We present a literature and empirical survey that critically assesses the state of the art in character-level modeling for machine translation (MT). Despite evidence in the literature that character-level systems are comparable with subword systems, they are virtually never used in competitive setups in WMT competitions. We empirically show that even with recent modeling innovations in character-level natural language processing, character-level MT systems still struggle to match their subword-based counterparts both in terms of translation quality and training and inference speed. Character-level MT systems show neither better domain robustness, nor better morphological generalization, despite being often so motivated. On the other hand, they tend to be more robust towards source side noise and the translation quality does not degrade with increasing beam size at decoding time.

通过文献和实证调查，我们批判性地评估了机器翻译（MT）中基于字符级建模的最新技术发展现状。尽管文献中有证据表明，与子词级方法相比，字符级系统相当可靠，但在竞争性的WMT比赛中几乎不被使用。我们的实证研究表明，尽管在自然语言处理的字符级建模方面取得了一些创新，但字符级MT系统仍然难以与基于子词的系统相匹配，也没有更好的领域稳健性或更好的形态概括性。但是，我们能够表现出对源端噪声的稳健性，并且在解码时随着Beam size的增加，翻译质量不会降低。

为什么人们不使用基于字符级的机器翻译？