Use of a sigmoidal-shaped function for noise attenuation in cochlear implants.

A new noise reduction algorithm is proposed for cochlear implants that applies attenuation to the noisy envelopes inversely proportional to the estimated signal-to-noise ratio (SNR) in each channel. The performance of the proposed noise reduction algorithm is evaluated with nine Clarion CII cochlear implant patients using IEEE sentences embedded in multi-talker babble and speech-shaped noise at 0-10 dB SNR. Results indicate that the sigmoidal-shaped weighting function produces significant improvements to speech recognition compared to the subjects' daily strategy. Much of the success of the proposed noise reduction algorithm is attributed to the improved temporal envelope contrast.

Effect of filter spacing on melody recognition: acoustic and electric hearing.

This paper assesses the effect of filter spacing on melody recognition by normal-hearing (NH) and cochlear implant (CI) subjects. A new semitone filter spacing is proposed for music. The quality of melodies processed by the various filter spacings is also evaluated. Results from NH listeners showed nearly perfect melody recognition with only four channels of stimulation, and results from CI users indicated significantly higher scores with a 12-channel semitone spacing compared to the spacing used in their daily processor. The quality of melodies processed by the semitone filter spacing was preferred over melodies processed by the conventional logarithmic filter spacing.

Use of S-shaped input-output functions for noise suppression in cochlear implants.

OBJECTIVES: The aim of this study is to assess the influence of the shape of the acoustic-to-electric mapping function on speech recognition in noise by cochlear implant listeners. DESIGN: A new acoustic-to-electric mapping function is proposed for cochlear implant users in noisy environments. The proposed s-shaped mapping function was expansive for low input levels up to a knee point level and compressive thereafter. The knee point of the mapping functions changed dynamically and was set proportional to the estimated noise floor level. The performance of the mapping function was evaluated on a sentence recognition task using IEEE sentences embedded in +5 to 10 dB SNR multitalker babble and in +5 dB SNR speech-shaped noise. Nine postlingually deafened cochlear implant users participated in the study. RESULTS: Results indicated that the same s-shaped mapping function did not yield significant improvements for all cochlear implant users. Significant benefits in speech intelligibility were observed, however, when the s-shaped mapping function was optimized to individual cochlear implant users. Significantly higher performance was achieved with the s-shaped mapping functions than the conventional log mapping function used by cochlear implant users in their daily strategy, in both multitalker (+5 and +10 dB SNR) and continuous speech-shaped (+5 dB SNR) conditions. CONCLUSIONS: These results clearly indicate that the shape of the nonlinear acoustic-to-electric mapping can have a significant effect on speech intelligibility in noise when it is optimized to individual cochlear implant users. The log functions currently used in most implant processors for mapping acoustic to electric amplitudes are not the best mapping functions to use in noisy environments. This is largely because compressive functions tend to amplify low-level segments of speech along with noise, thereby decreasing the spectral contrast and effective dynamic range. In contrast, the s-shaped mapping functions, which are partly compressive and partly expansive depending on the signal level, are more suitable for noisy environments and can produce significantly higher performance than the log-mapping functions.

The intelligibility of speech with "holes" in the spectrum.

The intelligibility of speech having either a single "hole" in various bands or having two "holes" in disjoint or adjacent bands in the spectrum was assessed with normal-hearing listeners. In experiment 1, the effect of spectral "holes" on vowel and consonant recognition was evaluated using speech processed through six frequency bands, and synthesized as a sum of sine waves. Results showed a modest decrease in vowel and consonant recognition performance when a single hole was introduced in the low- and high-frequency regions of the spectrum, respectively. When two spectral holes were introduced, vowel recognition was sensitive to the location of the holes, while consonant recognition remained constant around 70% correct, even when the middle- and high-frequency speech information was missing. The data from experiment 1 were used in experiment 2 to derive frequency-importance functions based on a least-squares approach. The shapes of the frequency-importance functions were found to be different for consonants and vowels in agreement with the notion that different cues are used by listeners to identify consonants and vowels. For vowels, there was unequal weighting across the various channels, while for consonants the frequency-importance function was relatively flat, suggesting that all bands contributed equally to consonant identification.