Unit19TheDigitalAudio

ContentsLearningObjectives1Words&Expressions2Text3Exercises4Learning

ObjectivesAfteryouhavereadthisunit,youshouldbefamiliarwith:1.MIDI.

2.Digitalaudio.

Words&ExpressionsSynthesizer(電子)合成器samplingrate采樣頻率Samplesize采樣深度，樣本大小quantization量化TextMIDIVersusDigitalAudioMIDI(Musicalinstrumentdigitalinterface)isacommunicationsstandarddevelopedintheearly1980sforelectronicmusicalinstrumentsandcomputers.Itallowsmusicandsoundsynthesizersfromdifferentmanufacturerstocommunicatewitheachotherbysendingmessagesalongcablesconnectedtothedevices.MIDIprovidesaprotocolforpassingdetaileddescriptionsofamusicalscore,suchasthenotes,sequencesofnotes,andwhatinstrumentwillplaythesenotes.ButMIDIdataarenotdigitizedsound;theyareshorthandrepresentationsofmusicstoredinnumericform.Digitalaudioisarecording,MIDIisascore–thefirstdependsonthecapabilitiesofyoursoundsystem,theotheronthequalityofyourmusicalinstrumentsandthecapabilitiesofyoursoundsystem.TextAMIDIfileisalistoftime-stampedcommandsthatarerecordingsofmusicalactions(thepressingdownofapianokeyorasustainpedal,forexample,orthemovementofacontrolwheelorslider)that,whensenttoaMIDIplaybackdevice,resultsinsound.MIDImessagecancauseacomplexsoundorsequenceofsoundtoplayonaninstrumentorsynthesizer,soMIDIfiletendtobesignificantlysmallerthanequivalentdigitizedwaveformfile.IncontrasttoMIDIdata,digitalaudiodataaretheaccrualrepresentationsofsound,storedintheformofthousandsofindividualnumbers(calledsamples).Thedigitaldatarepresenttheinstantaneousamplitude(orloudness)ofasoundatdiscreteslicesoftime.Becauseitisnotdevice-dependent,digitalaudiosoundsthesameeverytimeitisplayed.Butthatconsistencycomesataprice,largedatastoragefiles.DigitalsoundisusedformusicCDs.MIDIdataaretodigitalaudiodatawhatvectorordrawngraphicsaretobitmappedgraphics.Thatis,MIDIdataaredevice-dependent:digitaldataarenot.Justastheappearanceofvectorgraphicsdiffersdependingontheprinterdeviceordisplayscreen,thesoundsproducedbyMIDImusicfilesdependontheparticularMIDIdeviceusedforplayback.Similarly,arollofperforatedplayer-pianoscoreplayedonaconcertgrandwouldsounddifferentthenifplayedonahonky-tonkpiano.Digitaldata,ontheotherhand,producesoundsthataremoreorlessidenticalregardlessoftheplaybacksystem.TheMIDIstandardislikePostScript,lettinginstrumentcommunicateinawellunderstoodlanguage.DigitalaudioYoucandigitizesoundformamicrophone,asynthesizer,existingtaperecordings,liveradioandtelevisionbroadcasts,popularCDs,andyourfavoritetone-playingrecords.Inface,youcandigitizesoundsfromanysource,naturalorprerecorded.Digitizedsoundissampledsound.Everynthfractionofasecond,asampleofsoundistakenandstoredasdigitalinformationinbiteandbytes.Howoftenthesamplesaretakenisthesamplingrate,andtheamountofinformationstoredabouteachsampleisthesamplesize.Themoreoftenyoutakeasampleandthemoredatayoustoreaboutthatsample,thefinertheresolutionandqualityofthecapturedsoundwhenitisplayedback.

