Towards using hybrid word and fragment units for vocabulary independent LVCSR systems
Ariya Rastrow, Abhinav Sethy, Bhuvana Ramabhadran, Frederick Jelinek · 2009
This paper presents the advantages of augmenting a word-based system with sub-word units as a step towards building open vocabulary speech recognition systems. We show that a hybrid system which combines words and data-driven, variable length sub word units has a better phone accuracy than word only systems. In addition the hybrid system is better in detecting Out-Of-Vocabulary (OOV) terms and representing them phonetically. Results are presented on the RT-04 broadcast news and MIT Lecture data sets. An FSM-based approach to recover OOV words from the hybrid lattices is also presented. At an OOV rate of 2.5 % on RT-04 we observed a 8 % relative improvement in phone error rate (PER), 7.3 % relative improvement in oracle PER and 7 % relative improvement in WER after recovering the OOV terms. A significant reduction of 33 % relative in PER is seen in the OOV regions. Index Terms: hybrid LVCSR system, out-of-vocabulary, subword unit selection, phone recognition, OOV detection