Minimum Bayes Risk Decoding

CTC oracle gap anatomy: acoustic scoring saturates, linguistic MBR decoding recovers WER

A new arXiv paper systematically diagnoses why CTC-internal N-best rescoring fails to improve over greedy decoding on LibriSpeech, showing that blank-path proliferation causes a 53% degradation in rank correlation between CTC scores and WER as beam size grows. The authors demonstrate that the bottleneck is linguistic rather than acoustic: MBR decoding with RoBERTa pseudo-log-likelihood achieves 9% relative WER reduction on LibriSpeech test-other and generalizes across two architectures and three domains. The paper also analyzes MWER sequence-level fine-tuning failure at near-converged checkpoints, attributing collapse to a vanishingly small training oracle gap.