Simple and artefact-free spectral modifications for enhancing the intelligibility of casual speech

In this paper, the problem of modifying casual speech to reach the intelligibility level of clear speech is addressed. Unlike other studies, in this work modifications on casual speech both consider intelligibility and speech quality. To achieve this, the authors focus on human-like modifications inspired by clear speech. An acoustic analysis performed on clear and casual speech reveals energy differences on specific frequency bands between the two speaking styles. Then, a simple method is used to boost these frequency regions on casual speech. The proposed method, called mix-filtering, uses a multi-band filtering scheme to isolate the information of these frequency bands and then, add this information to the original signal. Our method is compared in terms of intelligibility and quality with unmodified casual speech and with a highly intelligible spectral modification technique, namely the Spectral Shaping and Dynamic Range Compression (SSDRC). Two different objective measures that are highly correlated with subjective intelligibility scores are used for estimating the intelligibility, whereas for evaluating the quality, preference listening tests are performed. Results show that the mix-filtering technique increases the intelligibility of casual speech while maintains its quality. On the other hand, while SSDRC outperforms on intelligibility, it degrades significantly the quality of casual speech.