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DistributedReal-TimeAudioProcessingNelsonPosseLagoAbstractComputersystemsforreal-timemultimediaprocessingrequirehighprocessingpower.Problemsthatdependonhighprocessingpowerareusuallysolvedbyusingparallelordistributedcomputingtechniques;however,thecombinationofthedicultiesofbothreal-timeandparallelprogramminghasledthedevelopmentofapplicationsforreal-timemultimediaprocessingforgeneralpurposecomputersystemstobebasedoncentralizedandsingle-processorsystems.Inseveralsystemsformultimediaprocessing,thereisaneedforlowlatencyduringtheinteractionwiththeuser,whichreinforcesthetendencytowardssingle-processordevelopment.Inthiswork,weimplementedamechanismforsynchronousanddistributedaudioprocessingwithlowlatencyonalocalareanetworkwhichmakestheuseofalowcostdistributedsystemforthiskindofprocessingpossible.Themaingoalistoallowtheuseofdistributedsystemsforrecordingandeditingofmusicalmaterialinhomeandsmallstudios,bypassingtheneedforhigh-costequipment.Thesystemweimplementedismadeoftwoparts:the rst,generic,implementedasamiddlewareforsynchronousanddistributedprocessingofcontinuousmediawithlowlatency;andthesecond,basedonthe rst,gearedtowardsaudioprocessingandcompatiblewithlegacyapplicationsbasedonthestandardLADSPAinterface.Weexpectthatfutureresearchandapplicationsthatsharetheneedsofthesystemdevelopedheremakeuseofthemiddlewarewedeveloped,bothforotherkindsofaudioprocessingaswellasfortheprocessingofothermediaforms,suchasvideo.1IntroductionInseveralcomputersystemsformultimediaprocessing(suchasinteractivesystemsforthecreationandeditionofmultimedia,particularlyaudioandmusic,orsystemsforpatternrecognitionincontinuousmedia),itishighlydesirabletobeabletodotheprocessingnotonlyinreal-time,butalsowithlowlatency.Lowlatencyprocessingmeansthatthetimeittakesforachangeintheinputdataofthecomputersystemtoproducethecorrespondingoutputshouldbeassmallaspossible;howsmallisenough,giventheusualgoalthatthelatencymustnotbeperceptiblebytheuser,variesalotwiththeapplicationandtheuser(seeSection2).Ininteractivesystems,forinstance,lowlatencyprocessingservesthepurposeofgivingtheusertheillusionthatthesystemperformsthecomputationsimmediately,whichisveryimportantsincetheusergenerallyadjustshisinputtothecomputersystemaccordingtotheoutputhereceivesfromthesystem.Lowlatencymayalsobeimportantifwewantpartofthedatatobeprocessedinreal-timebyexternaldevices(forinstance,wemaywanttorouteapreviouslycapturedaudiosignalintoananaloge ectsprocessorandrecordtheresultingsoundwithoutlosingthetiminginformationofthesignal).Asimpleexampleofasituationinwhichreal-timelow-latencyprocessingisdesirableistherecordingofanacousticmusicalinstrumentwithsomee ectprocessing(forinstance,anelectricguitarprocessedbyacustomdigitaldistorter):whileplayingtheinstrument,themusicianneedstohearthesoundbeingproduced;iftheprocessinglatencyistoolarge,themusicianwillhavedicultiestoperformcorrectly.Systemsformultimediaprocessinginreal-time,includingthosewherelowlatencyisdesirable,usu-allydemandlargeprocessingpowerfromthecomputersystem.Problemsthatdemandlargecomputing1power(asmultimediadoes)areusuallysolvedbyparallelordistributedprocessing.However,thereal-timeandlowlatencyrequirementsofmostmultimediaprocessingsystemscoupledwiththeneedforsometimesstrictsynchronizationbetweenseveralmediastreamsaswellasthecostofmultiprocessorsystemshavemademostmultimediaapplicationstobedevelopedforsingleprocessorsystems.Intheaudioandmusicprocessing eld,itisnotuncommonforsuchsystemstobecoupledwithdedicated,specializedhardwareinordertoboostthesystemperformance.Suchhardware,however,isusuallyproprietaryandexpensive:forexample,asingleprocessingboardfortheProToolsHDsystemcosts4timesasmuchasacompletemid-rangedesktopPCintheUSA1.Onsmallerstudioswithoutaccesstohigh-endequipment,itisrelativelycommonforthecomputingpowerduringaudioeditingtobeexceeded;whenthishappens,usuallypartoftheprocessingisdoneinnon-realtimemodeandtheprocessedresultissavedtodisk,whichisinconvenient,sincethepossibilityofinteractiveexperimentationislost.Giventheeconomicadvantageandexibilityo eredbygeneral-purposecomputersystems,beingabletoprocessmultimediadatainadistributedsystemwouldbeuseful,allowinguserstogobeyondtheperformancelimitsofsingleprocessorsystemsinamorecost-e ectiveway.Homeandsmallmusicrecordingstudios,whichusuallycannota ordtheexpensiveproprietarysolutions,wouldbene tfromtheuseofsmallclustersofolderandinexpensivecomputerstoincreasetheirprocessingpoweratalowcost.Inthiswork,wedevelopedamechanismfordistributedaudioprocessingonaLANwithlowlatencywhichiscompatiblewithlegacyapplicationsintheLinuxenvironmentusinggeneralpurpose,lowcosthardwareequipment.Linuxwaschosenforseveralreasonsbeyondits(many)technicalmerits,suchas(Kon,2001;Raymond,2001;SilveiraandCassino,2003):itslowcost;thefullavailabilityofthesourcecodeforboththeoperatingsystemkernelandtheapplicationsthatrunontopofit;thecommunityinvolvementinthedevelopmentofthesystem;thesocialbene tthatfreesoftwarecanbringaboutaspartofastrategyfordigitalinclusionofthepoorerpopulation;Thesystemisbasedontwolayers:the rst,generic,i