TechnicalEnglishForInformationScienceandElectronicEngineeringUnit10DigitalAudioCompressionPartIMPEGAudioLayer33NewWordsadvent到來，來臨sampler樣品album照相簿，歌曲集amplitude幅度，廣闊precision精度，精確的stereo立體聲megabyte兆字節modem調制解調器imperative勢在必行的psycho-acoustic心理聲學的pitch音調，程度threshold門限，閾值mono單一，單聲道bitrate比特率HuffmanencodingHuffman編碼bitcode比特字，比特碼underestimate低估minority少數mainstream主流genre流派，類型4NewWordsmultitude多數，大眾amultitudeof…種種的，眾多的unsignedband未簽約的樂隊promote推銷，促進rip撕，拉，劈royalty版稅，莊嚴，王權clout權力，影響，力量spell拼寫，迷住advocate提倡者，擁護者stem阻止，堵住rival對手lax寬松的，不嚴格的51WiththeadventoftheInternet,thereisadesireformoreandmoreinformationtobetransmittedacrossphonelines.Audioinformationisoneformthatisincreasinglydownloaded,beitasamplerforaband’salbum,aradioprogram,orsoundaspartofavideo.1

Asbandwidthinatelephonewireislimited,thishasledtoaneedforinformation(includingaudio)tobecompressed.音頻信息是一種愈來愈多被下載的（多媒體）形式，無論是樂隊的唱片選曲，無線電節目，還是視頻伴音。62Thetraditionalmethodofstoringdigitalaudio,usedinCDsanddigitalTV,samplestheamplitudeofthesoundasetnumberoftimespersecond,andrecordsthis.2

用于CD和數字電視中存儲數字音頻的傳統方法是每秒抽取并記錄一定次數的聲音幅度值。72Theprecisionoftheamplitudeisdeterminedbythenumberofbitsusedtostoretheamplitude.Sothebandwidth(ormemory)consumedbytheaudiosignalisdependentonthreefactors:thenumberofsamplestakenpersecond(Frequency),thenumberofbitsusedtostoretheamplitude(BitDepth)andthelengthofthesignal(Time).

每秒鐘的采樣次數（頻率），用于存儲幅度的比特位數（比特深度）以及信號的長度（時間）82Whenweknowthesethreethings,thememoryusedessimpletocalculate:Memory=FrequencyBitDepthTime.Additionally,ifthesignalisinstereo,thenthismustbedoubledastwosignalsareinfactused.如果信號是立體聲的，內存就乘以二，因為立體聲實際上用了兩個信號93Thisequationcanbeusedtodemonstratewhytransmittinghigh-qualityaudioacrosstheInternetrequirescompression.CDaudiouses16-bitstereosampledat44,100Hz.ThismeansthatoneminuteofCDaudiouses44,10016602=84,672,000bits,orslightlyover10megabytes.103Astandard56kbpsmodemwouldtake84,672,000/57344=1477secondsorabout25minutes!25minutesisalongtimetowaitforoneminuteofaudio,soanalternativeisimperative.ThatalternativewasMPEGAudioLayer3,orMP3.等待1分鐘的音頻要25分鐘這么長的時間，因此必須有另一個選擇114ThecodecThehumanearcanonlyhearalimitedrangeoffrequencies.Thecodecthereforeremovesallsoundsoutsidethisrange,astheywillnotbeheardanyway.125Apsycho-acousticmodelisthenappliedtothesound.Ifahigh-pitchedsoundisplayed,thenthedecibelthresholdforsoundsoflowerfrequenciestobemadeaudibleisincreased.3

