Large Vocabulary Speech Recognition Based on Statistical Methods
5.5 Acoustic Modeling
5.6.2 Decoding Strategies
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
&[PCOKE FGEQFKPI ECP DG EQODKPGF YKVJ GHſEKGPV RTWPKPI VGEJPKSWGU KP QTFGT VQ QDVCKP C UKPING RCUU FGEQFGT VJCV ECP RTQXKFG VJG CPUYGT WUKPI CNN VJG CXCKNCDNG KP HQTOCVKQP KG VJCV KP VJG OQFGNU KP C UKPING HQTYCTF FGEQFKPI RCUU QXGT QH VJG URGGEJ UKIPCN 6JKU MKPF QH FGEQFGT UWEJ CU VJG UVCEM FGEQFGT =? DCUGF QP VJG # CNIQTKVJO QT VJG QPGRCUU HTCOG U[PEJTQPQWU F[PCOKE PGVYQTM FGEQFGT =? KU XGT[
CVVTCEVKXG HQT TGCNVKOG CRRNKECVKQPU
5VCVKE FGEQFGTU TGSWKTG OWEJ OQTG OGOQT[ VJCP F[PCOKE FGEQFGTU YJGP WUGF YKVJ NQPI URCP NCPIWCIG OQFGNU ITCO QT JKIJGT QTFGT CPF CU C EQPUGSWGPEG VJG[ CTG OQUVN[ WUGF YKVJ UOCNNGT NCPIWCIG OQFGNU WUWCNN[ ITCOU QT EQPUVTCKPGF ITCO OCTU +V JCU DGGP TGEGPVN[ UJQYP VJCV D[ RTQRGT QRVKOK\CVKQP QH C ſPKVGUVCVG CW VQOCVQPÝ EQTTGURQPFKPI VQ C TGEQIPK\GT *// PGVYQTM UWDUVCPVKCN TGFWEVKQP QH VJG QXGTCNN PGVYQTM UK\G ECP DG QDVCKPGF GPCDNKPI UVCVKE FGEQFKPI YKVJ NQPI URCP ./U =? *QYGXGT VJG UK\G QH VJG QRVKOK\GF PGVYQTM TGOCKPU RTQRQTVKQPCN VQ VJG ./ UK\G
/WNVKRCUU FGEQFKPI ECP DG WUGF VQ RTQITGUUKXGN[ CFF MPQYNGFIG UQWTEGU KP VJG FG EQFKPI RTQEGUU VJWU CNNQYKPI VJG EQORNGZKV[ QH VJG KPFKXKFWCN FGEQFKPI RCUUGU VQ DG TGFWEGF CPF QHVGP TGUWNVKPI KP C HCUVGT QXGTCNN FGEQFGT =? (QT GZCORNG C ſTUV FG EQFKPI RCUU ECP WUG C ITCO NCPIWCIG OQFGN CPF UKORNG CEQWUVKE OQFGNU CPF NCVGT RCUUGU YKNN OCMG WUG QH ITCO CPF ITCO NCPIWCIG OQFGNU YKVJ OQTG EQORNGZ CEQWUVKE OQFGNU 6JKU OWNVKRNG RCUU RCTCFKIO TGSWKTGU C RTQRGT KPVGTHCEG DGVYGGP RCUUGU KP QTFGT VQ CXQKF NQUKPI KPHQTOCVKQP CPF GPIGPFGTKPI UGCTEJ GTTQTU +PHQT OCVKQP KU WUWCNN[ VTCPUOKVVGF XKC YQTF NCVVKEGUÞ QT YQTF ITCRJU UGG (KIWTG CNVJQWIJ UQOG U[UVGOU WUG 0DGUV J[RQVJGUGU YJKEJ CTG C NKUV QH VJG OQUV NKMGN[
Ý#P *//DCUGF URGGEJ TGEQIPK\GT ECP DG UGGP CU C VTCPUFWEVKQP ECUECFG YJKEJ EQPXGTVU VJG QDUGTXGF HGCVWTG XGEVQTU VQ C YQTF UVTKPI YJGTG VQ UQOG CRRTQZKOCVKQP GCEJ VTCPUFWEVKQP RJQPG OQFGN YQTF OQFGN QT NCPIWCIG OQFGN ECP DG TGRTGUGPVGF CU C ſPKVGUVCVG CWVQOCVQP
Þ.CVVKEGU CTG ITCRJU YJGTG PQFGU EQTTGURQPF VQ RCTVKEWNCT HTCOGU CPF YJGTG GFIGU TGRTGUGPVKPI YQTF J[RQVJGUKU JCXG CUUQEKCVGF CEQWUVKE CPF NCPIWCIG OQFGN UEQTGU
Example word lattice generated by a speech recognizer using a bigram language model for a 2.1s utterance. Each graph edge corresponds to a word hypothesis and a time interval (as specified by the time information on the nodes). In this example the word transcription with the highest likelihood is “sil IT WAS A GOOD PROGRAM sil” which happens to be what was said. (The acoustic and language model likelihoods are not given on the figure.)
