Cross-lingual and Multilingual Automatic Speech Recognition for Scandinavian Languages

Sammanfattning: Research into Automatic Speech Recognition (ASR), the task of transforming speech into text, remains highly relevant due to its countless applications in industry and academia. State-of-the-art ASR models are able to produce nearly perfect, sometimes referred to as human-like transcriptions; however, accurate ASR models are most often available only in high-resource languages. Furthermore, the vast majority of ASR models are monolingual, that is, only able to handle one language at a time. In this thesis, we extensively evaluate the quality of existing monolingual ASR models for Swedish, Danish, and Norwegian. In addition, we search for parallels between monolingual ASR models and the cognition of foreign languages in native speakers of these languages. Lastly, we extend the Swedish monolingual model to handle all three languages. The research conducted in this thesis project is divided into two main sections, namely monolingual and multilingual models. In the former, we analyse and compare the performance of monolingual ASR models for Scandinavian languages in monolingual and cross-lingual settings. We compare these results against the levels of mutual intelligibility of Scandinavian languages in native speakers of Swedish, Danish, and Norwegian to see whether the monolingual models favour the same languages as native speakers. We also examine the performance of the monolingual models on the regional dialects of all three languages and perform qualitative analysis of the most common errors. As for multilingual models, we expand the most accurate monolingual ASR model to handle all three languages. To do so, we explore the most suitable settings via trial models. In addition, we propose an extension to the well-established Wav2Vec 2.0-CTC architecture by incorporating a language classification component. The extension enables the usage of language models, thus boosting the overall performance of the multilingual models. The results reported in this thesis suggest that in a cross-lingual setting, monolingual ASR models for Scandinavian languages perform better on the languages that are easier to comprehend for native speakers. Furthermore, the addition of a statistical language model boosts the performance of ASR models in monolingual,  cross-lingual, and multilingual settings. ASR models appear to favour certain regional dialects, though the gap narrows in a multilingual setting. Contrary to our expectations, our multilingual model performs comparably with the monolingual Swedish ASR models and outperforms the Danish and Norwegian models. The multilingual architecture proposed in this thesis project is fairly simple yet effective. With greater computational resources at hand, further extensions offered in the conclusions might improve the models further.