  The recently proposed serialized output training (SOT) simplifies
multi-talker automatic speech recognition (ASR) by generating speaker
transcriptions separated by a special token. However, frequent speaker changes
can make speaker change prediction difficult. To address this, we propose
boundary-aware serialized output training (BA-SOT), which explicitly
incorporates boundary knowledge into the decoder via a speaker change detection
task and boundary constraint loss. We also introduce a two-stage connectionist
temporal classification (CTC) strategy that incorporates token-level SOT CTC to
restore temporal context information. Besides typical character error rate
(CER), we introduce utterance-dependent character error rate (UD-CER) to
further measure the precision of speaker change prediction. Compared to
original SOT, BA-SOT reduces CER/UD-CER by 5.1%/14.0%, and leveraging a
pre-trained ASR model for BA-SOT model initialization further reduces
CER/UD-CER by 8.4%/19.9%.