YQTF UGSWGPEGU YKVJ VJGKT TGURGEVKXG UEQTGU #V VJG RTKEG QH UQOG CEEGRVCDNG CR RTQZKOCVKQPU YQTF NCVVKEGU CPF 0DGUV NKUVU ECP DG IGPGTCVGF YKVJ NKVVNG QXGTJGCF CDQWV D[ OQFKH[KPI VJG DQQMMGGRKPI QH VJG RCTVKCN J[RQVJGUGU EQPUKFGTGF FWT KPI TGIWNCT FGEQFKPI =?
+V ECP UQOGVKOGU DG FKHſEWNV VQ CFF EGTVCKP MPQYNGFIG UQWTEGU KPVQ VJG FGEQFKPI RTQEGUU GURGEKCNN[ YJGP VJG[ FQ PQV ſV KP VJG /CTMQXKCP HTCOGYQTM 6JKU KU VJG ECUG YJGP VT[KPI VQ WUG UGIOGPVCN KPHQTOCVKQP QT VQ WUG ITCOOCVKECN KPHQTOCVKQP HQT NQPI VGTO CITGGOGPV 5WEJ KPHQTOCVKQP ECP DG OQTG GCUKN[ KPVGITCVGF KP C OWNVK RCUU U[UVGO D[ TGUEQTKPI VJG TGEQIPK\GT J[RQVJGUGU CHVGT CRRN[KPI VJG CFFKVKQPCN MPQYNGFIG UQWTEGU 'XKFGPVN[ VJG ſTUV RCUU WUGF VQ IGPGTCVG VJG KPKVKCN YQTF NCVVKEG OWUV DG CEEWTCVG GPQWIJ VQ PQV KPVTQFWEG NCVVKEG GTTQTU YJKEJ CTG WPTGEQXGTCDNG YKVJ HWTVJGT RTQEGUUKPI
+P CFFKVKQP VQ OWNVKRNG RCUU FGEQFKPI YQTF NCVVKEGU ECP DG WUGF VQ QXGTEQOG VJG 8KVGTDK CRRTQZKOCVKQP FKUEWUUGF CDQXG #U C OCVVGT QH HCEV VTWG /#2 FGEQFKPI KU C EQPUKFGTCDN[ GCUKGT VCUM QP C YQTF NCVVKEG VJCP QP VJG QTKIKPCN UGCTEJ URCEG #NQPI VJG UCOG NKPGU KV JCU DGGP RTQRQUGF VQ WUG YQTF NCVVKEGU VQ RGTHQTO C YQTF DCUGF /#2 FGEQFKPI KPUVGCF QH YQTF UGSWGPEG /#2 FGEQFKPI KG OKPKOK\KPI VJG YQTF GTTQT KPUVGCF QH VJG YQTF UGSWGPEG QT UGPVGPEG GTTQT TCVG =?
