Translating characters instead of words or word-fragments has the potential to simplify the processing pipeline for neural machine translation (NMT), and improve results by eliminating hyper-parameters and manual feature engineering. However, it results in longer sequences in which each symbol contains less information, creating both modeling and computational challenges. In this paper, we show that the modeling problem can be solved by standard sequence-to-sequence architectures of sufficient depth, and that deep models operating at the character level outperform identical models operating over word fragments. This result implies that alternative architectures for handling character input are better viewed as methods for reducing computation time than as improved ways of modeling longer sequences. From this perspective, we evaluate several techniques for character-level NMT, verify that they do not match the performance of our deep character baseline model, and evaluate the performance versus computation time tradeoffs they offer. Within this framework, we also perform the first evaluation for NMT of conditional computation over time, in which the model learns which timesteps can be skipped, rather than having them be dictated by a fixed schedule specified before training begins.

本论文通过设计深度模型对字符级信息进行序列到序列建模，并且验证了该模型优于传统的基于单词片段的模型，从而为字符级神经机器翻译提供了参考。同时，通过评估多种字符级NMT技术，发现它们不能与深层字符基线模型的表现相匹配。最后，我们还在该框架内进行了针对NMT的条件计算时间的第一次评估。

基于字符的神经机器翻译：容量和压缩的再思考