信号系统与电子线路-复试专业英语

TechnicalEnglishForInformationScienceandElectronicEngineeringUnit10DigitalAudioCompressionPartIMPEGAudioLayer33NewWordsadvent到来，来临sampler样品album照相簿，歌曲集amplitude幅度，广阔precision精度，精确的stereo立体声megabyte兆字节modem调制解调器imperative势在必行的psycho-acoustic心理声学的pitch音调，程度threshold门限，阈值mono单一，单声道bitrate比特率HuffmanencodingHuffman编码bitcode比特字，比特码underestimate低估minority少数mainstream主流genre流派，类型4NewWordsmultitude多数，大众amultitudeof…种种的，众多的unsignedband未签约的乐队promote推销，促进rip撕，拉，劈royalty版税，庄严，王权clout权力，影响，力量spell拼写，迷住advocate提倡者，拥护者stem阻止，堵住rival对手lax宽松的，不严格的51WiththeadventoftheInternet,thereisadesireformoreandmoreinformationtobetransmittedacrossphonelines.Audioinformationisoneformthatisincreasinglydownloaded,beitasamplerforaband’salbum,aradioprogram,orsoundaspartofavideo.1

Asbandwidthinatelephonewireislimited,thishasledtoaneedforinformation(includingaudio)tobecompressed.音频信息是一种愈来愈多被下载的（多媒体）形式，无论是乐队的唱片选曲，无线电节目，还是视频伴音。62Thetraditionalmethodofstoringdigitalaudio,usedinCDsanddigitalTV,samplestheamplitudeofthesoundasetnumberoftimespersecond,andrecordsthis.2

用于CD和数字电视中存储数字音频的传统方法是每秒抽取并记录一定次数的声音幅度值。72Theprecisionoftheamplitudeisdeterminedbythenumberofbitsusedtostoretheamplitude.Sothebandwidth(ormemory)consumedbytheaudiosignalisdependentonthreefactors:thenumberofsamplestakenpersecond(Frequency),thenumberofbitsusedtostoretheamplitude(BitDepth)andthelengthofthesignal(Time).

每秒钟的采样次数（频率），用于存储幅度的比特位数（比特深度）以及信号的长度（时间）82Whenweknowthesethreethings,thememoryusedessimpletocalculate:Memory=FrequencyBitDepthTime.Additionally,ifthesignalisinstereo,thenthismustbedoubledastwosignalsareinfactused.如果信号是立体声的，内存就乘以二，因为立体声实际上用了两个信号93Thisequationcanbeusedtodemonstratewhytransmittinghigh-qualityaudioacrosstheInternetrequirescompression.CDaudiouses16-bitstereosampledat44,100Hz.ThismeansthatoneminuteofCDaudiouses44,10016602=84,672,000bits,orslightlyover10megabytes.103Astandard56kbpsmodemwouldtake84,672,000/57344=1477secondsorabout25minutes!25minutesisalongtimetowaitforoneminuteofaudio,soanalternativeisimperative.ThatalternativewasMPEGAudioLayer3,orMP3.等待1分钟的音频要25分钟这么长的时间，因此必须有另一个选择114ThecodecThehumanearcanonlyhearalimitedrangeoffrequencies.Thecodecthereforeremovesallsoundsoutsidethisrange,astheywillnotbeheardanyway.125Apsycho-acousticmodelisthenappliedtothesound.Ifahigh-pitchedsoundisplayed,thenthedecibelthresholdforsoundsoflowerfrequenciestobemadeaudibleisincreased.3

Thepsycho-acousticmodelremovesanyandallsoundsthatwillbe‘masked’inthisway.在播放音调高的声音时，要提高使低频声能被听到的临界分贝数（分贝阈值）。136ThenextstageisJointStereo.Thehumanbrainisunabletoplacethedirectionsofsoundsatlowfrequencies,sosoundsbelowthisthresholdareencodedinmono.Ifsomesectionsofthesignalarestillabovetherequiredbitrate,thenthequalityofthesesectionswillbedecreased.Finally,Huffmanencodingisapplied.人脑无法估计低频声音的方向，所以这个阈值之下的声音都用单声道编码146Thisreplacesallbitcodeswithuniquebitcodesofvaryinglengthaccordingtothefrequencyofthepatternoccurrence.4Forinstance,themostcommonlyoccurredbitpatternwouldbeencodedas‘01’,whilethenextcommonwouldbe‘010’andthenextwouldbe‘011’,andsoon.将所有比特码字根据其出现的频率换成独特的变长比特码。157SocialandeconomiceffectsThesocialeffectsofMP3cannotbeunderestimated.Itisallowingnew,unsignedbandstodistributefreemusicovertheInternet.Thosewithminoritytastescanobtainexperimentaloralternativemusicfarmoreeasilythanpreviously,asmainstreamrecordshopsdonottendtostockthesegenresofmusic.5

