Finnish ASR with Deep Transformer Models

Recently, BERT and Transformer-XL based architectures have achieved strong results in a range of NLP applications. In this paper, we explore Transformer architectures — BERT and Transformer-XL — as a language model for a Finnish ASR task with different rescoring schemes. We achieve strong results in both an intrinsic and an extrinsic task with Transformer-XL. Achieving 29% better perplexity and 3% better WER than our previous best LSTM-based approach. We also introduce a novel three-pass decoding scheme which improves the ASR performance by 8%. To the best of our knowledge, this is also the first work (i) to formulate an alpha smoothing framework to use the non-autoregressive BERT language model for an ASR task, and (ii) to explore sub-word units with Transformer-XL for an agglutinative language like Finnish.