5.6.3 Efficiency
#U FKUEWUUGF CDQXG VJGTG CTG OCP[ GHſEKGPV UQNWVKQPU VQ VJG UGCTEJ RTQDNGO JQY GXGT ſPFKPI VJG QRVKOCN UQNWVKQP KU CNYC[U C VTCFGQHH DGVYGGP VJG OQFGN CEEWTCE[
CPF GHſEKGPV RTWPKPI +P IGPGTCN DGVVGT OQFGNU JCXG OQTG RCTCOGVGTU CPF VJGTGHQTG
TGSWKTG OQTG EQORWVCVKQP *QYGXGT UKPEG VJG OQFGNU CTG OQTG CEEWTCVG KV KU QHVGP RQUUKDNG VQ WUG C VKIJVGT RTWPKPI NGXGN VJWU TGFWEKPI VJG EQORWVCVKQPCN NQCF YKVJQWV CP[ NQUU KP CEEWTCE[
.KOKVCVKQPU QP VJG CXCKNCDNG EQORWVCVKQPCN TGUQWTEGU ECP UKIPKſECPVN[ CHHGEV VJG FG UKIP QH VJG CEQWUVKE CPF NCPIWCIG OQFGNU CU HQT GCEJ QRGTCVKPI RQKPV VJG TKIJV DCN CPEG DGVYGGP OQFGN EQORNGZKV[ CPF RTWPKPI NGXGN OWUV DG HQWPF #IITGUUKXG RTWP KPI KU IGPGTCNN[ PGGFGF VQ CEJKGXG TGCNVKOG QRGTCVKQP HQT .8%54 VCUMU QP EWTTGPVN[
CXCKNCDNG RNCVHQTOU 6JKU KPGXKVCDN[ KU C UQWTEG QH UGCTEJ GTTQTU CPF CU UWEJ OCP[
VGEJPKSWGU JCXG DGGP RTQRQUGF VQ TGFWEG VJGUG UGCTEJ GTTQTU CPF VQ NKOKV VJGKT GHHGEV QP VJG TGEQIPK\GT CEEWTCE[ 1PG QH VJG OQUV RQRWNCT FGEQFKPI UVTCVGIKGU HQT TGCN VKOG QRGTCVKQP KU VJG QPGRCUU HTCOGU[PEJTQPQWU F[PCOKE PGVYQTM FGEQFGT YJKEJ TGNKGU QP C RJQPGVKE VTGG QTICPK\CVKQP QH VJG FGEQFKPI PGVYQTM WUKPI ./ UVCVG EQP FKVKQPGF VTGG EQRKGU = ? 6JG UWEEGUU QH UWEJ C UKPING RCUU CRRTQCEJ KU JKIJN[
FGRGPFGPV QP VJG WUG QH GHſEKGPV RTWPKPI UVTCVGIKGU CUUQEKCVGF YKVJ C NCPIWCIG OQFGN NQQMCJGCF = ? /WNVKRCUU CRRTQCEJGU ECP CNUQ DG WUGF UWEEGUUHWNN[ HQT ENQUG VQ TGCNVKOG QRGTCVKQP D[ EJWPMKPI VJG FCVC CPF TWPPKPI VJG FKHHGTGPV RCUUGU KP RCTCNNGN YKVJ C UNKIJV FGNC[
(QT URGCMGTKPFGRGPFGPV .8%54 DCUGF QP )CWUUKCP OKZVWTG *// DGVYGGP CPF QH VJG TGEQIPKVKQP VKOG KU URGPV KP EQORWVKPI VJG *// UVCVG NKMGNKJQQFU YKVJ VJG TGOCKPKPI VKOG EQTTGURQPFKPI VQ VJG UGCTEJ RTQEGFWTG KVUGNH 6JKU KU FWG VQ VJG NCTIG PWODGT QH UVCVGU PGGFGF VQ TGRTGUGPV VJG EQPVGZVFGRGPFGPV RJQPG OQFGNU GXGP YJGP UVCVG V[KPI KU WUGF 6JKU EQORWVCVKQP ECP DG TGFWEGF GKVJGT D[ KORNGOGPVKPI C HCUV UVCVG NKMGNKJQQF EQORWVCVKQP YJKEJ WUWCNN[ TGSWKTGU OCMKPI UQOG CRRTQZKOC VKQPU QT D[ TGFWEKPI VJG