IEEE TRANS. ON SIGNAL PROCESSING 1 A Low-Complexit

IEEETRANS.ONSIGNALPROCESSING1ALow-ComplexityITU-CompliantDualToneMultipleFrequencyDetectorAmeyA.Deosthali,Member,IEEE,ShawnR.McCaslin,Member,IEEE,andBrianL.Evans,SeniorMember,IEEEA.A.Deosthali([e-mail])performedthisresearchwhileatTheUniversityofTexasinAustin,TX,andisnowwithSchlumbergerTechnologyCorporation,110SchlumbergerDr.,MD4,SugarLand,TX77478.S.R.McCaslin([e-mail])performedthisresearchwhileatCrystalSemiconductorinAustin,TX,andisnowwithCicadaSemiconductor,BartonOaksPlazaOne,901SouthMopacExpressway,Austin,TX78746.Prof.B.L.Evans([e-mail])isatENSBldg.,Dept.ofElectricalandComputerEng.,TheUniversityofTexas,Austin,TX78712-1084,rstdualtonemultiplefrequency(DTMF)signaldetectorthatmeetstheInternationalTelecommunicationsUnion(ITU)Q.24DTMFstandardwhenimplementedonan8-bitmicrocontroller.Keyinnovationsincludetheuseofadaptivenotchltersandsophisticateddecisionlogic.TheDTMFdetectoriswellsuitedforamultichanneldigitalsignalprocessorimplementation.Indexterms{touchtonedecoding,DTMFdetection,signalprocessingonmicrocontrollersI.IntroductionDualToneMultipleFrequency(DTMF)signalingisusedintelephonedialing,digitalanswermachines,andinteractivebankingsystems.DTMFsignalingrepresentseachsymbolonatele-phonetouchtonekeypad(0-9,*,#)usingtwosinusoidaltones,asshowninFig.1.Whenakeyispressed,aDTMFsignalconsistingofarowfrequencytoneplusacolumnfrequencytoneistransmitted.KeysA-Darenotoncommercialtelephonesets,butareusedinmilitaryandradiosignalingapplications.Themaximumdialingrateis10symbols/sintheBellcorestandard[1],[2]and12.5symbols/sintheInternationalTelecommunicationUnion(ITU)Q.24standard[3].TheITUDTMFspecications,whicharegiveninTableI,aremorestrictthantheBellcorespecications.So,ITU-complianceimpliesBellcore-compliance,butnotvice-versa.ITUspec-icationsrequirethatvalidDTMFsignalshavetheirhighandlowfrequencytoneswithinatoleranceof1:5%ofanidealDTMFfrequency.Ifthetoleranceofeithertoneisoutside3:5%,thenthesignalshouldberejectedasinvalid.ITUspecicationsplacerequirementsonthedu-rationofandpausesbetweenvalidDTMFsignals.ITUspecicationsrequire100%detectionofvalidDTMFsignalsat15dBSNR.BellcoreprovidestesttapestomeasuretheperformanceofaDTMFdetectoragainsttalk-o,whichisfalsedetectionofspeechsignalsasDTMFsignals.TheITU-compliantdetectorin[4]andBellcore-compliantdetectorsin[1],[5]arebasedontheDiscreteFourierTransform(DFT).Theyuse16-bitdata,require15{1716-bitmultiplicationsperinputsample,anddonotbuerinputdata.Fornchannels,theITU-compliantdetectorin[4]requiresnMIPS,120+30nwordsofdatamemory,and1000wordsofprogrammemory.WepresenttherstDTMFdetectorthatmeetstheITUspecicationsandpassestheBellcoretalk-otestswhenimplementedonan8-bitmicrocontroller.Thedetectorusestwoconstrainedadaptivenotchltersandtwofrequencyestimatorsforeachchannel,anddoesnotrequirebuer-ingofinputdata.Thedetectorprovidesanoptionaldialtonesuppressorforcentraloceappli-DRAFTOctober1,1999DEOSTHALI,MCCASLINANDEVANS:LOW-COMPLEXITYDTMFDETECTION3cations.Foreachinputsample,thedetectorrequires3:58-bitmultiplications.Forsinglechanneldetection,weimplementedthedetectoronaa5-MIP8-bitMicrochipPIC16C711microcontrollerusing64bytesofdatamemoryand800bytesofprogrammemory.Forsimultaneouslydetect-ingnchannels,weestimatethattheimplementationonadigitalsignalprocessor(DSP)wouldrequire0:7nMIPS,19+45nwordsofdatamemory,and1000wordsofprogrammemory.So,a30-MIP16-bitTexasInstrumentsTMS320C54DSPwith16kwordsofRAMand4kwordsofROMcoulddecode42time-divisionmultiplexedchannels,whichisplentyforanE1line(32channels)orT1line(24channels).II.PreviousDTMFDecodersDecodingDTMFsignalsamountstodetectingtwosinusoidsinnoise.DTMFdetectorstypi-callyconsistofasignalanalysisfrontendfollowedbyadecisionlogicbackend.Thebackendenforcescompliancewithastandard.Integratedcircuitdetectorsmaymixafrontendconsistingofeightbandpassresonatorsimplementedinswitchedcapacitortechnologyfollowedbyadigitalpost-processortomeasuretonedurationandprovidecorrectlycodeddigitaloutputs[6].PopularDFT-baseddigitalmethodsforDTMFdetectionalsouseeightresonatingbandpassIIRlters[1],[5].ForDFToflengthM,theMthoutputofeachdigitallteristheDFTcoecientcomputedusingtheGoertzelalgorithm[7].TheGoertzelalgorithmuses1wordofROMand2wordsofRAM,andrequiresM+2multiplicationsand2M+2additionsforeveryMsamples[1].TheabsolutevalueofthelteroutputissquaredtomeasurethesignalenergypresentattheDTMFfrequency.ADTMFtonepairisconsideredvalidifenoughenergyexistsintheDTMFfrequenciesandifthatenergyexceedstheotherDTMFtonepairenergies[1].IncreasingtheDFTlengthincreasesfrequencyresolutionbutdecreasestimingresolution.NosingleDFTlength,however,cansimultaneouslymeetthetimingandfrequencyspecicationsintheITUDTMFstandard[4],[5].FortheBellcoreDTMFstandard,manysingleDFTlengthswork.ADFTlengththatsimpliesthedetectionlogicistheminimumDTMFsignaldur