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DistributedReal-TimeAudioProcessingNelsonPosseLagoAbstractComputersystemsforreal-timemultimediaprocessingrequirehighprocessingpower.Problemsthatdependonhighprocessingpowerareusuallysolvedbyusingparallelordistributedcomputingtechniques;however,thecombinationofthedicultiesofbothreal-timeandparallelprogramminghasledthedevelopmentofapplicationsforreal-timemultimediaprocessingforgeneralpurposecomputersystemstobebasedoncentralizedandsingle-processorsystems.Inseveralsystemsformultimediaprocessing,thereisaneedforlowlatencyduringtheinteractionwiththeuser,whichreinforcesthetendencytowardssingle-processordevelopment.Inthiswork,weimplementedamechanismforsynchronousanddistributedaudioprocessingwithlowlatencyonalocalareanetworkwhichmakestheuseofalowcostdistributedsystemforthiskindofprocessingpossible.Themaingoalistoallowtheuseofdistributedsystemsforrecordingandeditingofmusicalmaterialinhomeandsmallstudios,bypassingtheneedforhigh-costequipment.Thesystemweimplementedismadeoftwoparts:therst,generic,implementedasamiddlewareforsynchronousanddistributedprocessingofcontinuousmediawithlowlatency;andthesecond,basedontherst,gearedtowardsaudioprocessingandcompatiblewithlegacyapplicationsbasedonthestandardLADSPAinterface.Weexpectthatfutureresearchandapplicationsthatsharetheneedsofthesystemdevelopedheremakeuseofthemiddlewarewedeveloped,bothforotherkindsofaudioprocessingaswellasfortheprocessingofothermediaforms,suchasvideo.1IntroductionInseveralcomputersystemsformultimediaprocessing(suchasinteractivesystemsforthecreationandeditionofmultimedia,particularlyaudioandmusic,orsystemsforpatternrecognitionincontinuousmedia),itishighlydesirabletobeabletodotheprocessingnotonlyinreal-time,butalsowithlowlatency.Lowlatencyprocessingmeansthatthetimeittakesforachangeintheinputdataofthecomputersystemtoproducethecorrespondingoutputshouldbeassmallaspossible;howsmallisenough,giventheusualgoalthatthelatencymustnotbeperceptiblebytheuser,variesalotwiththeapplicationandtheuser(seeSection2).Ininteractivesystems,forinstance,lowlatencyprocessingservesthepurposeofgivingtheusertheillusionthatthesystemperformsthecomputationsimmediately,whichisveryimportantsincetheusergenerallyadjustshisinputtothecomputersystemaccordingtotheoutputhereceivesfromthesystem.Lowlatencymayalsobeimportantifwewantpartofthedatatobeprocessedinreal-timebyexternaldevices(forinstance,wemaywanttorouteapreviouslycapturedaudiosignalintoananalogeectsprocessorandrecordtheresultingsoundwithoutlosingthetiminginformationofthesignal).Asimpleexampleofasituationinwhichreal-timelow-latencyprocessingisdesirableistherecordingofanacousticmusicalinstrumentwithsomeeectprocessing(forinstance,anelectricguitarprocessedbyacustomdigitaldistorter):whileplayingtheinstrument,themusicianneedstohearthesoundbeingproduced;iftheprocessinglatencyistoolarge,themusicianwillhavedicultiestoperformcorrectly.Systemsformultimediaprocessinginreal-time,includingthosewherelowlatencyisdesirable,usu-allydemandlargeprocessingpowerfromthecomputersystem.Problemsthatdemandlargecomputing1power(asmultimediadoes)areusuallysolvedbyparallelordistributedprocessing.However,thereal-timeandlowlatencyrequirementsofmostmultimediaprocessingsystemscoupledwiththeneedforsometimesstrictsynchronizationbetweenseveralmediastreamsaswellasthecostofmultiprocessorsystemshavemademostmultimediaapplicationstobedevelopedforsingleprocessorsystems.Intheaudioandmusicprocessingeld,itisnotuncommonforsuchsystemstobecoupledwithdedicated,specializedhardwareinordertoboostthesystemperformance.Suchhardware,however,isusuallyproprietaryandexpensive:forexample,asingleprocessingboardfortheProToolsHDsystemcosts4timesasmuchasacompletemid-rangedesktopPCintheUSA1.Onsmallerstudioswithoutaccesstohigh-endequipment,itisrelativelycommonforthecomputingpowerduringaudioeditingtobeexceeded;whenthishappens,usuallypartoftheprocessingisdoneinnon-realtimemodeandtheprocessedresultissavedtodisk,whichisinconvenient,sincethepossibilityofinteractiveexperimentationislost.Giventheeconomicadvantageandexibilityoeredbygeneral-purposecomputersystems,beingabletoprocessmultimediadatainadistributedsystemwouldbeuseful,allowinguserstogobeyondtheperformancelimitsofsingleprocessorsystemsinamorecost-eectiveway.Homeandsmallmusicrecordingstudios,whichusuallycannotaordtheexpensiveproprietarysolutions,wouldbenetfromtheuseofsmallclustersofolderandinexpensivecomputerstoincreasetheirprocessingpoweratalowcost.Inthiswork,wedevelopedamechanismfordistributedaudioprocessingonaLANwithlowlatencywhichiscompatiblewithlegacyapplicationsintheLinuxenvironmentusinggeneralpurpose,lowcosthardwareequipment.Linuxwaschosenforseveralreasonsbeyondits(many)technicalmerits,suchas(Kon,2001;Raymond,2001;SilveiraandCassino,2003):itslowcost;thefullavailabilityofthesourcecodeforboththeoperatingsystemkernelandtheapplicationsthatrunontopofit;thecommunityinvolvementinthedevelopmentofthesystem;thesocialbenetthatfreesoftwarecanbringaboutaspartofastrategyfordigitalinclusionofthepoorerpopulation;Thesystemisbasedontwolayers:therst,generic,i
本文标题:Distributed Real-Time Audio Processing
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