Thepsycho-acousticmodelremovesanyandallsoundsthatwillbe‘masked’inthisway.在播放音調高的聲音時，要提高使低頻聲能被聽到的臨界分貝數（分貝閾值）。136ThenextstageisJointStereo.Thehumanbrainisunabletoplacethedirectionsofsoundsatlowfrequencies,sosoundsbelowthisthresholdareencodedinmono.Ifsomesectionsofthesignalarestillabovetherequiredbitrate,thenthequalityofthesesectionswillbedecreased.Finally,Huffmanencodingisapplied.人腦無法估計低頻聲音的方向，所以這個閾值之下的聲音都用單聲道編碼146Thisreplacesallbitcodeswithuniquebitcodesofvaryinglengthaccordingtothefrequencyofthepatternoccurrence.4Forinstance,themostcommonlyoccurredbitpatternwouldbeencodedas‘01’,whilethenextcommonwouldbe‘010’andthenextwouldbe‘011’,andsoon.將所有比特碼字根據其出現的頻率換成獨特的變長比特碼。157SocialandeconomiceffectsThesocialeffectsofMP3cannotbeunderestimated.Itisallowingnew,unsignedbandstodistributefreemusicovertheInternet.Thosewithminoritytastescanobtainexperimentaloralternativemusicfarmoreeasilythanpreviously,asmainstreamrecordshopsdonottendtostockthesegenresofmusic.5

具有非主流口味（喜好）的人獲取實驗性或不同尋常的音樂比以往容易得多，主流唱片店一般不經銷這些流派的音樂。167PortablehardwareMP3playersmaynowbepurchasedatlowprices,andthepriceisstillfalling.SitessuchasdistributefreeMP3sfromamultitudeofunsignedbands.Otherssellalbumsfromunsignedbands,withafreeMP3trackfromsomealbums,allowingtheconsumertosampletheband’smusicbeforepurchase.另外一些網站銷售未簽約樂隊的專輯，用一些專輯上的免費MP3音樂讓消費者在購買之前進行試聽177Somemainstreamactshavealsotakentodistributingsomefreetracksinordertopromotealbums.Thereis,unfortunately,adarkersidetothisrevolution.Illegalsitesdistributetracksillegally‘ripped’fromthealbumsofestablishedartists,whosubsequentlyloseoutonroyalties.6

非法網站發行從已成名藝術家的唱片中非法竊取的音樂，這些藝術家因而失去版稅。188TheeconomicimplicationsofMP3arecloselytiedtoitssocialeffects.Themajorrecordcompaniesareessentiallyrunningscaredoftheeffectthisislikelytohave.7

主要的唱片公司差不多因為這可能產生的社會影響而驚恐萬狀。198TheyrefusetosellMP3albums,exceptinthosecaseswheretheartisthasenoughclouttoforcethem.Theyarehurriedlytryingtoestablishastandardfora‘secure’musicformatthatcannotbeusedonmorethanonemachine.8

