Envelope vocoders simulate cochlear implant processing by splitting sound into amplitude-modulated subbands

Evidence

The speech and the masker were mixed and low-pass filtered at 4000-Hz, and were either presented unprocessed or were passed through a tone-excited envelope vocoder that simulates certain aspects of CI processing (Dorman et al. 1998 ; Whitmal et al. 2007 ). The stimulus was divided into 16 frequency subbands, with the same center frequencies as the 16 tone maskers. The temporal envelope from each subband was extracted using a Hilbert transform, and then the resulting envelope was lowpass filtered with a 4th-order Butterworth filter and a cutoff frequency of 50 Hz. This cutoff frequency was chosen to reduce possible voicing periodicity cues, and to reduce the possibility that the vocoding produced spectrally resolved components via the amplitude modulation. — Dijk, Pim van, Physiology, Psychoacoustics and…, 2177

The degraded stimuli were created using a noise-band-vocoder to simulate CI processing. The stimuli were first bandlimited to 80–6000 Hz, and were subsequently bandpass-filtered into 6 channels. Sixth order Butterworth filters were used, with a spacing equal to the distances in the cochlea as determined using the Greenwood function. The slow-varying amplitude envelopes were extracted from each channel via lowpass filtering, and these envelopes were then used to modulate carrier wideband noise, the resulting 6 channels were finally bandpass filtered once more using the same 6 bandpass filters. The processed stimuli were the summed signals from the output of all channels. This manipulation lead to stimuli with unnatural spectrotemporally degraded form, hence stimuli that simulate the signal conveyed via CIs. — Dijk, Pim van, Physiology, Psychoacoustics and…, 3966

The stimuli were first bandlimited to 80–6000 Hz, and were subsequently bandpass-filtered into 6 channels. Sixth order Butterworth filters were used, with a spacing equal to the distances in the cochlea as determined using the Greenwood function. The slow-varying amplitude envelopes were extracted from each channel via lowpass filtering, and these envelopes were then used to modulate carrier wideband noise, the resulting 6 channels were finally bandpass filtered once more using the same 6 bandpass filters. The processed stimuli were the summed signals from the output of all channels. This manipulation lead to stimuli with unnatural spectrotemporally degraded form, hence stimuli that simulate the signal conveyed via CIs. — Dijk, Pim van, Physiology, Psychoacoustics and…, 3959

Two listening conditions were used in the experiment; natural speech (NS) and degraded speech (DS). The degraded stimuli were created using a noise-band-vocoder to simulate CI processing. The stimuli were first bandlimited to 80–6000 Hz, and were subsequently bandpass-filtered into 6 channels. Sixth order Butterworth filters were used, with a spacing equal to the distances in the cochlea as determined using the Greenwood function. The slow-varying amplitude envelopes were extracted from each channel via lowpass filtering, and these envelopes were then used to modulate carrier wideband noise, the resulting 6 channels were finally bandpass filtered once more using the same 6 bandpass filters. The processed stimuli were the summed signals from the output of all channels. This manipulation lead to stimuli with unnatural spectrotemporally degraded form, hence stimuli that simulate the signal conveyed via CIs. — Dijk, Pim van, Physiology, Psychoacoustics and…, 3966

Categorization

• DEFINE: Explains the technical method of envelope vocoding used to simulate CI processing — useful for understanding what CI users actually hear via amplitude-modulated envelopes rather than fine structure.
• XREF: Connects to cochlear implant signal processing literature; the Greenwood frequency-position map on the basilar membrane underlies channel spacing.
• corroborated: 4 independent passages support this claim — Dijk, Pim van, Physiology, Psychoacoustics and…, 2177; Dijk, Pim van, Physiology, Psychoacoustics and…, 3966; Dijk, Pim van, Physiology, Psychoacoustics and…, 3959