Speech recognition using on-line estimation of speaking rate (1997)

SPEECHRECOGNITIONUSINGON-LINEESTIMATIONOFSPEAKINGRATENelsonMorgan,EricFosler,andNikkiMirghaforiInternational Computer Science Institute, 1947Center St, Berkeley, CA94704University of California atBerkeley, EECSDepartment, Berk, CA 94720Tel:(510)643-9153,FAX: (510)643-7684,Email:fmorgan, fosler, nikkig@icsi.b erkeley.eduABSTRACTInthispap er,wedescrib earateofsp eechestima-tor that is deriveddirectly from theacoustic signal.Thismeasurehasb eendevelop edasanalternativetolexicalmeasuresofsp eakingratesuchasphonesor syllables p er second,which, in previous work, weestimated using a rst recognition pass; the accuracyofourearlierlexical rateestimatedep endedonthequalityofrecognition.Hereweshothatournewmeasureis a go o d predictorof worderrorrate,andinaddition,correlatesmo deratelywellwithlexicalsp eech rate.We also show that a simple mo di cationofthemo deltransitionprobabilitiesbasedonthismeasure can reduce the error rate almost as much asusing lexical phones p er second calculated from man-ually transcrib ed data.When we categorized test ut-terances based on sp eaking rate thresholds computedfromthetrainingset,weobservedthatadi erenttransition probability value was required to minimizetheerrorrateineachsp eakingbin.However,thereductionoferrorprovidedbythisapproachisstill small in comparison withtheincreasesin errorobserved for unusually fast or slow sp eech.1.INTRODUCTIONInpreviousworke[1]andothers[3,4]havob-servedstrongcorrelationsb etweenthep erformanceofsp eechrecognitionsystemsanddeviationthetest data from the average rate of sp eech observed intraining data.Wehave also do cumentedthatsomefairlysimplechangestothesystem(e.g.,mo di ca-tion of state transition probabilities) can b eused toimprove p erformance in this case.However, our strat-egy was based on alexical ly-basedmeasure of sp eak-ingrate.Theb estmeasuresthatwefounderebasedoncountingphoneticunitsfollowing recogni-tion.This required a recognizer that p erformed rea-sonably well in the rst place.For dicult problemssuch as the recognition of conversational sp eech, p er-formancemay b eto op o orforthistob eareliabletechnique.Onesolution to this problem is to try toestimate phoneb oundarieswithout using afull rec-ognizer; for instance, Verhasselt and Martens [5] em-ployed a multi-laer p erceptronapproach with mea-surable successon the TIMIT database.However, in conversational sp eechcommon phoneticunitscorresp ondingtoallophonesorallophoneseg-ments may b e signi cantly transformed or even disap-p ear, and so may not b e reliable measures of sp eakingrate.We b elieve that it would b e desirable to developa measure of sp eaking rate that is directly based onsignal pro cessingof the sp eech,without referencetolexical units or requiring the use of a recognition sys-temforestimation.Suchameasureshouldalsob econtinuouslycomputable,andyetcorrelatereason-ably well with lexically-based measures such as phonecount (although some errors are inevitable due to thephone transformation mentioned ab ove).In the work describ ed here, we have b egun to addresstheseconcerns.Inparticular,wehavdevelop edasimple estimator of sp eech rate based on 1-2 secondsofthesp eechsignal.Wehavcomputedmea-sure for a sample of data from the Switchb oard Cor-pusthathasb eenphoneticallytranscrib ed,aswellas for a phonetically transcrib edp ortion of the OGINumb ers corpus from the Oregon Graduate Institute.In eachcasewe have determined thecorrelation co-ecientb etweenawindoedphonecounandthenewmeasureforthissample.Wealsohavexam-ined the relationship b etween this measure and worderrorrate.Finally, wehavimplemented aprelim-inaryrecognitionstrategyinwhichweonlymo difythe transition probabilities of a hybrid system giventherateestimate, andhave observedresultsfor theNumb ers corpus.2.SIGNAL ANALYSISPrevious work [2] has demonstrated that the sp eechsignal is signi cantly altered for varying sp eaking rates.Themost obvious change to thesp eech,however,isthe variation in the energy envelop e.In other words,theenergyenvelop eofsp eechsimply hasmorerapid change when the sp eaking rate is higher.This