Investigations on Phoneme-Based End-To-End Speech Recognition
Table 2: On Switchboard 300h, comparing decoding. All phoneme models here have word-disambiguate symbols. Phoneme single is with end-of-word (EOW) symbol. Grapheme single is with whitespace, which is like a EOW symbol. The optional LSTM LM is on word-level. The advanced decoder is also restricted on the lexicon, and more specifically the unique greedy BPE-split.
['Labels Unit', 'Labels Type', 'Decoder LM', 'Decoder Beam', 'Decoder Impl.', 'WER[%] Hub5’00', 'WER[%] Hub5’00', 'WER[%] Hub5’00']
For all further experiments, we need to use our advanced decoder, which allows for a word-level LM. It also restricts the search to only label sequences which occur in the lexicon, including only the BPE-splits seen during training, in contrast to the simple decoder, which does not have this restriction. We see that in the case of single phone or grapheme labels, i.e. where we have a weaker model, the restriction on the lexicon by the advanced decoder is helpful, while it is hurtful for the BPE variants, esp. in the case of phoneme-BPE. We also see the effect of the external LM combination, which is helpful (as expected).