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JOINTFILTERBANKSTRUCTURESFORINTEGRATINGAUDIOCODINGINTOHEARING AIDAPPLICATIONS RongDong,DavidHermann,RobertBrennan,EdwardChau

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Content	JOINTFILTERBANKSTRUCTURESFORINTEGRATINGAUDIOCODINGINTOHEARING AIDAPPLICATIONS RongDong,DavidHermann,RobertBrennan,EdwardChau AMISemiconductor,611KumpfDrive,Unit200,Waterloo,Ontario,CanadaN2V1K8 ABSTRACT Tohelpdevelopultra-lowpowerwirelesshearingaidproducts, weinvestigatetheintegrationofsubbandaudiocodingwithhear- ingaidapplications.Boththeaudiocodingandthehearingaid applicationusesubbandprocessing,buttheirrequirementsforthe ﬁlterbanksaretotallydifferent.Thehearingaidapplicationtypi- callyusesanoversampledﬁlterbanktoreducethealiasingineach subband,whereastheaudiocodecneedsacriticallysampledﬁlter- bankformaximalcodingefﬁciency.Ajointﬁlterbankstructureis proposedinthispapertosatisfythesecontradictiveﬁlterbankre- quirements.Withthisstructure,thetwoﬁlterbanksarecombined intoasinglestereoﬁlterbankoperation,whichcanbeefﬁcientlyim- plementedonaﬁlterbankcoprocessor.Thisstructuresubstantially reducesthecomputationcomplexity,powerconsumptionandmem- oryusage. IndexTerms—Channelbankﬁlters,transformcoding,audio coding,hearingaids,coprocessors 1.INTRODUCTION Addingwirelessfunctionalitytohearingaidsbringsmanybeneﬁts totheusers[1].Forexample,ahearingaidcanreceiveanaudio streamfromaremotedevice(e.g.,amicrophone,television,tele- phoneoranotherhearingaidontheoppositeear)wirelesslytore- ducethenegativeeffectsofdistance,reverberationandbackground noise. Inordertoreducethetransmissionbandwidth,hencethepower consumption,foraudiostreaming,anaudiocodecisnecessaryto providedatacompression.Theaudiocompressioncanbeperformed ineithertimedomainorasubbanddomainbasedontime-frequency transforms.However,withsubbandcodec,wecanallocatemore bitstoperceptuallyimportantsubbandsandachievehighercoding gainthanthetime-domaincodecs[2].Thus,asubbandcodecisan integralpartofthenewgenerationofwireless-capablehearingaids. However,inahearingaiddevice,eventhoughboththecodec andthehearingaidalgorithmsusesubbandsignalprocessing,their ﬁlterbankrequirementsareverydifferent.Thehearingaidalgo- rithmstypicallyapplywidemagnitudeadjustmentsineachsubband, sotheyrequireanoversampledﬁlterbanktoachieveapropertrade- offintermsofaliasingreductionineachsubband,groupdelay,and computationcomplexity[3].Ontheotherhand,thesubbandcodec requiresacriticallysampledﬁlterbanktominimizeredundantin- formationandmaximizethecompressionrate.Unfortunately,itis well-knownthat,inacriticallysampledﬁlterbank,theoverlapping betweenadjacentbandsgeneratesaliasingdistortionsthatcanbe cancelledoutonlyifthereislittleornomodiﬁcationtothesub- bandsignal.Thisisobviouslyincompatiblewithhearingaidsignal processing,sochallengesarisewhenweattempttointegratethese twodifferenttypesofsubbandprocessingalgorithms.Therefore,in thispaper,weinvestigateandpresentanovelapproachfortheefﬁ- cientintegrationofasubbandcodecintoahearingaidapplication, basedonanultra-lowpowerDSPsystemincludinganoptimizedﬁl- terbankcoprocessor.Ourproposedsolutionaimsforminimizingthe computationcomplexity,powerconsumptionandmemoryusage. Therestofthepaperisorganizedasfollows.Section2describes thearchitectureoftheDSPsystemandwhatthecoprocessoriscapa- bleof.InSection3,weproposeanapplicationframeworktosolve theproblemoftheconﬂictingﬁlterbankrequirements.InSection 4,weproposeaselectionofﬁlterbanksthatareappropriateinthis framework,anddescribetheimplementationoftheﬁlterbanksusing thecoprocessor.Finally,Section5presentsasummaryofthepaper. 2.THEDSPSYSTEM TheDSPsystemisdesignedforultra-lowpowerandminiatureaudio processingapplications.Figure1showsatop-leveloverviewofthe DSPsystem.Thesystemiscenteredaroundthreemajordigitalcom- ponents:aﬁxed-pointgeneralpurposeDSP,acoprocessorandan I/Oprocessor.Thesethreecomponentsruninparallelandcommuni- catethroughinterruptsignalsandsharedmemory.Thegeneralpur- poseDSPisfullyprogrammable.Itprovidestheﬂexibilityneeded toimplementawiderangeofsignalprocessingalgorithms.The coprocessorisspecializedforﬁlterbankoperationsandvectorop- erations.Itcanperformtheseoperationswithoutinterventionfrom theDSP.Inparticular,thecoprocessorhardwareishighlyoptimized fortheimplementationofanoversampledcomplexmodulatedﬁlter- bank.Withsomeadaptation,itcanalsogeneratecritically-sampled ﬁlterbanks[4][5][6].Theﬁlterbankishighlyconﬁgurableinorder tomeettherequirementsofvariousapplications.Itallowsforodd orevenstacking,differentnumberofsubbands,anddifferenttype ofanalysisandsynthesisprototypeﬁlters.Inaddition,itcanalso operateinstereomode,withtworeal-valuedinputsignalsandasep- arateprototypeﬁlterforeachinput.Withthisdesign,thewholeDSP systemisparticularlyefﬁcientforoversampledsubbandprocessing. TheI/Oprocessorcontrolstheinput/outputdataﬂowbetweenthe input/outputstages(i.e.,theADC,DACandassociatedcircuits)and theFIFOmemory.Itoperatesonblocksofdatainthebackground andonlyinterruptstheDSPwhennecessary.Itworksinconjunction withmultipleI/Ochannelsintheinput/outputstageconcurrently. Finally,theDSPsystemalsoincludesclockcircuitsandanum- berofotheron-chipperipherals.Therefore,itcanbeeasilyinte- gratedintominiaturedeviceswithoutmanyextraexternalcompo- nents.Anumberofanaloganddigitalinterfacesarealsoprovided thatallowcommunicationswithawidevarietyofexternalsystems, suchasawirelesschip.Whenusedintypicalhearingaidapplica- tions,theDSPsystemoperatesat1.25Vsupplyandconsumesless than1mW. Copyright 2008 IEEE. Published in the 2008 International Conference on Acoustics, Speech, and Signal Processing (ICASSP 2008), scheduled for March 30 - April 4, 2008 in Las Vegas, Nevada. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works, must be obtained from the IEEE. Contact: Manager, Copyrights and Permissions / IEEE Service Center / 445 Hoes Lane / P.O. Box 1331 / Piscataway, NJ 08855-1331, USA. Telephone: + Intl. 908- 562-3966.
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