具有非主流口味（喜好）的人获取实验性或不同寻常的音乐比以往容易得多，主流唱片店一般不经销这些流派的音乐。167PortablehardwareMP3playersmaynowbepurchasedatlowprices,andthepriceisstillfalling.SitessuchasdistributefreeMP3sfromamultitudeofunsignedbands.Otherssellalbumsfromunsignedbands,withafreeMP3trackfromsomealbums,allowingtheconsumertosampletheband’smusicbeforepurchase.另外一些网站销售未签约乐队的专辑，用一些专辑上的免费MP3音乐让消费者在购买之前进行试听177Somemainstreamactshavealsotakentodistributingsomefreetracksinordertopromotealbums.Thereis,unfortunately,adarkersidetothisrevolution.Illegalsitesdistributetracksillegally‘ripped’fromthealbumsofestablishedartists,whosubsequentlyloseoutonroyalties.6

非法网站发行从已成名艺术家的唱片中非法窃取的音乐，这些艺术家因而失去版税。188TheeconomicimplicationsofMP3arecloselytiedtoitssocialeffects.Themajorrecordcompaniesareessentiallyrunningscaredoftheeffectthisislikelytohave.7

主要的唱片公司差不多因为这可能产生的社会影响而惊恐万状。198TheyrefusetosellMP3albums,exceptinthosecaseswheretheartisthasenoughclouttoforcethem.Theyarehurriedlytryingtoestablishastandardfora‘secure’musicformatthatcannotbeusedonmorethanonemachine.8

他们急欲建立一种“安全”音乐格式标准，使之不能用于一台以上的机器（仅能在一台机器上播放）。208Microsoftrecentlyattempted,andfailed,todojustthat.Theirformat,WMT4,was‘cracked’withinlessthan24hoursofitsrelease.Infact,thesametechniquesusedtocrackWMT4canbeappliedtoanymusicformat,nomatterhowsecure.PeoplebelievethatMP3mayspelltheendforthetraditionalrecordcompany.它们的格式WMT4，在发布之后不到24小时就被破解了人们相信，MP3可能是传统唱片公司的末日218Most,however,foreseetherecordcompaniesrealizingthattheycannotwinthisparticularwar,andbeginningtodistributeMP3albumsthemselves.Infact,inthisageMP3isprobablynotthatmuchlesssecurethantheCDitself.实际上，在这个时代，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.尽管唱片公司拒绝对其进行支持，但是它们也没办法阻止这个潮流2410TheMP3fileformatisanextremelyefficientcompressionstandard,whichhasalreadyseenoffchallengesfromWMT4andotherrivalstandards.9Duetoitsextremelylaxlicensingterms,MP3seemsunlikelytoloseitspopularity.MP3文件格式是一种效率极高的压缩标准，它已经赢得（告别）了WMT4和其他对手发起的挑战。PartIIDigitalAudioCompressionStandardAC326NewWordscoordination协调voluntary自发的，自愿的executive执行的，执行者annex附件multiplex多样的，多路复用herein在此，如此motivation动机，推动力algorithm算法dynamicrange动态范围fractional部分的，分数的woofer低音喇叭transponder应答器，转发器demodulate解调terrestrial地面的，地球上的consistent一致的normative规范的，标准的syntax句法informative提供信息的overlap重叠decimate抽取27NewWordscoefficient系数exponent指数mantissa尾数envelope包络allocation分配，指定synchronize使同步，同时发生resolution分辨率parameter参数inverse反转的，逆unpack解开rematrix重新进行矩阵变换dematrix求矩阵反变换281ForewordTheUnitedStatesAdvancedTelevisionSystemsCommittee(ATSC)†wasformedbythememberorganizationsoftheJointCommitteeonInterSocietyCoordination(JCIC)‡,recognizingthattheprompt,efficientandeffectivedevelopmentofacoordinatedsetofnationalstandardsisessentialtothefuturedevelopmentofdomestictelevisionservices.