他們急欲建立一種“安全”音樂格式標準，使之不能用于一臺以上的機器（僅能在一臺機器上播放）。208Microsoftrecentlyattempted,andfailed,todojustthat.Theirformat,WMT4,was‘cracked’withinlessthan24hoursofitsrelease.Infact,thesametechniquesusedtocrackWMT4canbeappliedtoanymusicformat,nomatterhowsecure.PeoplebelievethatMP3mayspelltheendforthetraditionalrecordcompany.它們的格式WMT4，在發布之后不到24小時就被破解了人們相信，MP3可能是傳統唱片公司的末日218Most,however,foreseetherecordcompaniesrealizingthattheycannotwinthisparticularwar,andbeginningtodistributeMP3albumsthemselves.Infact,inthisageMP3isprobablynotthatmuchlesssecurethantheCDitself.實際上，在這個時代，MP3在安全性上大概并不比CD差很多229ConclusionInconclusion,MP3cancompressdigitalaudiobyahighfactor.ThismakesitidealforthedistributionofaudioacrosstheInternet.Ithasfoundpopularityamongunsignedandexperimentalbands,aswellassomeestablishedartistswhoareadvocatesofthetechnology.239Althoughtherecordcompaniesrefusetobackit,thereislittletheycandotostemthetide.TheyhavefoundthemselvesverymuchinthepositionofKingCanute,althoughthiswillundoubtedlychangeinthelongterm.KingCanutewasaBritishmonarchwhobelievedthathecouldforcethetideback.Funnilyenough,hefailed.盡管唱片公司拒絕對其進行支持，但是它們也沒辦法阻止這個潮流2410TheMP3fileformatisanextremelyefficientcompressionstandard,whichhasalreadyseenoffchallengesfromWMT4andotherrivalstandards.9Duetoitsextremelylaxlicensingterms,MP3seemsunlikelytoloseitspopularity.MP3文件格式是一種效率極高的壓縮標準，它已經贏得（告別）了WMT4和其他對手發起的挑戰。PartIIDigitalAudioCompressionStandardAC326NewWordscoordination協調voluntary自發的，自愿的executive執行的，執行者annex附件multiplex多樣的，多路復用herein在此，如此motivation動機，推動力algorithm算法dynamicrange動態范圍fractional部分的，分數的woofer低音喇叭transponder應答器，轉發器demodulate解調terrestrial地面的，地球上的consistent一致的normative規范的，標準的syntax句法informative提供信息的overlap重疊decimate抽取27NewWordscoefficient系數exponent指數mantissa尾數envelope包絡allocation分配，指定synchronize使同步，同時發生resolution分辨率parameter參數inverse反轉的，逆unpack解開rematrix重新進行矩陣變換dematrix求矩陣反變換281ForewordTheUnitedStatesAdvancedTelevisionSystemsCommittee(ATSC)?wasformedbythememberorganizationsoftheJointCommitteeonInterSocietyCoordination(JCIC)?,recognizingthattheprompt,efficientandeffectivedevelopmentofacoordinatedsetofnationalstandardsisessentialtothefuturedevelopmentofdomestictelevisionservices.

認識到迅速有效地制定一套相互協調的國家標準對于美國電視業今后的發展至關重要291?TheUnitedStatesAdvancedTelevisionSystemsCommittee,December20,1995?TheJCICispresentlycomposedof:theElectronicIndustriesAssociation(EIA),theInstituteofElectricalandElectronicEngineers(IEEE),theNationalAssociationofBroadcasters(NAB),theNationalCableTelevisionAssociation(NCTA),andtheSocietyofMotionPictureandTelevisionEngineers(SMPTE).302OneoftheactivitiesoftheATSCisexploringtheneedforand,whereappropriate,coordinatingthedevelopmentofvoluntarynationaltechnicalstandardsforAdvancedTelevisionSystems(ATV).1

ATSC（先進電視系統委員會）的工作之一就是尋求對ATV（先進電視系統）的非強制性國家技術標準的需要并在適當的情況下協調這種開發工作。312TheATSCExecutiveCommitteeassignedtheworkofdocumentingtheU.S.ATVstandardtoanumberofspecialistgroupsworkingundertheTechnologyGrouponDistribution(T3).TheAudioSpecialistGroup(T3/S7)waschargedwithdocumentingtheATVaudiostandard.起草美國ATV標準的工作323ThisdocumentwaspreparedinitiallybytheAudioSpecialistGroupaspartofitseffortstodocumenttheUnitedStatesAdvancedTelevisionbroadcaststandard.ItwasapprovedbytheTechnologyGrouponDistributiononSeptember26,1994,andbythefullATSCMembershipasanATSCStandardonNovember10,1994.作為制定美國ATV廣播標準的一部分333AnnexA,“AC-3ElementaryStreamsinanMPEG-2Multiplex,”wasapprovedbytheTechnologyGrouponDistributiononFebruary23,1995,andbythefullATSCMembershiponApril12,1995.AnnexB,“AC-3DataStreaminIEC958Interface,”andAnnexC,“AC-3KaraokeMode,”wereapprovedbytheTechnologyGrouponDistributiononOctober24,1995andbythefullATSCMembershiponDecember20,1995.343ATSCStandardA/53,DigitalTelevisionStandardforHDTVTransmission,referencesthisdocumentanddescribeshowtheaudiocodingalgorithmdescribedhereinisappliedintheU.S.ATVstandard.引用了本文件并敘述了本文件所述的音頻編碼算法應如何用于美國ATV標準354Atthetimeofreleaseofthisdocument,thesystemdescriptioncontainedhereinhadnotbeenverifiedbythetransmissionofsignalsfromindependentlydevelopedencoderstoseparatelydevelopeddecoders.本文所述系統尚未經過由獨立開發的編碼器到分別開發的解碼器之間進行信號傳輸的驗證365MotivationInordertomoreefficientlybroadcastorrecordaudiosignals,theamountofinformationrequiredtorepresenttheaudiosignalsmaybereduced.需要減少用于表示音頻的信息量375Inthecaseofdigitalaudiosignals,theamountofdigitalinformationneededtoaccuratelyreproducetheoriginalpulsecodemodulation(PCM)samplesmaybereducedbyapplyingadigitalcompressionalgorithm,resultinginadigitallycompressedrepresentationoftheoriginalsignal.2

