Not all the questions related to the semi-supervised training of hybrid ASR system with DNN acoustic model were already deeply investigated. In this paper, we focus on the question of the granularity of confidences (per-sentence, per-word, per-frame), the question of how the data should be used (data-selection by masks, or in mini-batch SGD with confidences as weights). Then, we propose to re-tune the system with the manually transcribed data, both with the ‘frame CE’ training and ‘sMBR’ training.

Our preferred semi-supervised recipe which is both simple and efficient is following: we select words according to the word accuracy we obtain on the development set. Such recipe, which does not rely on a grid-search of the training hyper-parameter, generalized well for: Babel Vietnamese (transcribed 11h, untranscribed 74h), Babel Bengali (transcribed 11h, untranscribed 58h) and our custom Switchboard setup (transcribed 14h, untranscribed 95h). We obtained the absolute WER improvements 2.5% for Vietnamese, 2.3% for Bengali and 3.2% for Switchboard.