A channel-selection criterion for suppressing reverberation in cochlear implants

Abstract

Little is known about the extent to which reverberation affects speech intelligibility by cochlear implant (CI) listeners. Experiment 1 assessed CI users' performance using Institute of Electrical and Electronics Engineers (IEEE) sentences corrupted with varying degrees of reverberation. Reverberation times of 0.30, 0.60, 0.80, and 1.0 s were used. Results indicated that for all subjects tested, speech intelligibility decreased exponentially with an increase in reverberation time.A decaying-exponential model provided an excellent fit to the data. Experiment 2 evaluated (offline) a speech coding strategy for reverberation suppression using a channel-selection criterion based on the signal-to-reverberant ratio (SRR) of individual frequency channels. The SRR reflects implicitly the ratio of the energies of the signal originating from the early (and direct) reflections and the signal originating from the late reflections. Channels with SRR larger than a preset threshold were selected, while channels with SRR smaller than the threshold were zeroed out. Results in a highly reverberant scenario indicated that the proposed strategy led to substantial gains (over 60 percentage points) in speech intelligibility over the subjects' daily strategy. Further analysis indicated that the proposed channel-selection criterion reduces the temporal envelope smearing effects introduced by reverberation and also diminishes the self-masking effects responsible for flattened formants.

Sentence recognition in noise promoting or suppressing masking release by normal-hearing and cochlear-implant listeners

“…Considering that amplitude fluctuations in the speech waveform are often markers of segmental boundaries and play an important role in speech recognition (e.g., Drullman et al, 1994a,b), compromised word and syllable boundaries in the þMR condition combined with less perceptual difference between speech and noise, might account for the lack of MR in CI listeners. This interpretation is in accord with the suggestion that the detrimental influence of reverberation on CI-user performance can be attributed to a filling in of low speech-intensity segments and resulting segmentation difficulties (Kokkinakis et al, 2011;Hazrati and Loizou, 2012).…”

Blind binary masking for reverberation suppression in cochlear implants

“…The results obtained from the proposed BRM are compared against those obtained by testing the subjects with the unprocessed reverberant stimuli in three moderate to relatively high reverberant conditions (T 60 ¼ 0.3, 0.6, and 0.8 s). Scores obtained from the IRM-processing (Kokkinakis et al, 2011) are also given for comparison to provide the upper bound in performance. The average intelligibility score obtained in anechoic quiet condition was 82.6% for the six tested CI listeners.…”

“…5(b)]. This is evident in unvoiced segments of speech, and consequently causes a decrease in intelligibility (Kokkinakis et al, 2011). As shown in panels (c) of Figs.…”

“…The early and late reflections fill the gaps in the temporal envelope of speech and reduce envelope modulation depth (Assmann and Summerfield, 2004). Although speech intelligibility is not affected much by reverberation for normal hearing (NH) listeners, it is degraded significantly for hearing impaired, cochlear implant (CI) users, and elderly people (Nabelek and Letowski, 1988;Kokkinakis et al, 2011;Nabelek and Letowski, 1985). Reverberation also poses a detrimental impact on the performance of automatic speech recognition (ASR) (Palom€ aki et al, 2004), and speaker identification (SID) systems (Sadjadi and Hansen, 2011).…”

“…The ideal reverberant mask (IRM) proposed by Kokkinakis et al (2011), has previously been shown to result in substantial intelligibility gains for both CI users and NH listeners (Kokkinakis et al, 2011;Hazrati and Loizou, 2012).…”