电子与通信工程专业英语Unit 2 Pulse Code Modulation-修改

电子与通信工程专业英语Unit2PulseCodeModulationOneoftheproblemsassociatedwiththetransmissionofspeechsignalsisthat,asthedistanceoverwhichthesignalistransmittedisincreased,sodoesthenoiseinducedintothetransmissioncircuit．Unfortunately，sincetheintroductionoftelephonyjustoveracenturyago,thenumberofsourcesofelectro-magneticnoisehavealsoincreasedconsiderably．Amplifyingsignalscannotbeusedasatechniquetoovercomenoisebecausethenoisegetsamplifiedalongwiththesignal．Ontheotherhand,thenumberofbinarydigitsrequiredtorepresentthequantizationlevelsisrelatedtothenumberoflevelsinthequantizationprocess．Thustoofineaquantizationleadstoanexcessiverequirementonthenumberofbinarydigitsrequiredtorepresenteachamplitudesample．Ithasbeenfoundinpracticethat256amplitudelevelsisadequateforacceptablespeechquality,whichrequires

log2256=8binarydigitspersample．Thequantizationlevels,however,arenotequallyspacedthroughoutthefullamplituderangeofthesignal,butarechosentobeofincreasingcoarsenessasthesignalamplitudeincreases．Thisensuresthatthesignal-to-quantizationnoiseratioremainssubstantiallyconstantthroughoutthefulldynamicrangeofspeechsignals．Becausetheeffectofthenon-linearquantizationscaleistocompressthelargeamplitudesignalsfortransmissionandtoexpandthemagainonreceipt,theprocesshasbecomeknownas“companding”.

3．Companding

Toobtaincompressionofthelargeamplitudesignals,alogarithmiclawwouldseemappropriate．Unfortunately,aswecanseefromFig2.2,thefunctiony

=

logxdoesnotpassthroughtheorigin．Itisthereforenecessarytosubstitutealinearportiontothecurveforlowervaluesofx．MostpracticalcompandingsystemsarebasedonsuggestedbyK.W.Cattermole,namely．

Fig2.2Functiony=logx

Aisthecompressioncoefficientandthecurveiscontinuousatx

=

1/A．ThelawisillustratedinFig2.3.

Fig2.3Cattermoles’scompandinglaw

Thepracticalimplementationofthislawwouldrequirenonlinearsignalprocessingfollowedbylinearquantization．Theimplementationofthenon-linearity,togetherwiththecomplementarynon-linearityatthereceiver,presentsconsiderableproblems．Itisusual,therefore,toimplementapiece-wiselinearsegmentalapproximationtothelaw．Twopiece-wiselinearlawsareingeneraluse．ThatusedinEuropeisknownastheCCITTcompandinglawA．A-lawcompandingconsistsofeightlinearsegmentsforeachpolarity,theslopehalvingforeachsegment,exceptforthelowesttwosegments．ThelawisillustratedinFig2.4．Threebitsarerequiredtodefinethesegment．Withineachsegmentthereare16linearsteps,definedbyafurther4bits．Finally,thepolarityofthesignalisdefinedbyasinglebit,makingatotalof8bitsinall．Becausethefullcurvehas13linearsegments,A-lawcompandingissometimesreferredtoas13segmentcompanding．ThecompressioncoefficientforA-lawcompandingisapproximatelyequalto87.6．AslightlydifferentcompandinglawisusedintheUSA,knownas-lawcompanding．In-lawcompandingtheslopeishalvedoveralleightsegmentsasshowninFig2.4,otherwisetheprocedureisverysimilartoA-law．Becausethefullcurvecomprises15linearsegments．-lawissometimesreferredtoas15segmentcompanding．

Fig2.4A-lawandμ-lawcompandingThepiece-wiselinearcompandingrulescanbereadilyimplementedwithintheanalogue-to-digitalconversionprocess．Asthesignalsamplepassesupfromonesegmenttoanother,allthatisrequiredisadoublingofthestepsizeoftheanalogue-to-digitalconverter．

