Are Subtitling Corpora really Subtitle-like?

Growing needs in translating multimedia content have resulted in Neural Machine Translation (NMT) gradually becoming an established practice in the field of subtitling. Contrary to text translation, subtitling is subject to spatial and temporal constraints, which greatly increase the post-processing effort required to restore the NMT output to a proper subtitle format. In this work, we explore whether existing subtitling corpora conform to the constraints of: 1) length and reading speed; and 2) proper line breaks. We show that the process of creating parallel sentence alignments removes important time and line break information and propose practices for creating resources for subtitling-oriented NMT faithful to the subtitle format.