對于數字音頻信號，用于精確重建原始脈沖編碼調制樣本所需要的數字信息量可以通過應用數字壓縮算法來減少，由此產生原信號的數字壓縮形式。385

(Thetermcompressionusedinthiscontextmeansthecompressionoftheamountofdigitalinformationwhichmustbestoredorrecorded,andnotthecompressionofdynamicrangeoftheaudiosignal.)壓縮一詞在這里是指壓縮必須存儲或記錄的數字信息量，而不是壓縮音頻信號的動態范圍395Thegoalofthedigitalcompressionalgorithmistoproduceadigitalrepresentationofanaudiosignalwhich,whendecodedandreproduced,

soundsthesameastheoriginalsignal,whileusingaminimumofdigitalinformation(bitrate)forthecompressed(orencoded)representation.在解碼和回放時，給出與原始信號相同的聽覺效果，而其壓縮（或編碼）形式使用最少的數字信息量（比特率），405TheAC-3digitalcompressionalgorithmspecifiedinthisdocumentcanencodefrom1to5.1channelsofsourceaudiofromaPCMrepresentationintoaserialbitstreamatdataratesrangingfrom32kbpsto640kbps.The0.1channelreferstoafractionalbandwidthchannelintendedtoconveyonlylowfrequency(subwoofer)signals.可以將1至5.1聲道的PCM形式音頻源信號編碼為32kbps至640bps的串行比特流416AtypicalapplicationofthealgorithmisshowninFigure10.1.Inthisexample,a5.1channelaudioprogramisconvertedfromaPCMrepresentationrequiringmorethan5Mbps(6channels×48kHz×18bits=5.184Mbps)intoa384kbpsserialbitstreambytheAC-3encoder.SatellitetransmissionequipmentconvertsthisbitstreamtoanRFtransmissionwhichisdirectedtoasatellitetransponder.衛星發射設備將比特流轉換成射頻，送到衛星收發器426Figure10.1ExampleapplicationofAC-3tosatelliteaudiotransmission436Theamountofbandwidthandpowerrequiredbythetransmissionhasbeenreducedbymorethanafactorof13bytheAC-3digitalcompression.Thesignalreceivedfromthesatelliteisdemodulatedbackintothe384kbpsserialbitstream,anddecodedbytheAC-3decoder.Theresultistheoriginal5.1channelaudioprogram.下降了13倍以上447Digitalcompressionofaudioisusefulwhereverthereisaneconomicbenefittobeobtainedbyreducingtheamountofdigitalinformationrequiredtorepresenttheaudio.Typicalapplicationsareinsatelliteorterrestrialaudiobroadcasting,deliveryofaudioovermetallicoropticalcables,orstorageofaudioonmagnetic,optical,semiconductor,orotherstoragemedia.只要…具有經濟效益經過電纜或光纜傳輸音頻，或在磁、光、半導體或其它存儲介質上存儲音頻信號458EncodingTheAC-3encoderacceptsPCMaudioandproducesanencodedbitstream

