Proceedings Fonetik 2009 - Institutionen för lingvistik

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Proceedings, FONETIK 2009, Dept. of Linguistics, Stockholm University3. the Mahalanobis distance (Taylor, 2008)for the MFCCsF0 distance was weighted by 0.5. A penalty of10 was given to those segments from the targetcorpus where the vowel following the initialplosive was not /a/, i.e. a different vowel thanthe one in the remainder corpus. The F0weighting factor and the vowel-penalty valuewere arrived at after iterative tuning. The distanceswere calculated using a combination ofPerl and Microsoft Excel.ConcatenationFor each possible segment combination((/p|t|k/) + (/ap|at|ak/), i.e. 9 possible combinationsin total), the join costs were ranked. Thefive combinations with the lowest costs withineach of these nine categories were then concatenatedusing the Snack tool CONCAT. Concatenationpoints were located to zero-crossingswithin a range of 15 samples after the middle ofthe vowel following the initial plosive. (And ifno zero-crossing was found within that range,the concatenation point was set to the middle ofthe vowel.)Evaluation7 adult subjects were recruited to perform a listeningtest. All subjects were native Swedes,with no known hearing problems and naïve inthe sense that they had not been involved in anywork related to speech synthesis development.A listening test was constructed in Tcl/Tk topresent the 45 stimuli (i.e. the five concatenationswith the lowest costs for each of the ninedifferent syllables) and 9 original recordings ofthe different syllables. The 54 stimuli were allrepeated twice (resulting in a total of 108items) and presented in random order. The taskfor the subjects was to decide what syllablethey heard (by selecting one of the nine possiblesyllables) and judge the naturalness of theutterance on a scale from 0 to 100. The subjectshad the possibility to play the stimuli as manytimes as they wanted. Before starting the actualtest, 6 training items were presented, afterwhich the subjects had the possibility of askingquestions regarding the test procedure.Statistical analysisInter-rater agreement was assessed via the intraclasscorrelation (ICC) coefficient (2, 7) forsyllable identification accuracy and naturalnessrating separately.Pearson correlations were used to assess intra-rateragreement for each listener separately.ResultsThe results of the evaluation are presented inTable 1.Table 1. Evaluation results for the concatenatedand original speech samples. The first column displaysthe percentage of correctly identified syllables,and the second column displays the averagenaturalness judgments (max = 100).% correct syll NaturalnessConcatenated 94% 49 (SD: 20)Original 100% 89 (SD: 10)The listeners demonstrated high inter-rateragreement on naturalness rating (ICC = 0.93),but lower agreement on syllable identificationaccuracy (ICC = 0.79).Average intra-rater agreement for all listenerswas 0.71 on naturalness rating, and 0.72 onsyllable identification accuracy.DiscussionConsidering that the purpose of this study wasto study the possibilities of generating understandableand close to natural sounding concatenationsof segments from different speakers,the results are actually quite promising.The listeners’ syllable identification accuracyof 94% indicates that comprehensibility is not abig problem. Although the total naturalnessjudgement average of 49 (of 100) is not at allimpressive, an inspection of the individualsamples reveals that there are actually someconcatenated samples that receive higher naturalnessratings than original samples. Thus, theresults confirm that it is indeed possible to generateclose to natural sounding samples by concatenatingspeech from different speakers.However, when considering that the long-termgoal is a working system that can be implementedand used to assist phonological therapywith children, the system is far from complete.As of now, the amount of manual interventionrequired to run the re-synthesis process islarge. Different tools were used to completedifferent steps (various Snack tools, MicrosoftExcel), and Perl scripts were used as interfacesbetween these steps. Thus, there is still a longway to real-time processing. Moreover, it isstill limited to voiceless plosives in sentenceinitialpositions, and ideally, the system should200
