A confidence measure based on agreement among multiple LVCSR models - correlation between pair of acoustic models and confidence

{utsuro,tetsu}@cl.ics. tut.ac. jp, {nisizaki,nakagawa}@slp.ics. tut.ac. jpABSTRACTFor many practicalapplications ofspeech recognition systems, it isquite desirable to have an estimate of conﬁdence for each hypoth-esized word. Unlike previous works on conﬁdence measures, thispaper studies features for conﬁdence measures that are extractedfrom outputs of more than one LVCSR models. More speciﬁcally,this paper experimentally evaluates the agreement among the out-puts of multiple Japanese LVCSR models, with respect to whetherit is effective as an estimate of conﬁdence for each hypothesizedword. The results of experimental evaluation show that the agree-ment between the outputs with two LVCSR models with differ-ent decoders and acoustic models can achieve quite reliable con-ﬁdence. Furthermore, among various features of acoustic modelsbased on Gaussian mixture HMMs, it is concluded that ones suchas whether or not to have short pause models, as well as differentunits in HMMs (e.g., triphone model or syllable model) are themost effective in achieving highly reliable conﬁdence. 1. INTRODUCTIONSince current speech recognizers’ outputs are far from perfect andalways include a certain amount of recognition errors, it is quitedesirable to have an estimate of conﬁdence for each hypothesizedword. This is especially true for many practical applications ofspeech recognition systems such as word selection for unsuper-vised adaptation schemes, automatic weighting of additional, non-speech knowledge sources, keyword based speech understanding,and recognition error rejection – conﬁrmation in spoken dialoguesystems.Most of previous works on conﬁdence measures (e.g., [1, 2])are based on features available in a single LVCSR model. How-ever, it is well known that a voting scheme such as ROVER (Rec-ognizeroutputvoting error reduction) forcombining multiple speechrecognizers’ outputs can achieve word error reduction [3, 4]. Con-sidering the success of a simple voting scheme such as ROVER, italso seems quite possible to improve reliability of previously stud-ied features for conﬁdence measures by simply exploiting morethan one speech recognizers’ outputs. From this observation, un-like those previous works on conﬁdence measures, this paper stud-ies features for conﬁdence measures that are extracted from out-puts of more than one LVCSR models.For the purpose of estimating conﬁdence for each hypothe-sized word, it is more important to examine which combination ofexisting LVCSR models can achieve high conﬁdence and whichcombination can not, although even simple voting schemes canachieve word error reduction. Therefore, in this paper, we exper-imentally evaluate the agreement among the outputs of multipleJapanese LVCSR models, with respect to whether it is effectiveas an estimate of conﬁdence for each hypothesized word. In thisevaluation of existing Japanese LVCSR models, we concentrate onevaluating conﬁdence of the agreement among outputs with dif-ferent decoders and/or different acoustic models. The results ofexperimental evaluation show that the agreement between the out-puts with two LVCSR models with different decoders and acousticmodels can achieve quite reliable conﬁdence. Furthermore,amongvarious features of acoustic models based on Gaussian mixtureHMMs, it is concluded that ones such as whether or not to haveshort pause models, as well as different units in HMMs (e.g., tri-phone model or syllable model) are the most effective in achievinghighly reliable conﬁdence. It is also shown that it is better to com-bine various features including those most effective ones than touse one of those most effective features alone.2. SPECIFICATION OF JAPANESE LVCSR SYSTEMS2.1. DecodersAs the decoders of Japanese LVCSR systems, we use the onenamed Julius, which is provided by IPA Japanese dictation freesoftware project [5], as well as the one named SPOJUS [6], whichhas been developed in our laboratory. Both decoders are com-posed of two decoding passes, where the ﬁrst pass uses the wordbigram, and the second pass uses the word trigram. Julius is withword-trellis searches and hence has much broader search spacethan SPOJUS, which is with N-best searches.2. 2. Acoustic ModelsThe acoustic models of Japanese LVCSR systems are based onGaussian mixture HMM. We evaluate phoneme-based HMMs aswell as syllable-based HMMs.2. 2.1. Acoustic Models with the Decoder J