信號系統(tǒng)與電子線路-復試專業(yè)英語

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(jié)modem調(diào)制解調(diào)器imperative勢在必行的psycho-acoustic心理聲學的pitch音調(diào)，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m(xù)inority少數(shù)mainstream主流genre流派，類型4NewWordsmultitude多數(shù)，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(quán)clout權(quán)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(jié)目，還是視頻伴音。62Thetraditionalmethodofstoringdigitalaudio,usedinCDsanddigitalTV,samplestheamplitudeofthesoundasetnumberoftimespersecond,andrecordsthis.2

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

每秒鐘的采樣次數(shù)（頻率），用于存儲幅度的比特位數(shù)（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(nèi)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(diào)高的聲音時，要提高使低頻聲能被聽到的臨界分貝數(shù)（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(jù)其出現(xiàn)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(jīng)銷這些流派的音樂。167PortablehardwareMP3playersmaynowbepurchasedatlowprices,andthepriceisstillfalling.SitessuchasdistributefreeMP3sfromamultitudeofunsignedbands.Otherssellalbumsfromunsignedbands,withafreeMP3trackfromsomealbums,allowingtheconsumertosampletheband’smusicbeforepurchase.另外一些網(wǎng)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(wǎng)站發(fā)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(chǎn)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(fā)布之后不到24小時就被破解了人們相信，MP3可能是傳統(tǒng)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(jīng)贏得（告別）了WMT4和其他對手發(fā)起的挑戰(zhàn)。PartIIDigitalAudioCompressionStandardAC326NewWordscoordination協(xié)調(diào)voluntary自發(fā)的，自愿的executive執(zhí)行的，執(zhí)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(tài)范圍fractional部分的，分數(shù)的woofer低音喇叭transponder應答器，轉(zhuǎn)發(fā)器demodulate解調(diào)terrestrial地面的，地球上的consistent一致的normative規(guī)范的，標準的syntax句法informative提供信息的overlap重疊decimate抽取27NewWordscoefficient系數(shù)exponent指數(shù)mantissa尾數(shù)envelope包絡allocation分配，指定synchronize使同步，同時發(fā)生resolution分辨率parameter參數(shù)inverse反轉(zhuǎn)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(xié)調(diào)的國家標準對于美國電視業(yè)今后的發(fā)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(tǒng)委員會）的工作之一就是尋求對ATV（先進電視系統(tǒng)）的非強制性國家技術標準的需要并在適當?shù)那闆r下協(xié)調(diào)這種開發(fā)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(tǒng)尚未經(jīng)過由獨立開發(fā)的編碼器到分別開發(fā)的解碼器之間進行信號傳輸?shù)尿炞C365MotivationInordertomoreefficientlybroadcastorrecordaudiosignals,theamountofinformationrequiredtorepresenttheaudiosignalsmaybereduced.需要減少用于表示音頻的信息量375Inthecaseofdigitalaudiosignals,theamountofdigitalinformationneededtoaccuratelyreproducetheoriginalpulsecodemodulation(PCM)samplesmaybereducedbyapplyingadigitalcompressionalgorithm,resultinginadigitallycompressedrepresentationoftheoriginalsignal.2

對于數(shù)字音頻信號，用于精確重建原始脈沖編碼調(diào)制樣本所需要的數(shù)字信息量可以通過應用數(shù)字壓縮算法來減少，由此產(chǎn)生原信號的數(shù)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(shù)字信息量，而不是壓縮音頻信號的動態(tài)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(shù)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(wèi)星發(fā)射設備將比特流轉(zhuǎn)換成射頻，送到衛(wèi)星收發(fā)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(jīng)濟效益經(jīng)過電纜或光纜傳輸音頻，或在磁、光、半導體或其它存儲介質(zhì)上存儲音頻信號458EncodingTheAC-3encoderacceptsPCMaudioandproducesanencodedbitstream

consistentwiththisstandard.Thespecificsoftheaudioencodingprocessarenotnormativerequirementsofthisstandard.產(chǎn)生符合本標準的比特流468Nevertheless,theencodermustproduceabitstreammatchingthesyntaxdescribedinSection5,which,whendecodedaccordingtoSections6and7,producesaudioofsufficientqualityfortheintendedapplication.Section8containsinformativeinformationontheencodingprocess.Theencodingprocessisbrieflydescribedbelow.對于具體應用產(chǎn)生音質(zhì)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(zhuǎn)換為一系列頻率系數(shù)的塊被乘以一個時間窗函數(shù)509Duetotheoverlappingblocks,eachPCMinputsampleisrepresentedintwosequentialtransformedblocks.Thefrequencydomainrepresentationmaythenbedecimatedbyafactoroftwosothateachblockcontains256frequencycoefficients.Theindividualfrequencycoefficientsarerepresentedinbinaryexponentialnotationasabinaryexponentandamantissa.3

各頻域系數(shù)以二進制指數(shù)形式表示成二進制指數(shù)和二進制尾數(shù)。519Thesetofexponentsisencodedintoacoarserepresentationofthesignalspectrumwhichisreferredtoasthespectralenvelope.Thisspectralenvelopeisusedbythecorebitallocationroutinewhichdetermineshowmanybitstousetoencodeeachindividualmantissa.信號頻譜的粗放表示形式對每一尾數(shù)編碼時所需要的比特數(shù)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(xiàn)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(shù)等。5410Errordetectioncodesareinsertedinordertoallowthedecodertoverifythatareceivedframeofdataiserrorfree.Theanalysisfilterbankspectralresolutionmaybedynamicallyalteredsoastobettermatchthetime/frequencycharacteristicofeachaudioblock.5Thespectralenvelopemaybeencodedwithvariabletime/frequencyresolution.分析濾波器組的頻譜分辨率可以動態(tài)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(shù)，以生成更優(yōu)的比特分配在低比特率工作時實現(xiàn)更高的編碼增益5610Inthetwo-channelmodearematrixingprocessmayb