电子信息工程专业英语翻译清华出版社English for IT and EE-10

信息科学与电子工程专业英语TechnicalEnglishForInformationScienceandElectronicEngineering信息科学与电子工程专业英语Unit10DigitalAudioCompression信息科学与电子工程专业英语PartIMPEGAudioLayer33NewWordsadvent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.1Asbandwidthinatelephonewireislimited,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,thememoryusedbecomessimpletocalculate:Memory=FrequencyBitDepthTime.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.ThismeansthatoneminuteofCDaudiouses44,10016602=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.3Thepsycho-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.SitessuchasMP3.comdistributefreeMP3sfromamultitudeofunsignedbands.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,MP3cancompressdigitala