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1、CHAPTERAudioProcessing22Audioprocessingcoversmanydiversefields,allinvolvedinpresentingsoundtohumanlisteners.Threeareasareprominent:(1)highfidelitymusicreproduction,suchasinaudiocompactdiscs,(2)voicetelecommunications,anothernamefortelephonenetworks,and(3)sy
2、ntheticspeech,wherecomputersgenerateandrecognizehumanvoicepatterns.Whiletheseapplicationshavedifferentgoalsandproblems,theyarelinkedbyacommonumpire:thehumanear.DigitalSignalProcessinghasproducedrevolutionarychangesintheseandotherareasofaudioprocessing.Human
3、HearingThehumanearisanexceedinglycomplexorgan.Tomakemattersevenmoredifficult,theinformationfromtwoearsiscombinedinaperplexingneuralnetwork,thehumanbrain.Keepinmindthatthefollowingisonlyabriefoverview;therearemanysubtleeffectsandpoorlyunderstoodphenomenarela
4、tedtohumanhearing.Figure22-1illustratesthemajorstructuresandprocessesthatcomprisethehumanear.Theoutereariscomposedoftwoparts,thevisibleflapofskinandcartilageattachedtothesideofthehead,andtheearcanal,atubeabout0.5cmindiameterextendingabout3cmintothehead.Thes
5、estructuresdirectenvironmentalsoundstothesensitivemiddleandinnerearorganslocatedsafelyinsideoftheskullbones.Stretchedacrosstheendoftheearcanalisathinsheetoftissuecalledthetympanicmembraneoreardrum.Soundwavesstrikingthetympanicmembranecauseittovibrate.Themid
6、dleearisasetofsmallbonesthattransferthisvibrationtothecochlea(innerear)whereitisconvertedtoneuralimpulses.Thecochleaisaliquidfilledtuberoughly2mmindiameterand3cminlength.AlthoughshownstraightinFig.22-1,thecochleaiscurledupandlookslikeasmallsnailshell.Infact
7、,cochleaisderivedfromtheGreekwordforsnail.351352TheScientistandEngineer'sGuidetoDigitalSignalProcessingWhenasoundwavetriestopassfromairintoliquid,onlyasmallfractionofthesoundistransmittedthroughtheinterface,whiletheremainderoftheenergyisreflected.Thisisbeca
8、useairhasalowmechanicalimpedance(lowacousticpressureandhighparticlevelocityresultingfromlowdensityandhighcompressibility),whileliquidhasahighmechanicalimpedance.Inlesstechnicalterms,itrequiresmoreeffor
