe write laser moves in e&actl$ the same wa$ as the read laser' 6t moves outward while thedisc spins. The %ottom plastic la$er has grooves pre/pressed into it to guide the laser alongthe correct path. #$ cali%rating the rate of spin with the movement of the laser assem%l$ the%urner keeps the laser running along the track at a constant rate of speed. To record thedata the %urner simpl$ turns the laser writer on and off in s$nch with the pattern of +s ands. The laser dar)ens the material to encode a and leaves it transl(cent to encode a ,.

The machinery in a CD b(rner loo)s pretty m(ch the same as themachinery in any CD player" There is a mechanism that spins the

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disc and another mechanism that slides the laser assembly"

"ost CD %urners can create CDs at multiple speeds. At +& speed the CD spins at a%out thesame rate as it does when the pla$er is reading it. This means it would take $ou a%out ,minutes to record , minutes of music. At -& speed it would take $ou a%out half an hour torecord , minutes and so on.

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!n a CD-RW disc' the reflecting lands and non-reflecting b(mps of a conentional CD arerepresented by phase shifts in a special compo(nd" When the compo(nd is in a crystallinestate' it is transl(cent' so light can shine thro(gh to the metal layer aboe and reflect bac)to the laser assembly" When the compo(nd is melted into an amorpho(s state' it becomes

opa2(e' ma)ing the area non-reflectie"

5hase/change Compounds6n phase-change compo(nds these shifts in form can %e 2locked into place2' The$ persisteven after the material cools down again. 6f $ou heat the compound in CD/1W discs to themelting temperature and let it cool rapidl$ it will remain in a fluid amorphous state eventhough it is %elow the cr$stalli3ation temperature. 6n order to cr$stalli3e the compound $ouhave to keep it at the cr$stalli3ation temperature for a certain length of time so that it turnsinto a solid %efore it cools down again.

6n the compound used in CD/1W discs the cr$stalline form is translucent while theamorphous fluid form will a%sor% most light. n a new %lank CD all of the material in thewrita%le area is in the cr$stalline form so light will shine through this la$er to the reflectivemetal a%ove and %ounce %ack to the light sensor. To encode information on the disc the CD%urner uses its write laser which is powerful enough to heat the compound to its meltingtemperature. These 2melted2 spots serve the same purpose as the %umps on a conventionalCD and the opa;ue spots on a CD/1' The$ %lock the 2read2 laser so it won0t reflect off themetal la$er. @ach non-reflectie areaindicates a in the digital code. @ver$ spot thatremains cr$stalline is still reflectie indicating a +.

The @rase =aserAs with CD/1s the read laserdoes not have enough power to change the state of thematerial in the recording la$er // it0s a lot weaker than the write laser. The erase laserfallssomewhere in %etween' While it isn0t strong enough to melt the material it does have thenecessar$ intensit$ to heat the material to the cr$stalli3ation point. #$ holding the material atthis temperature the erase laser restores the compound to its cr$stalline state effectivel$erasing the encoded . This clears the disc so new data can %e encoded.

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CD/1W discs do not reflect as much light as older CD formats so the$ cannot %e read %$most older CD pla$ers and CD/1" drives. :ome newer drives and pla$ers including allCD/1W writers can adust the read laser to work with different CD formats. #ut since CD/1Ws will not work on man$ CD pla$ers these are not a good choice for music CDs.

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@ncoding DataThere are a num%er of important aspects involved in making a CD reada%le to a CD pla$er'

The CD track is marked with a sort of timecode which tells the CD pla$er what part

of the disc it is reading at an$ particular time. Discs are also encoded with a table ofcontents located at the %eginning of the track (the center of the disc) which tells thepla$er where particular songs (or files) are written onto the disc.

The data track is %roken up %$ e&tra filler so there are no long strings of +s or s.

Without fre;uent shifts from + to there would %e large sections without a changingpattern of reflectivit$. This could cause the read laser to 2lose its place2 on the disc.The filler data %reaks up these large sections.

@&tra data %its are included to help the pla$er recogni3e and fi& a mista)e. 6f the

read laser misreads a single %it the pla$er is a%le to correct the pro%lem using theadditional encoded data.

1ecorded information is not encoded se;uentiall$4 it is interlacedin a set pattern.

This reduces the risk of losing whole sections of data. 6f a scratch or piece of de%rismakes a part of the track unreada%le it will damage separate %its of data fromdifferent parts of the song or file instead of eliminating an entire segment ofinformation. :ince onl$ small pieces of each file segment are unreada%le it0s easierfor the CD pla$er to correct the pro%lem or recover from it.

Trailer TrackCD/1s and CD/1Ws have acomponent that ordinar$ musicCDs do not have // an e&tra %itof track at the %eginning of theCD %efore time 3ero(')which is the starting pointrecogni3ed %$ CD pla$ers. Thisadditional track space includesthe power memory area(5"A) and the powercalibration area(5CA). The5"A stores a temporar$ ta%leof contents for the individualpackets on a disc that has%een onl$ partiall$ recorded.When $ou complete the discthe %urner uses thisinformation to create the finalta%le of contents.

The 5CA is a sort of testingground for the CD %urner. 6norder to ensure that the write

laser is set at the right levelthe %urner will make a series oftest mar)salong the 5CAsection of track. The %urner willthen read over these markschecking for the intensit$ ofreflection in marked areas ascompared to unmarked areas.#ased on this information the%urner determines the optimumlaser setting for writing onto thedisc.