认识到迅速有效地制定一套相互协调的国家标准对于美国电视业今后的发展至关重要291†TheUnitedStatesAdvancedTelevisionSystemsCommittee,December20,1995‡TheJCICispresentlycomposedof:theElectronicIndustriesAssociation(EIA),theInstituteofElectricalandElectronicEngineers(IEEE),theNationalAssociationofBroadcasters(NAB),theNationalCableTelevisionAssociation(NCTA),andtheSocietyofMotionPictureandTelevisionEngineers(SMPTE).302OneoftheactivitiesoftheATSCisexploringtheneedforand,whereappropriate,coordinatingthedevelopmentofvoluntarynationaltechnicalstandardsforAdvancedTelevisionSystems(ATV).1

ATSC（先进电视系统委员会）的工作之一就是寻求对ATV（先进电视系统）的非强制性国家技术标准的需要并在适当的情况下协调这种开发工作。312TheATSCExecutiveCommitteeassignedtheworkofdocumentingtheU.S.ATVstandardtoanumberofspecialistgroupsworkingundertheTechnologyGrouponDistribution(T3).TheAudioSpecialistGroup(T3/S7)waschargedwithdocumentingtheATVaudiostandard.起草美国ATV标准的工作323ThisdocumentwaspreparedinitiallybytheAudioSpecialistGroupaspartofitseffortstodocumenttheUnitedStatesAdvancedTelevisionbroadcaststandard.ItwasapprovedbytheTechnologyGrouponDistributiononSeptember26,1994,andbythefullATSCMembershipasanATSCStandardonNovember10,1994.作为制定美国ATV广播标准的一部分333AnnexA,“AC-3ElementaryStreamsinanMPEG-2Multiplex,”wasapprovedbytheTechnologyGrouponDistributiononFebruary23,1995,andbythefullATSCMembershiponApril12,1995.AnnexB,“AC-3DataStreaminIEC958Interface,”andAnnexC,“AC-3KaraokeMode,”wereapprovedbytheTechnologyGrouponDistributiononOctober24,1995andbythefullATSCMembershiponDecember20,1995.343ATSCStandardA/53,DigitalTelevisionStandardforHDTVTransmission,referencesthisdocumentanddescribeshowtheaudiocodingalgorithmdescribedhereinisappliedintheU.S.ATVstandard.引用了本文件并叙述了本文件所述的音频编码算法应如何用于美国ATV标准354Atthetimeofreleaseofthisdocument,thesystemdescriptioncontainedhereinhadnotbeenverifiedbythetransmissionofsignalsfromindependentlydevelopedencoderstoseparatelydevelopeddecoders.本文所述系统尚未经过由独立开发的编码器到分别开发的解码器之间进行信号传输的验证365MotivationInordertomoreefficientlybroadcastorrecordaudiosignals,theamountofinformationrequiredtorepresenttheaudiosignalsmaybereduced.需要减少用于表示音频的信息量375Inthecaseofdigitalaudiosignals,theamountofdigitalinformationneededtoaccuratelyreproducetheoriginalpulsecodemodulation(PCM)samplesmaybereducedbyapplyingadigitalcompressionalgorithm,resultinginadigitallycompressedrepresentationoftheoriginalsignal.2

对于数字音频信号，用于精确重建原始脉冲编码调制样本所需要的数字信息量可以通过应用数字压缩算法来减少，由此产生原信号的数字压缩形式。385

(Thetermcompressionusedinthiscontextmeansthecompressionoftheamountofdigitalinformationwhichmustbestoredorrecorded,andnotthecompressionofdynamicrangeoftheaudiosignal.)压缩一词在这里是指压缩必须存储或记录的数字信息量，而不是压缩音频信号的动态范围395Thegoalofthedigitalcompressionalgorithmistoproduceadigitalrepresentationofanaudiosignalwhich,whendecodedandreproduced,

soundsthesameastheoriginalsignal,whileusingaminimumofdigitalinformation(bitrate)forthecompressed(orencoded)representation.在解码和回放时，给出与原始信号相同的听觉效果，而其压缩（或编码）形式使用最少的数字信息量（比特率），405TheAC-3digitalcompressionalgorithmspecifiedinthisdocumentcanencodefrom1to5.1channelsofsourceaudiofromaPCMrepresentationintoaserialbitstreamatdataratesrangingfrom32kbpsto640kbps.The0.1channelreferstoafractionalbandwidthchannelintendedtoconveyonlylowfrequency(subwoofer)signals.可以将1至5.1声道的PCM形式音频源信号编码为32kbps至640bps的串行比特流416AtypicalapplicationofthealgorithmisshowninFigure10.1.Inthisexample,a5.1channelaudioprogramisconvertedfromaPCMrepresentationrequiringmorethan5Mbps(6channels×48kHz×18bits=5.184Mbps)intoa384kbpsserialbitstreambytheAC-3encoder.SatellitetransmissionequipmentconvertsthisbitstreamtoanRFtransmissionwhichisdirectedtoasatellitetransponder.卫星发射设备将比特流转换成射频，送到卫星收发器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…具有经济效益经过电缆或光缆传输音频，或在磁、光、半导体或其它存储介质上存储音频信号458EncodingTheAC-3encoderacceptsPCMaudioandproducesanencodedbitstream