NEWWORDSANDPHRASES

PulseCodeModulation(PCM) 脉冲编码调制

regeneratevt.,vi. 还原，再生，更新，(使)恢复

impair vt.,n. 削弱，损害，减少

recognition n. 认出，辨别，判明

beyondrecognition 不能辨认，认不出来

deterioration n. 变质，退化，变坏

quantization n. 量化，分层equivalent adj. 相等的，等效的

signal-to-quantization 量化信噪比

companding n. 压扩，展缩，压伸

coefficient n. 系数，折算率，程度

implementation n. 装置，仪器，履行，实现

complementary adj.余的，补的，补充，互补，辅助

threshold n. 槛，门限，定值

analogue-to-digitalconverter 模/数转换器

NOTES

[1]Providedthesepulseswereregeneratedbeforetheybecomeimpairedbeyondrecognition,thenitwouldbepossible,inprinciple,totransmitthesignaloverinfinitelylongdistanceswithnodeterioration,excepttheapproximationinherentinconvertingananaloguesignalintoadigitalsignalandbackagain．

本句中，“provided…recognition”是以“provided”为引导词的条件状语从句，“provided”是“只要”的意思。主句是“itwouldbepossible…deterioration”，其中“inprinciple”为插入语；“exceptthe…backagain”是介词短语作状语。本句可译为：只要在脉冲变得无法识别之前重新再生它，那么，除了把模拟信号转换为数字信号及把数字信号转换为模拟信号时内在的近似过程外，原则上信号可以无衰减地传输无限长的距离。

[2]Thesamplingtheoremtellsusthatweneedtosamplethesignalatarateequivalenttoatleasttwicethehighestfrequencypresentinthesignalifwearetoreconstitutethesignalatthereceiverwithoutdistortion．本句是复杂句，句中“thatweneed…withoutdistortion”是宾语从句；“ifwe…withoutdistortion”是宾语从句中的条件状语从句。

本句可译为：抽样定律告诉我们，要想在接收端无失真地重建信号，抽样频率至少是信号中最高频率的两倍。

[3]Thenextquestionwehavetoanswerishowmanyquantizationlevelsdoweneedtoprovideinordertorepresentthesignalamplitudewithsufficientaccuracy?该句是个复杂句。“Wehavetoanswer”是“question”的定语从句(省略了关系代词“that”)。“howmany…withsufficientaccuracy”是表语从句，它与“is”一起构成复合谓语。为了加强语气，强调谓语动词“need”表示的动作，“do”提到“weneed”之前。

本句可译为：接下来要解决的问题是：需要多少个量化等级才能足够准确地表示信号幅值呢？

EXERCISES

I.Trueorfalse.

(1)

TheoverallbinarydataraterequiredtotransmitaPCMspeechsignalis64

kb/s．()

(2)

Thequantizationlevelsareequallyspacedthroughoutthefullamplituderangeofspeechsignals．()

(3)

APCMbasisframehas32time-slots．()

II．TranslatethefollowingparagraphsintoChinese.

(1)Thisdeteriorationisknownasquantizationnoiseandwillbediscussedfurtherinasubsequentsection．Ofcourse,regeneratorsarenecessary,butthesearenomoredifficulttobuildthanamplifiersanditdosenotappearnecessarytoprovidethesearenomoredifficulttobuildthanamplifiersanditdosenotappearnecessarytoprovidetheseanymorefrequentlythanitisalreadynecessarytoprovideamplifierstoovercomethetransmissionlossesinthetransmissionpath．ThetechniqueofconvertingtheanaloguesignalintoadigitalpulsestreambecameknownasPulseCodeModulation(PCM)．(2)Thismeansthatsignalingmessagesrelatingtoaspecificchannelareonlyrequiredwhenthere1sachangeinstatusonthatchannel．Therepetitivetransmissionofsignalingmessages,characteristicofchannelassociatedsignaling，isthereforeavoided．Thistechniqueisknownascommonchannelsignaling．TheCCITTcommonchannelsignalingstandardforusewithdigitalswitchingnetworksisknownassignalingsystemNo.7(SS7)．TheNo.7signalingsystemoperatesonadatapacketconcept．