consistentwiththisstandard.Thespecificsoftheaudioencodingprocessarenotnormativerequirementsofthisstandard.產生符合本標準的比特流468Nevertheless,theencodermustproduceabitstreammatchingthesyntaxdescribedinSection5,which,whendecodedaccordingtoSections6and7,producesaudioofsufficientqualityfortheintendedapplication.Section8containsinformativeinformationontheencodingprocess.Theencodingprocessisbrieflydescribedbelow.對于具體應用產生音質足夠好的音頻信號479TheAC-3algorithmachieveshighcodinggain(theratiooftheinputbit-ratetotheoutputbit-rate)bycoarselyquantizingafrequencydomainrepresentationoftheaudiosignal.AblockdiagramofthisprocessisshowninFigure10.2.通過對音頻信號的頻域形式進行粗量化489AnalysisFilterBankMantissaQuantizationAC-3FrameFormattingBitAllocationSpectralEnvelopEncodingPCMTimeSamplesExponentsBitAllocationInformationMantissasQuantizedMantissasEncodedSpectralEnvelopeEncodedAC-3Bit-StreamFigure10.2TheAC-3encoder499ThefirststepintheencodingprocessistotransformtherepresentationofaudiofromasequenceofPCMtimesamplesintoasequenceofblocksoffrequencycoefficients.Thisisdoneintheanalysisfilterbank.Overlappingblocksof512timesamplesaremultipliedbyatimewindowandtransformedintothefrequencydomain.將音頻信號從一系列PCM時域樣本形式轉換為一系列頻率系數的塊被乘以一個時間窗函數509Duetotheoverlappingblocks,eachPCMinputsampleisrepresentedintwosequentialtransformedblocks.Thefrequencydomainrepresentationmaythenbedecimatedbyafactoroftwosothateachblockcontains256frequencycoefficients.Theindividualfrequencycoefficientsarerepresentedinbinaryexponentialnotationasabinaryexponentandamantissa.3

各頻域系數以二進制指數形式表示成二進制指數和二進制尾數。519Thesetofexponentsisencodedintoacoarserepresentationofthesignalspectrumwhichisreferredtoasthespectralenvelope.Thisspectralenvelopeisusedbythecorebitallocationroutinewhichdetermineshowmanybitstousetoencodeeachindividualmantissa.信號頻譜的粗放表示形式對每一尾數編碼時所需要的比特數529Thespectralenvelopeandthecoarselyquantizedmantissasfor6audioblocks(1536audiosamples)areformattedintoanAC-3frame.TheAC-3bitstreamisasequenceofAC-3frames.5310TheactualAC-3encoderismorecomplexthanindicatedinFigure10.2.Thefollowingfunctionsnotshownabovearealsoincluded:Aframeheaderisattachedwhichcontainsinformation(bit-rate,samplerate,numberofencodedchannels,etc.)requiredtosynchronizetoanddecodetheencodedbitstream.4在每一幀前面加上頭部，其中包含與編碼比特流實現同步并將它解碼所需的信息，如比特率、采樣頻率、編碼聲道數等。5410Errordetectioncodesareinsertedinordertoallowthedecodertoverifythatareceivedframeofdataiserrorfree.Theanalysisfilterbankspectralresolutionmaybedynamicallyalteredsoastobettermatchthetime/frequencycharacteristicofeachaudioblock.5Thespectralenvelopemaybeencodedwithvariabletime/frequencyresolution.分析濾波器組的頻譜分辨率可以動態地改變以便更好地匹配每一音頻（信號）塊的時頻特性。5510

Amorecomplexbitallocationmaybeperformed,andparametersofthecorebitallocationroutinemodifiedsoastoproduceamoreoptimumbitallocation.Thechannelsmaybecoupledtogetherathighfrequenciesinordertoachievehighercodinggainforoperationatlowerbit-rates.可以進行更復雜的比特分配，可以修改核心比特分配例程的參數，以生成更優的比特分配在低比特率工作時實現更高的編碼增益5610Inthetwo-channelmodearematrixingprocessmayb