Combined effects of frequency compression-expansion and shift on speech recognition.

Objective: To explore combined acute effects of frequency shift and compression-expansion on speech recognition, using noiseband vocoder processing.

Design: Recognition of vowels and consonants, processed with a noiseband vocoder, was measured with five normal-hearing subjects, between the ages of 27 and 35 yr. The speech signal was filtered into 8 or 16 analysis bands and the envelopes were extracted from each band. The carrier noise bands were modulated by the envelopes and resynthesized to produce the processed speech. In the baseline matched condition, the frequency ranges of the corresponding analysis and carrier bands were the same. In the shift only condition, the frequency ranges of the carrier bands were shifted up or down relative to the analysis bands. In the compression and expansion only conditions, the analysis band range was made larger or smaller, respectively, than the carrier band range. By applying the shift to carrier bands and compression or expansion to analysis bands simultaneously, the combined effects of the two spectral distortions on speech recognition were explored.

Results: When the spectral distortions of compression-expansion or shift were applied separately, the performance was reduced from the baseline matched condition. However, when the two spectral degradations were applied simultaneously, a compensatory effect was observed; the reduction in performance was smaller for some combinations compared to the reduction observed for each distortion individually.

Conclusions: The results of the present study are consistent with previous vocoder studies with normal-hearing subjects that showed a negative effect of spectral mismatch between analysis and carrier bands on speech recognition. The present results further show that matching the frequency ranges of 1 to 2 kHz, which contain important speech information, can be more beneficial for speech recognition than matching the overall frequency ranges, in certain conditions.