Monotonic Infinite Lookback Attention for Simultaneous Machine Translation
Summary
Simultaneous machine translation starts producing output before the speaker finishes a sentence. It must decide when to read more source tokens and when to write target tokens, trading quality against delay. Earlier systems either learned an adaptive schedule on top of an NMT model trained only on full sentences, or trained NMT jointly with a rigid schedule such as wait-k. The authors propose Monotonic Infinite Lookback (MILk) attention for an RNN encoder-decoder. A hard monotonic head decides how much of the source has been read. A soft head can then attend to every token up to that point. Because unlimited lookback removes any built-in incentive to stream, they add a latency term to the training loss. This term is a differentiable, expectation-based version of Average Lagging. The whole model trains end to end as a substitute for standard soft attention. On German-English and English-French (WMT14), MILk beats wait-k, MoChA and Monotonic Attention across a wide range of lags. It keeps full-attention quality at noticeably lower delays, which shows the value of schedules that adapt to the input.
Key points
- MILk is presented as the first simultaneous NMT system that learns an adaptive read/write schedule jointly with a model attending over all source tokens read so far. Differentiable Average Lagging (DAL) is used as the latency objective.
- DeEn: MILk holds full-attention quality (28.4 BLEU) down to a lag of about 8.5 tokens, whereas wait-k degrades below 13.3. At a lag of 4 tokens, MILk leads by 1.5 BLEU.
- EnFr (WMT14): wait-k drops sharply at k = 8 (DAL 8.4, BLEU 39.8), while MILk stays flat until a lag of 7.2 (BLEU 40.5). At a lag of 4.5, MILk leads by 2.3 BLEU.
- MILk's advantage grows under the AL and AP metrics. However, the tested configuration could not reach lags below 4, while wait-k can still operate at very low latency.
- MILk's schedule adapts to the input: its initial delays vary widely compared with the nearly constant wait-6, despite similar DAL (5.8 vs 6.5). It tends to wait until noun or verb phrases are complete, which helps it reorder words correctly.
Topics Machine translation
Institutions
Cite
Cite as Naveen Arivazhagan, Colin Cherry, Wolfgang Macherey, Chung-Cheng Chiu, Semih Yavuz, Ruoming Pang, Wei Li and Colin Raffel. 2019. Monotonic Infinite Lookback Attention for Simultaneous Machine Translation. Proceedings of the 57th Annual Meeting of the Association for Computational Linguistics.
@inproceedings{arivazhagan2019monotonic,
title = {{Monotonic Infinite Lookback Attention for Simultaneous Machine Translation}},
author = {Naveen Arivazhagan, Colin Cherry, Wolfgang Macherey, Chung-Cheng Chiu, Semih Yavuz, Ruoming Pang, Wei Li and Colin Raffel},
booktitle = {Proceedings of the 57th Annual Meeting of the Association for Computational Linguistics},
year = {2019},
doi = {10.18653/v1/p19-1126},
url = {https://doi.org/10.18653/v1/p19-1126}
}References
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