International Workshop on Spoken Language Translation (IWSLT) 2011
San Francisco, CA, USA
In this paper we describe some of our recent investigations
into ASR and SMT coupling issues from an ASR perspective.
Our study was motivated by several areas: Firstly, to
understand how standard ASR tuning procedures effect the
SMT performance and whether it is safe to perform this tuning
in isolation. Secondly, to investigate how vocabulary and
segmentation mismatches between the ASR and SMT system
effect the performance. Thirdly, to uncover any practical issues
that arise when using a WFST based speech decoder for
tight coupling as opposed to a more traditional tree-search
On the IWSLT07 Japanese-English task we found that larger language model weights only helped the SMT performance when the ASR decoder was tuned in a sub-optimal manner. When we considered the performance with suitable wide beams that ensured the ASR accuracy had converged we observed the language model weight had little influence on the SMT BLEU scores.
After the construction of the phrase table the actual SMT vocabulary can be less than the training data vocabulary. By reducing the ASR lexicon to only cover the words the SMT system could accept, we found this lead to an increase in the ASR error rates, however the SMT BLEU scores were nearly unchanged. From a practical point of view this is a useful result as it means we can significantly reduce the memory footprint of the ASR system.
We also investigated coupling WFST based ASR to a simple WFST based translation decoder and found it was crucial to perform phrase table expansion to avoid OOV problems. For the WFST translation decoder we describe a semiring based approach for optimizing the log-linear weights.
Bibliographic reference. Dixon, Paul R. / Finch, Andrew / Hori, Chiori / Kashioka, Hideki (2011): "Investigation on the effects of ASR tuning on speech translation performance", In IWSLT-2011, 167-174.