ThethreesamplingfrequenciesmostoftenusedinmmultimediaareCD-quality44.1kHz(kilohertz),22.05kHz,and11.025kHz.Samplesizesareeither8bitsor16bits.Thelargerthesamplesize,thebetterthedatadescribetherecordedsound.An8bitsamplesize,ontheotherhand,providesastaggering65536equalunitstodescribethedynamicrange.Thevalueofeachsampleisroundedofftotheneatestinteger(quantization),andiftheamplitudeisgreaterthantheintervalsavailable,clippingofthetopandbottomofthewaveoccurs.Quantizationcanproduceanunwantedbackgroundhissingnoise,andclippingmayseverelydistortthesound.Exercises1.Answerthefollowingquestionaccordingtothetext.（1）WhatisMIDI?（2）Aredigitalaudiodatatheactualrepresentationsofsound?（3）Isdigitalsounddevicedependent?HowaboutMIDI?（4）Whatisthesamplingrate?Whatisthesamplesize?（5）Howmanyequalunitsdoes8-bitsamplesizeprovide?

2.TranslatethefollowingtermsorphrasesfromEnglishintoChineseandviceversa:(1)synthesizer(6)數(shù)字音頻(2)samplingrate(7)麥克風(fēng)(3)samplesize(8)波形(4)quantization(9)背景(5)MIDI(10)數(shù)幅值key3.Fillintheblankswiththeinformationgiveninthetext:

(1)________isacommunicationsstandarddevelopedintheearly1980sforelectronicmusicalinstrumentsandcomputers.(2)IncontrasttoMIDIdata,________dataaretheaccrualrepresentationsofsound,storedintheformofthousandsofindividualnumbers(calledsamples).(3)MIDIdataaretodigitalaudiodatawhat________ordrawngraphicsaretobitmappedgraphics.(4)Youcandigitizesoundformamicrophone,a________,existingtaperecordings,liveradioandtelevisionbroadcasts,popularCDs,andyourfavoritetone-playingrecords.(5)________areeither8bitsor16bits.

key4.Choosethebestonefromtheitemsgivenbelowtocompletethefollowingpassage.

AVSvideostandardis(1)bytheAudioVideoCodingStandardWorkingGroupofChina,whichwasapproved(2)June2003.AVSworkinggrouphas(3)thefirstversionofAVSvideo

(4)in2003.AVSvideostandardprovidesagoodtrade-offsolution(5)complexityandcodingefficiency.(1)A.madeB.developedC.improvedD.Went(2)A.inB.onC.duringD.after(3)A.producedB.providedC.finishedD.Given(4)A.numberB.systemC.representationD.standard(5)A.amongB.betweenC.inD.onkey5.MarkthefollowingstatementswithT(true)orF(false)accordingtothetext.(1)MIDIisacommunicationsstandarddevelopedforelectronicmusicalinstrumentsandcomputers.(2)MIDIdataaredigitizedsound;theyareshorthandrepresentationsofmusicstoredinnumericform.(3)Digitalaudiodataaretheactualrepresentationsofsound,storedintheformofthousandsofindividualnumbers(calledsamples).(4)An16-bitsamplesizeprovides256equalunitstodescribethedynamicrangeoramplitude.(5)Quantizationcanproduceanunwantedbackgroundhissingnoise,andclippingmayseverelydistortthesound.

key6.Selecttheoneanswertoeachquestion.

(1)TheCDservesasmultimedia’schiefstorageand_______medium.A.changeB.makeC.saveD.exchange(2)Ifyouwantseparateanordinarycomputerfrommultimedia,theonlythingsareasoundboardandaCD-ROM_______.A.runnerB.driverC.drawerD.painter(3)_______fact,Multimediaisjusttwomedia:soundandpictures.A.ofB.atC.onD.in(4)It________youcantailoramultimediapresentationtoyourownneeds.A.isB.meansC.wasD.seems(5)Youdon'thavetobeapassiverecipient_______multimedia.A.inB.aboutC.withD.outkey7.TranslatethefollowingsentencesintoChinese.(1)Digitalaudioisarecording,MIDIisascore–thefirstdependsonthecapabilitiesofyoursoundsystem,theotheronthequalityofyourmusicalinstrumentsandthecapabilitiesofyoursoundsystem.(2)MIDImessagecancauseacomplexsoundorsequenceofsoundtoplayonaninstrumentorsynthesizer,soMIDIfiletendtobesignificantlysmallerthanequivalentdigitizedwaveformfile.(3)Thedigitaldatarepresenttheinstantaneousamplitude(orloudness)ofasoundatdiscreteslicesoftime.(4)Similarly,arollofperforatedplayer-pianoscoreplayed