Updating the SRMR-CI Metric for Improved Intelligibility Prediction for Cochlear Implant Users

When compared to intrusive speech intelligibility metrics, non-intrusive ones show a stronger dependency on speech content, given the lack of a reference signal for distortion level computation. Reduction of this dependency is an important step needed to develop reliable metrics. In this paper, two different updates to SRMR-CI, a recently-proposed speech intelligibility metric tailored for cochlear implant users, are applied. First, modulation energy thresholding is proposed to reduce the variability caused by the differences in modulation spectral representations for different phonemes and speakers, as well as speech enhancement algorithm artifacts. Second, a narrower range of modulation filters is employed to reduce fundamental frequency effects. Experimental results show that the updated metric outperforms two benchmark metrics, namely ModA and ANIQUE+, by as much as 15% in terms of correlation between objective and subjective ratings, and a relative decrease of 47% in root mean square error compared to the previously-proposed SRMR-CI metric.