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Hearing impaired listeners using cochlear implants (CIs) suffer from a decrease in speech intelligibility (SI) in adverse listening conditions. Time-frequency masks are often applied to perform noise suppression in an attempt to increase SI. Two important masks are the so-called ideal binary mask (IBM) with its binary weights and the ideal Wiener filter (IWF) with its continuous weights. It is unclear which of the masks has the highest potential for SI and speech quality enhancement in CI users. In this study, both approaches for SI and quality enhancement were compared. The investigations were conducted in normal-hearing (NH) subjects listening to noise vocoder CI simulations and in CI users. The potential for SI improvement was assessed in a sentence recognition task with ideal mask estimates in multitalker babble and with an interfering talker. The robustness of the approaches was evaluated with simulated estimation errors. CI users assessed the speech quality in a preference rating. The IWF outperformed the IBM in NH listeners. In contrast, no significant difference was obtained in CI users. Estimation errors degraded SI in CI users for both approaches. In terms of quality, the IWF outperformed, slightly, the IBM processed signals. The outcomes of this study suggest that the mask pattern is not that crucial for CIs. Results of speech enhancement algorithms obtained with NH subjects listening to vocoded or normally processed stimuli do not translate to CI users. This outcome means that the effect of new strategies has to be quantified with the user group considered.

Recent studies have shown that transient parts of a speech signal contribute most to speech intelligibility in normal-hearing listeners. In this study, the influence of enhancing the onsets of the envelope of the speech signal on speech intelligibility in noisy conditions using an eight channel cochlear implant vocoder simulation was investigated. The enhanced envelope (EE) strategy emphasizes the onsets of the speech envelope by deriving an additional peak signal at the onsets in each frequency band. A sentence recognition task in stationary speech shaped noise showed a significant speech reception threshold (SRT) improvement of 2.5 dB for the EE in comparison to the reference continuous interleaved sampling strategy and of 1.7 dB when an ideal Wiener filter was used for the onset extraction on the noisy signal. In a competitive talker condition, a significant SRT improvement of 2.6 dB was measured. A benefit was obtained in all experiments with the peak signal derived from the clean speech. Although the EE strategy is not effective in many real-life situations, the results suggest that there is potential for speech intelligibility improvement when an enhancement of the onsets of the speech envelope is included in the signal processing of auditory prostheses.

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Despite many considerable technical advances in the field of hearing aids and cochlear implants, people using auditory prostheses still have major problems with speech understanding in the presence of interfering sounds and with directional hearing. Both abilities are dependent on sound stream segregation in real-world listening environments. In this paper, two timely and important issues related to sound stream segregation in auditory prostheses are addressed, namely the coding of monaural and binaural cues. Several state-of-the-art signal processing algorithms used in cochlear implants (CI) and in hearing aids (HA) are introduced. A review is given of some recent proposals to improve temporal coding in monaural CIs, and of recent work to improve the transmission of binaural cues in both HAs, CIs and combined acoustic and electric hearing (bimodal hearing). The ultimate aim is to improve speech and music perception, and additionally the preservation of binaural cues to preserve directional hearing.