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1、UnitEightAuxiliaryStoragePassageACDandCD-ROMPassageBHardDisksandOpticalDisksPassageCIntroductiontoCD-ROMPassageACDandCD-ROMAcompactdisc(CD)isalaser-read(alsotermed“opticallyread”)datastoragedeviceonwhichaudio,video,ortextualmaterialcanbestored.Althoughithasbeenus
2、edprimarilytorecordstereophonicsoundandhassupplantedthelong-playingPHONOGRAPHrecordastheprincipalmediumformusicstorage,itoffersahugepotentialasthemediumforthestorageofmassiveamountsofmanytypesofinformation.[1]Unliketheconventionalphonographrecord,theCDstoresinfor
3、mationindigitalform.Stereophonic(two-channel)soundsignalsaredigitallysampledatarateof44,100timespersecondperchannel.Eachsampleisexpressedasabinarynumbervalueconsistingof16binarydigits,orbits.Thesampleddigitalvalues,alongwitherrorcorrectiondata,trackingcodes,andcu
4、eingdata,arerecordedonadigitaltape,whichisusedtomakecompression-moldedplasticdiscs12cm(4.72 in)indiameter,eachcoveredbyathin,reflectivemetalliclayerandprotectedbyaclearplasticcoating.[2]Duringplayback,alow-poweredlaserbeamreadsthedigitaldatathroughthereflectivere
5、arsurfaceofthedisk.[1]TheCDimprovesoverconventionalrecordsandtaperecordingswithitsmoreuniformandaccuratefrequencyresponse,acompleteabsenceofbackgroundnoise,awiderdynamicrange(thedifferencebetweenthesoftestandloudestrecordablemusicalsounds),andlongerwear—sincenoth
6、ingmechanicaltouchesthesurfaceofthediscwhenitisplayed.Introducedin1992,theSonyMiniDiscmeasuresonly6.35 cm (2.5 in)indiameter,butitcanbeusedtorecordaswellasplayupto74minutesofmusic.Itssmallsizeismadepossiblebyasystemofdatareductionthatusespsycho-acousticprinciples
7、toeliminatedatathatisnotnormallyheardbythelistener-therebyreducingthedatadensitytoone-fifththatrequiredforconventionalCDs.Mini-Discscanbeusedonlywiththeirownrecorder-player.AsecondimportantapplicationofCDtechnologyistheCD-ROM(CompactDiscReadOnlyMemory),atext/grap
8、hics/soundstoragemediumthatisaccessedthroughtheCD-ROMdriveinacomputer.ACD-ROMcanstoreinexcessof500megabytes(millionsofbytes)ofdata—ascomparedwithacomputer’shar