OQFGN UK\G YJKEJ JCU VJG CFFKVKQPCN CFXCPVCIG QH TGFWEKPI VJG OGOQT[ TGSWKTGOGPVU # YKFGN[ WUGF VGEJPKSWG HQT URGGFKPI WR VJG UVCVG NKMG NKJQQF EQORWVCVKQP KU XGEVQT SWCPVK\CVKQP QH VJG HGCVWTG XGEVQT URCEG KP QTFGT VQ RTGRCTG C )CWUUKCP UJQTV NKUV HQT GCEJ *// UVCVG CPF GCEJ TGIKQP QH VJG SWCPVKſGF HGCVWTG URCEG =? 9KVJ VJKU VGEJPKSWG VJG PWODGT QH )CWUUKCP NKMGNKJQQFU VQ DG EQORWVGF FWTKPI FGEQFKPI HQT GCEJ KPRWV HTCOG CPF GCEJ UVCVG ECP DG TGFWEGF VQ C HTCEVKQP QH VJG PWODGT QH )CWUUKCPU EQTTGURQPFKPI VQ VJG CEVKXG UVCVGU YKVJ QPN[ C UOCNN NQUU KP CEEWTCE[
/QFGN CPF UVCVG V[KPI CTG EQOOQPN[ WUGF VQ KORTQXG VJG OQFGN CEEWTCE[ DWV QRVKOCN V[KPI HTQO VJG CEEWTCE[ RQKPV QH XKGY ECP UVKNN TGUWNV KP C XGT[ NCTIG OQFGN YKVJ M VQ M UVCVGU YJGP NCTIG COQWPVU QH VTCKPKPI FCVC CTG CXCKNCDNG 2CTCOGVGT V[KPI KU CNUQ RQYGTHWN VGEJPKSWG VQ TGFWEG VJG PWODGT QH RCTCOGVGTU CPF ECP DG CRRNKGF VQ CNN VJG NGXGNU QH VJG OQFGN UVTWEVWTG CNNQRJQPG OQFGN UVCVG CPF )CWUUKCP =?
*QYGXGT OQTG ƀGZKDKNKV[ KU CXCKNCDNG HQT )CWUUKCP 2&( V[KPI KP VJCV NCTIG OQFGN TGFWEVKQPU ECP DG QDVCKPGF YKVJQWV UCETKſEKPI VQQ OWEJ KP VGTOU QH U[UVGO CEEWTCE[
6JKU KU GZGORNKſGF D[ VJG UWDURCEG FKUVTKDWVKQP V[KPI CRRTQCEJ = ? YJKEJ KP KVU OQUV GNGOGPVCT[ KORNGOGPVCVKQP ECP DG UGGP CU C SWCPVK\CVKQP QH VJG OQFGN RCTCOGVGTU
6JG NCPIWCIG OQFGN WUWCNN[ C ITCO QT ITCO DCEMQHH ./ KP UVCVGQHVJGCTV U[UVGOU ECP JCXG C XGT[ NCTIG PWODGT QH RCTCOGVGTU QXGT OKNNKQP CPF VJGTG HQTG OC[ TGSWKTG RTQJKDKVKXG COQWPVU QH OGOQT[ 1PG QH VJG CVVTCEVKXG RTQRGTVKGU QHnITCO OQFGNU KU VJG RQUUKDKNKV[ QH TGN[KPI OQTG QP VJG DCEMQHH EQORQPGPVU D[
KPETGCUKPI VJG EWVQHHU QP VJGITCO EQWPVU VJWU TGFWEKPI UKIPKſECPVN[ VJG ./ UK\G EH 5GEVKQP /QTG GNCDQTCVGITCO RTWPKPI VGEJPKSWGU JCXG CNUQ DGGP RTQ RQUGF = ? VQ UWDUVCPVKCNN[ TGFWEG VJG ./ UK\G YKVJ PGINKIKDNG NQUU KP CEEWTCE[
#P CNVGTPCVKXG CRRTQCEJ VQ NKOKV VJG OGOQT[ TGSWKTGOGPVU KU VQ MGGR OQUV QH VJG ./
RCTCOGVGTU QP VJG FKUM UKPEG OQUVITCOU CTG PGXGT WUGF EQODKPGF YKVJ C ECEJG QH VJG UEQTGU HQT CEEGUUGF ./ UVCVGU =?