Modeling Non-Linguistic Contextual Signals in LSTM Language Models Via Domain Adaptation

Language Models (LMs) for Automatic Speech Recognition (ASR) can benefit from utilizing non-linguistic contextual signals in modeling. Examples of these signals include the geographical location of the user speaking to the system and/or the identity of the application (app) being spoken to. In practice, the vast majority of input speech queries typically lack annotations of such signals, which poses a challenge to directly train domain-specific LMs. To obtain robust domain LMs, generally an LM which has been pre-trained on general data will be adapted to specific domains. We propose four domain adaptation schemes to improve the domain performance of Long Short-Term Memory (LSTM) LMs, by incorporating app based contextual signals of voice search queries. We show that most of our adaptation strategies are effective, reducing word perplexity up to 21 % relative to a fine-tuned baseline on a held-out domain-specific development set. Initial experiments using a state-of-the-art Italian ASR system show a 3 % relative reduction in WER on top of an unadapted 5-gram LM. In addition, human evaluations show significant improvements on sub-domains from using app signals.