The intelligibility of noise-vocoded speech: spectral information available from across-channel comparison of amplitude envelopes

Abstract

Noise-vocoded (NV) speech is often regarded as conveying phonetic information primarily through temporal-envelope cues rather than spectral cues. However, listeners may infer the formant frequencies in the vocal-tract output-a key source of phonetic detail-from across-band differences in amplitude when speech is processed through a small number of channels. The potential utility of this spectral information was assessed for NV speech created by filtering sentences into six frequency bands, and using the amplitude envelope of each band ( 30 Hz) to modulate a matched noise-band carrier (N). Bands were paired, corresponding to F1 (%N1 þ N2), F2 (%N3 þ N4) and the higher formants (F3 0 % N5 þ N6), such that the frequency contour of each formant was implied by variations in relative amplitude between bands within the corresponding pair. Three-formant analogues (F0 ¼ 150 Hz) of the NV stimuli were synthesized using frame-by-frame reconstruction of the frequency and amplitude of each formant. These analogues were less intelligible than the NV stimuli or analogues created using contours extracted from spectrograms of the original sentences, but more intelligible than when the frequency contours were replaced with constant (mean) values. Across-band comparisons of amplitude envelopes in NV speech can provide phonetically important information about the frequency contours of the underlying formants.

Phase-Locked Responses to Speech in Human Auditory Cortex are Enhanced During Comprehension

“…However, this confound is not present in the 4 channel and 4 channel rotated conditions, which differ in intelligibility but are well matched for spectral complexity. Our comparison of responses with 4 channel and spectrally rotated 4 channel vocoded sentences thus demonstrates that it is intelligibility, rather than dynamic spectral change created by multiple amplitude envelopes (Roberts et al 2011), that is critical for enhancing cerebro-acoustic coherence. Our results show significantly increased cerebro-acoustic coherence for the more-intelligible, nonrotated 4 channel sentences in the left temporal cortex.…”

Temporal coding in the auditory cortex

“…Such speech signals, containing only envelope but no fine structure information, are called vocoded speech (Faulkner et al, 2000). The remarkable comprehension level reached by most patients with cochlear implants, in whom about 15-20 electrodes replace 3000 hair cells, remains the best empiric demonstration that the spectral content of speech can be degraded with tolerable alteration of speech perception (Roberts et al, 2011). Such findings have led to the idea that the temporal envelope is sufficient to yield speech comprehension (Rosen, 1992;Drullman et al, 1994a, b;Shannon et al, 1995;Giraud et al, 2004;Scott et al, 2006;Loebach and Wickesberg, 2008;Souza and Rosen, 2009).…”

Across-formant integration and speech intelligibility: Effects of acoustic source properties in the presence and absence of a contralateral interferer

“…The frequencies of the first three formants and their patterns of change over time are a critical source of information for identifying the phonetic segments being articulated by a talker and hence for understanding speech (see, e.g., Roberts et al, 2011). Precisely how the information carried by different formants is integrated across frequency into a phonetic percept is not fully understood, especially in contexts where more than one talker is speaking at once (see, e.g., Darwin, 2008).…”