consistentwiththisstandard.Thespecificsoftheaudioencodingprocessarenotnormativerequirementsofthisstandard.产生符合本标准的比特流468Nevertheless,theencodermustproduceabitstreammatchingthesyntaxdescribedinSection5,which,whendecodedaccordingtoSections6and7,producesaudioofsufficientqualityfortheintendedapplication.Section8containsinformativeinformationontheencodingprocess.Theencodingprocessisbrieflydescribedbelow.对于具体应用产生音质足够好的音频信号479TheAC-3algorithmachieveshighcodinggain(theratiooftheinputbit-ratetotheoutputbit-rate)bycoarselyquantizingafrequencydomainrepresentationoftheaudiosignal.AblockdiagramofthisprocessisshowninFigure10.2.通过对音频信号的频域形式进行粗量化489AnalysisFilterBankMantissaQuantizationAC-3FrameFormattingBitAllocationSpectralEnvelopEncodingPCMTimeSamplesExponentsBitAllocationInformationMantissasQuantizedMantissasEncodedSpectralEnvelopeEncodedAC-3Bit-StreamFigure10.2TheAC-3encoder499ThefirststepintheencodingprocessistotransformtherepresentationofaudiofromasequenceofPCMtimesamplesintoasequenceofblocksoffrequencycoefficients.Thisisdoneintheanalysisfilterbank.Overlappingblocksof512timesamplesaremultipliedbyatimewindowandtransformedintothefrequencydomain.将音频信号从一系列PCM时域样本形式转换为一系列频率系数的块被乘以一个时间窗函数509Duetotheoverlappingblocks,eachPCMinputsampleisrepresentedintwosequentialtransformedblocks.Thefrequencydomainrepresentationmaythenbedecimatedbyafactoroftwosothateachblockcontains256frequencycoefficients.Theindividualfrequencycoefficientsarerepresentedinbinaryexponentialnotationasabinaryexponentandamantissa.3

各频域系数以二进制指数形式表示成二进制指数和二进制尾数。519Thesetofexponentsisencodedintoacoarserepresentationofthesignalspectrumwhichisreferredtoasthespectralenvelope.Thisspectralenvelopeisusedbythecorebitallocationroutinewhichdetermineshowmanybitstousetoencodeeachindividualmantissa.信号频谱的粗放表示形式对每一尾数编码时所需要的比特数529Thespectralenvelopeandthecoarselyquantizedmantissasfor6audioblocks(1536audiosamples)areformattedintoanAC-3frame.TheAC-3bitstreamisasequenceofAC-3frames.5310TheactualAC-3encoderismorecomplexthanindicatedinFigure10.2.Thefollowingfunctionsnotshownabovearealsoincluded:Aframeheaderisattachedwhichcontainsinformation(bit-rate,samplerate,numberofencodedchannels,etc.)requiredtosynchronizetoanddecodetheencodedbitstream.4在每一帧前面加上头部，其中包含与编码比特流实现同步并将它解码所需的信息，如比特率、采样频率、编码声道数等。5410Errordetectioncodesareinsertedinordertoallowthedecodertoverifythatareceivedframeofdataiserrorfree.Theanalysisfilterbankspectralresolutionmaybedynamicallyalteredsoastobettermatchthetime/frequencycharacteristicofeachaudioblock.5Thespectralenvelopemaybeencodedwithvariabletime/frequencyresolution.分析滤波器组的频谱分辨率可以动态地改变以便更好地匹配每一音频（信号）块的时频特性。5510

Amorecomplexbitallocationmaybeperformed,andparametersofthecorebitallocationroutinemodifiedsoastoproduceamoreoptimumbitallocation.Thechannelsmaybecoupledtogetherathighfrequenciesinordertoachievehighercodinggainforoperationatlowerbit-rates.可以进行更复杂的比特分配，可以修改核心比特分配例程的参数，以生成更优的比特分配在低比特率工作时实现更高的编码增益5610Inthetwo-channelmodearematrixingprocessmayb