Proceedings, FONETIK 2009, Dept. of Linguistics, Stockholm Universitybe more general. However, considering thatchildren usually master speech sounds in wordinitialand word-final positions later than inword-medial positions (Linell & Jennische,1980), this limitation should not be disqualifyingon its own.The speech data in this work came fromadult speakers. New challenges can be expectedwhen faced with children’s voices, e.g. increasedvariability in the speech database(Gerosa et al, 2007). Moreover, variability inthe speech of the intended user - the child in thetherapy room - can also be expected. (Not tomention the variability from child to child inmotivation and ability and will to comply withthe therapist’s intervention plans.)The evaluation showed that there is muchroom for improving naturalness, and fortunately,some improvement strategies can besuggested. First, more manipulations withweighting factors might be a way to assure thatthe combinations that are ranked the highest arealso the ones that sound the best. As of now,this is not always the case. During the course ofthis investigation, attempts were made at increasingthe size of the target corpus, by includingword-initial voiceless plosives within utterancesas well. However, these efforts did notimprove the quality of the output concatenatedspeech samples. The current system does notinvolve any spectral smoothing; this might be away to polish the concatenation joints to improvenaturalness.Looking beyond the context of modified resynthesisto assist therapy with children withphonological impairments, the finding that it isindeed possible to generate natural soundingconcatenations of segments from differentspeakers might be valuable in concatenativesynthesis development in general. This mightbe useful in the context of extending a speechdatabase if the original speaker is no longeravailable, e.g. with new phonemes. However, itseems reasonable to assume that the method isonly applicable to voiceless segments.AcknowledgementsThis work was funded by The Swedish GraduateSchool of Language Technology (GSLT).ReferencesCarlson, R. & Granström, B. (2005) Datadrivenmultimodal synthesis. Speech Communication47, 182-193.Gerosa, M., Gioliani, D. & Brugnara, F. (2007)Acoustic variability and automatic recognitionof children’s speech. Speech Communication49, 847-835.Hewlett, N. (1992) Processes of developmentand production. In Grunwell, P. (ed.) DevelopmentalSpeech Disorders, 15-38. London:Whurr.Hunt, A. and Black, A. (1996) Unit selection ina concatenative speech synthesis system usinga large speech database. Proceedings ofICASSP 96 (Atlanta, Georgia), 373-376.Iskra, D., Grosskopf, B., Marasek, K., Van DenHeuvel, H., Diehl, F., and Kiessling, A.(2002) Speecon - speech databases for consumerdevices: Database specification andvalidation.Linell, P. & Jennische, M. (1980) Barns uttalsutveckling,Stockholm: Liber.Locke, J.L. & Pearson, D.M. (1992) VocalLearning and the Emergence of PhonologicalCapacity. A Neurobiological Approach.In C.A. Ferguson, L. Menn & C. Stoel-Gammon (Eds.), Phonological Development.Models, research, implications.,York: York Press.Protopapas, A. (1998) Modified LPC resynthesisfor controlling speech stimulus discriminability.136th Annual Meeting of theAcoustical Society of America, Norfolk,VA, October 13-16.Ramus, F. & Mehler, J. (1999) Language identificationwith suprasegmental cues: A studybased on speech resynthesis, Journal of theAcoustical Society of America 105, 512-521.Shuster, L. I. (1998) The perception of correctlyand incorrectly produced /r/. Journalof Speech, Language and Hearing Research41, 941-950.Sjölander, K. The Snack sound toolkit, Departmentof Speech, Music and Hearing,KTH, Stockholm, Sweden. Online:http://www.speech.kth.se/snack/, 1997-2004, accessed on April 12, 2009.Sjölander, K. (2003) An HMM-based systemfor automatic segmentation and alignmentof speech. Proceedings of Fonetik 2003(Umeå University, Sweden), PHONUM 9,93-96.Taylor, P. (2008) Text-to-Speech Synthesis,Cambridge University Press.Wik, P. (2004) Designing a virtual languagetutor. Proceedings of Fonetik 2004 (StockholmUniversity, Sweden), 136-139.201
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