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Speech perception in noise with a harmonic complex excited vocoder.

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Understanding degraded speech with cochlear implants (CI) is improved by randomizing carrier phases in vocoders. This technique enhances speech intelligibility for normal hearing listeners, potentially improving CI technology.

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Area of Science:

  • Auditory Neuroscience
  • Speech Processing
  • Hearing Technology

Background:

  • Cochlear implants (CI) deliver speech information by modulating electrical pulse trains based on acoustic envelope information.
  • Vocoders simulate CI processing by modulating acoustic carriers with speech envelope information.
  • Understanding normal hearing (NH) listeners' perception of vocoded speech can elucidate factors influencing CI performance.

Purpose of the Study:

  • To investigate the impact of carrier phase dispersion on speech envelope processing and intelligibility in vocoded signals.
  • To explore parameters that best simulate electric hearing and identify differences between NH and CI performance.
  • To assess the relationship between neural envelope representation and speech recognition scores.

Main Methods:

  • Utilized a vocoder with harmonic complex carriers (f0 = 100 Hz) and varied degrees of random starting phase dispersion.
  • Tested NH listeners on closed-set word recognition in noise using vocoded speech.
  • Employed two sets of synthesis filters to simulate varying degrees of CI current spread.
  • Performed cross-correlogram analyses on auditory nerve model simulations.

Main Results:

  • Speech intelligibility was highest when vocoder carriers had maximally dispersed starting phases.
  • Maximal phase dispersion flattened temporal envelopes, potentially enhancing speech understanding.
  • Auditory nerve model simulations showed improved neural envelope representation with dispersed phases.
  • Neural metrics did not consistently predict speech recognition scores across all conditions.

Conclusions:

  • Randomly dispersing carrier phases in vocoders significantly improves speech intelligibility for NH listeners.
  • The benefits of phase dispersion may stem from altered temporal envelope characteristics and central processing.
  • Current neural metrics may not fully capture the complexities of speech perception in vocoded signals.
  • Central auditory mechanisms might be less sensitive to the fine structure exploited by vocoders.