参考译文

第二单元脉冲编码调制

话音信号在传输过程中会遇到这样的问题，随着传输距离的增加，传输电路中引入的噪声也会随之增加。然而，自从一个多世纪前电话技术问世时起，各种各样的电磁干扰源与日俱增。单纯地使用信号放大技术并不能抑制噪声，因为信号放大的同时噪声也放大了。早在1983年，AlecReeves就提出了一种话音长距离传输中噪声的解决方法，即将模拟话音信号转换为一组数字脉冲信号串。只要在脉冲变得无法识别之前重新再生它，那么，除了把模拟信号转换为数字信号及把数字信号转换为模拟信号时内在的近似过程外，原则上信号可以无衰减地传输无限长距离。这种信号的降质称为量化噪声，这将在后面的部分进一步讨论。将模拟信号转换为数字脉冲串的技术称为脉冲编码调制(PCM)，下面详细介绍PCM。

1．采样

PCM的基本原理是：首先将代表声波的模拟信号按照一定的周期采样，然后将每个采样的幅值用数字表示。这将涉及量化过程，即用有限个数字量化等级来表示无限种幅值大小。量化过程如图2.1所示。

图2.1模拟信号的量化

采样定理告诉我们，要想在接收端无失真地重建信号，抽样频率至少是信号中最高频率的两倍。由于话音频带的带宽为300

Hz～3400Hz，因此PCM的采样频率取为8000个采样/秒。

2．量化

接下来要解决的问题是：需要多少个量化等级才能足够准确地表示信号幅值呢？如果量化太粗，则会由于量化噪声导致信号损坏。一方面，由于量化过程固有的近似，造成实际信号和再生信号存在差异，从而导致了量化噪声。另一方面，表示量化等级所需的二进制位数与量化过程的等级数有关。这样，如果量化级差分得太细，则用于表示每个采样幅值的二进制编码数量过多。通过实践发现，256个幅值等级就足以达到可接受的话音质量，每个采样需log2256

=

8个二进制数表示。然而，在信号的所有幅值范围内，量化级差并不是均匀的，信号幅值越大级差间隔越大。这将确保话音信号在全动态范围内的量化比大体上为一个恒定值。

因为非线性量化等级的作用是在传输时压缩大的幅值信号，而在接收端进行扩展，所以这个过程称为“压缩扩展”。

3．压缩扩展

为了实现对大幅值信号的压缩，可采用对数函数的特性。然而，函数y

=

logx不能通过原点，如图2.2所示。因此有必要将低x值曲线部分替换成线性的。大多数实际的压缩扩展系统都是基于K.W.Cattermole提出的A律实现的。

A为压缩系数，曲线在x

=

1/A处连续，如图2.3所示。

图2.2函数y

=

logx

图2.3Cattermole的压缩扩展律

在实际使用A律时，要求对非线性信号处理后再线性量化。非线性变换和在接收端进行的非线性反变换存在很多问题。因此，通常采用分段线性近似表示A律。常采用的有两种分段线性律。欧洲使用的是CCITT的A律压缩扩展。A律压缩扩展的每一级由8个线性段组成，除最低的两段外，每一段的斜率都减半，如图2.4所示。

图2.4A律和律压缩扩展

用3比特表示段，段又分为16个线性级，用另外4比特表示；最后，用1比特定义信号的极性，这样共用8比特表示完整的一段。因为全部曲线共有13个线性段，故A律压缩扩展有时被称为13段压缩扩展。A律压缩扩展的压缩系数约为87.6。美国使用的是μ律压缩扩展，与A律略有不同。如图2.4所示，μ律8段的斜率又减半了，其他部分与A律近似。因为全部曲线共有15个线性段，故μ律压缩扩展有时被称为15段压缩扩展。