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The actual arrangement of information on music CDs is incredi%l$ comple&. And CD/1":// compact discs that contain computer files rather than song tracks // have even moree&tensive error/correction s$stems. This is %ecause an error in a computer file could corruptan entire program while a small uncorrected error on a music CD onl$ means a %it of fu33 ora skipping noise. 6f $ou are interested in the various wa$s that data is arranged on differentt$pes of CDs check outAudio Compact Disc / Writing and 1eading the Data.

With some writa%le CD formats $ou have to prepareall of the information %efore $ou %egin%urning. This limitation is %uilt into the original format of CDs as well as the ph$sical designof the disc itself. After all the long track forms one continuous connected string of +s and sand it0s difficult to %reak this up into separate sections. With newer disc formats $ou canrecord files one 2pac)et2 at a time adding the ta%le of contents and other unif$ing structuresonce $ou0ve filled up the disc.

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CD %urners are an ama3ing piece of technolog$ and the inner workings are certainl$fascinating. #ut to the t$pical computer user the most compelling aspect of %urners is what$ou can do with them. 6n the ne&t section we0ll find out how $ou can put all of this technolog$to work and make $our own music mi&.

Creating ?our wn CDs' :oftwareWhile CD/1s can store all sorts of digital information the most widespread application theseda$s is making m(sic-mi& CDswith a computer. 6f $ou0re new to the world of CD %urnersthis can seem like a daunting task. #ut it0s actuall$ ver$ simple once $ou have the rightsoftware and know the general procedure.

6f $ou have alread$ hooked up $our CD %urner the first step in making a CD is loading thesoftware $ou need. This music/management software serves several functions'

6t converts songs to the correct format for %urning.

6t allows $ou to arrange the songs for $our mi&.

6t controls the encoding process for writing to the CD.

These da$s most %urners are packaged with one or more music programs %ut $ou can also%u$ programs or download them over the 6nternet. ?ou ma$ need separate mediaapplications to handle different elements in the process %ut there are some good programsthat handle ever$thing (see %elow). Click hereto do a search for software related to %urning$our own CDs.

Creating ?our wn CDs' "usicWhen $ou have all of the software $ou need it0s time to gather some songs. ?ou ma$ wantto take songs directl$ from $our CD collection. To do this $ou need to 2rip2 the songs // cop$them from $our CD to $our computer0s hard drive. ?ou0ll need an e&traction program to do

this. To cop$ a particular track insert the CD into $our %uilt/in CD/1" drive (or the CD/%urner itself) and select the song $ou want through the e&traction program. @ssentiall$ theprogram will pla$ the song and re-recordit into a usa%le data format. 6t0s legal to makecopies of songs $ou own as long as the CD is onl$ for $our personal use.

?ou can also gather "5!sover the 6nternet. ?ou can download "5!s from pa$/for/musicsites or with file/sharing programs. :ome "5!s are free and can %e legall$ downloaded andcopied onto a CD. "ost are illegal copies however and it is a cop$right violation todownload them and %urn them onto a CD. To search for "5!/related We% sites click here.

"5!s are compressed files and $ou must e&pand (decode) them in order to %urn themonto a CD. :tandard music/management programs can decode these files. 6f $ou don0t have

the right software there are a num%er of decoding programs that $ou can download over the6nternet.

nce $ou0ve gathered the songs $ou can use $our music manager to arrange them in theorder $ou want. Beep in mind that $ou have a limited amount of disc space to work with. CD/1s have var$ing capacities measured in %oth mega%$tes and minutes. These da$s mostCD/1s are either 7 minutes or minutes long. #efore $ou move on to %urning $our CD$ou should make sure that $our mi& isn0t too long for the %lank disc.

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Creating ?our wn CDs' #urningnce the mi& is complete and $ou have saved it all $ou need to do is insert a %lank CD/1disc into the %urner and choose the 2%urn2 or 2write2 option in $our music/managementsoftware. #e sure to select 2m(sic CD2 rather than 2data CD2 or $ou won0t %e a%le to pla$the disc on ordinar$ CD pla$ers. ?ou0ll also need to choose the speedat which $ou want to

%urn the disc. T$picall$ a slowerspeed reduces the chance of a maor error during thewriting process.

A lot of things can go wrong when $ou0re %urning a CD so don0t %e surprised if some of themdon0t come out right. :ince CD/1s can not %e overwritten an$ irreversi%le mistake means$ou0ll have to unk the whole disc. Among the CD/%urning set this is called 2ma)ing acoaster2 as that0s prett$ much all $ou can do with the damaged CD.

6f $ou continuall$ have pro%lems %urning CDs $our drive ma$ %e defective or $our music/management program ma$ %e fault$. #efore $ou return $our %urner tr$ out some otherprograms and see if the$ $ield %etter results.

To make a CD-RO8 $ou0ll go through a similar process // %ut $ou0ll code the disc as a dataCD not a music CD. :ome newer CD pla$ers and D>D pla$erscan read untranslated "5!data files and $ou ma$ %e a%le to make CD/1" music mi&es this wa$. :ince "5!s arecompressed files $ou can fit a lot more of them on a single disc which means $ou can makea longer mi&. The draw%ack of course is that $our disc won0t work in the vast maorit$ of CDpla$ers.

CD %urners have opened up a whole new world to the average computer user. ?ou canrecord music that will run in most an$%od$0s CD pla$er or $ou can put together CD/1"scontaining photos We% pagesor movies. With a piece of e;uipment a%out the si3e of a carstereo and a%out the price of a cheap %ic$cle $ou can set up $our own multimediaproduction compan$

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