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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 114

MPEG

Byfar,themostpopularhighfidelityaudiocodersrelyupontechniquesdevelopedunderthe

MPEGumbrella.MPEGstandsforMotionPicturesExpertGroup,aJointCommitteeofthe

InternationalStandardsOrganisation(ISO)andtheInternationalElectrotechnicalCommission.

Overadecadeago,whentheCDhadjustbeenintroduced,thefirstproposalsforaudiocodin

g

weregreetedwithsuspicionanddisbelief.Therewaswidespreadagreementthatitwould

simplynotbepossibletosatisfygoldenearswhiledeleting80%ormoreofthedigitalaudio

data.ButtheaudiocodingpioneerswerepersistentandtheMPEGaudiogroupwasformed.

Since1988,theyhavebee

nworkingonthestandardizationofhighqualityaudiocoding.Today,

almostallagreenotonlythataudiobitratereductioniseffectiveanduseful,butthattheMPEG

processhasbeensuccessfulatpickingthebesttechnologyandencouragingcompatibilityacross

awidevarietyofequipment.

TheMPE

Gprocessisopenandcompetitive.Acommitteeofindustryrepresentativesand

researchersmeettodeterminegoalsfortargetbitrate,qualitylevels,applicationareas,testing

procedures,etc.Interestedorganizationsthathavesomethingtocontributeareinvitedto

submittheirbestwork.Acarefuldoubleblindlisteningtestseriesisth

enconductedto

determinewhichoftheentrant'stechnologiesdeliversthehighestperformance.Thesubjective

listeningevaluationsaredoneatvariousvolunteerorganizationsaroundtheworldthathave

accesstobothexperiencedandinexperiencedtestsubjects.Broadcastersarethemost

commonparticipantswithmanyoftheimportanttestseriesconductedattheBBCinEn

gland,

theCBCinCanada,andNHKinJapan.Finally,resultsaretabulated,areportisdraftedand

ultimatelyastandardisissued.

In1992,underMPEG1(thefirstoftheMPEGstandards),thisprocessresultedintheselectionof

threerelatedaudiocodingmethods,eachtargetedtodifferentbi

tratesandapplications.These

arethefamouslayers:1,2and3.Asthelayernumbergoesup,sodoesperformanceand

implementationcomplexity.Layer1isnotmuchused.Layer‐2iswidelyusedforDABinEurope,

audioforvideo,andbroadcastplayoutsystems.Layer‐3–w

hichTeloswasthefirsttouseinthe

Zephyr–iswidelyusedinbroadcastcodecsandhasgoneontosignificantInternetand

consumerelectronicsfameunderthemonikerderivedfromthefileextension:MP3.MPEG2

openedthedoorfornewwork,andsomeminorimprovementswereaddedtobothLayers2

and3.In199

7,thefirstintheAACfamilywasaddedtotheMPEG2standard.MPEG4isongoing

now,butithasalreadybeendecidedthatAACwillbethe“generalaudio”coderunderthis

umbrella.(MPEG3wasskippedforreasonsunknown.)



MPEG2 AAC (Advanced Audio Coding)

TheMPE

G2AACsystemisthenewestaudiocodingmethodselectedbyMPEGandbecamean

InternationalstandardinApril1997.Itisafullystate‐of‐the‐artaudiocompressiontoolthat

providesperformancesuperiortoanyknownapproachatbitratesgreaterthan64kbpsand

excellentperformancerelativetothealternativesatbitratesr

eachingaslowas16kbps.

TheideathatledtoAACwasnotonlytostartfresh,butalsotocombinethebestworkfromthe

world’sleadingaudiocodinglaboratories.Fraunhofer,Dolby,Sony,andAT&Tweretheprimary

collaboratorsthatofferedcomponentsforAAC.Th

ehopedforresultwasITU(International

Tele‐communicationsUnion)“indistinguishablequality“at64kbpspermonochannel.Thatis,

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