在模/数转换过程中，分段线性压缩扩展很容易实现。当信号采样从一段到另一段时，只需对模/数转换器的级差大小加倍即可。

EXTENSIVETEXT

PSTN

PSTN,whichistheEnglishabbreviationforpublicswitchedtelephonenetwork,isaglobalvoicecommunicationcircuit-switchednetwork,andhaveasmanyas800millionusersaround.Initially,itisakindofanalogyfixed-linetelephonenetwork,butnow,almostallofthecurrentPSTNuseddigitaltelephonenetworksincludingmobileandfixedtelephonelines.Inthefollowing,wewillintroducePSTNinmoredetail.ThePublicSwitchedTelephoneNetwork(PSTN)istheconcatenationoftheworld’spubliccircuit-switchedtelephonenetworks,inmuchthesamewaythattheInternetistheconcatenationoftheworld’spublicIP-basedpacket-switchednetworks.Itisaninterconnectionnetworkbetweenthecenterandusers,whichoffersvoicedial-upbetweenanytwousersconnectedtothePSTN.Inaddition,overseasconnectionstoothercountriesareincluded.ThetechnicalstandardsofthePSTNarelargelygovernedbytheITU-T,anduseE.l63/E.164addresses(knownmorecommonlyastelephonenumbers)foraddressing.Originally,thePSTNadoptsmanualanaloguecircuitswitchingsystems(plug-boards).Thewholeprocessistheuseofelectromechanicalswitches.Butnowthisisalmostcompletelymadedigital,exceptforthefinalconnectiontothesubscriber(the“lastmile”).Thesignalscomingoutofthephonesetareallanalogue.Itisusuallytransmittedoveratwistedpaircablestillasananaloguesignal.Atthetelecommunicationofficethisanaloguesignalisusuallydigitized,using8000samplespersecondand8bitspersample,yieldinga64kb/sdatastream.SothebasicdigitalcircuitinthePSTNisa64-kilobit-per-secondchannel,originallydesignedbyBellLabs,calleda“DS0”orDigitalSignal0.TheDS0isthebasicunitatwhichswitchingtakesplaceinatelephoneexchange.DS0isalsoknownastimeslotsbecausetheyaremultiplexedtogetherinatime-divisionfashion.MultipleDS0saremultiplexedtogetheronhighercapacitycircuits,suchthat24DS0smakeaDS1signal,whichwhencarriedoncopperisthewell-known,T-carriersystem,T1(theEuropeanequivalentisanEl,containing3264kb/schannels).Inmodemnetworks,thismultiplexingmovesmoreandmoreclosertotheenduser,cabinetusuallyplacedbytheroadsideofresidents’areaorcommercialbuildings.

TheimportanttelecommunicationservicesthePSTNnetworkprovidesareasfollows:

(1)

fixedtelephony

(2)

cordlesstelephony,or“fixedtelephonywithlimitedterminalmobility”(3)

fax(viaabuilt-inmodem)

(4)

datacommunicationservices

ThevolumesofPSTNnetworkdataandfaxtraffichaveaverygreatimpactontheamountofinformationnetworkdesign.Butformanyyears,therealitysituationofthedatacommunicationonthephonenetworkisjustaprofessionaltechnicalterm.Thetelephonenetworktransmissionqualityvariesgreatly,anddatacommunicationoverthetelephonecommunicationsforthebiterrorraterequirementsaremuchmorestringent.Therefore,wecaneasilyunderstandwhydedicateddigitaldatanetworksaredeveloped.WearespeakinghereofnetworkssuchasX.25andframerelay,orNarrowbandISDN(N-ISDN).Now,therearealotofobstaclesforthePSTNondatatraffic,suchasthelimitedbandwidthandlongconnectionset-uptime.ThelimitedbandwidthofthenetworkrepresentsabottlenecktotheintroductionofvideoandmultimediaservicesintothePSTN.Video-telephonyisuptonowtheonlymultimediaservicePSTNoffers,butithasnotbeenwidespreadaccepted.Thisispartlyowingtothelackofavideophonestandard(Differentmanufacturers’videophonesdonotalwaysoperatewelltogether).Butstandardizationisunderway,andthedevelopmentofdatacompressionandmodemtechniquesseemstobearousingnewinterestinvideo-telephony.Meanwhile,fiberextensiveinstallationsandmodemcodinganderrorcorrectiontechnologyforthePSTNnetworkhavegreatlycontributedtoimprovementsindatatransmissioncapabilitie