Recognition of temporally interrupted and spectrally degraded sentences with additional unprocessed low-frequency speech

Abstract

Recognition of periodically interrupted sentences (with an interruption rate of 1.5 Hz, 50% duty cycle) was investigated under conditions of spectral degradation, implemented with a noiseband vocoder, with and without additional unprocessed low-pass filtered speech (cutoff frequency 500 Hz). Intelligibility of interrupted speech decreased with increasing spectral degradation. For all spectral degradation conditions, however, adding the unprocessed low-pass filtered speech enhanced the intelligibility. The improvement at 4 and 8 channels was higher than the improvement at 16 and 32 channels: 19% and 8%, on average, respectively. The Articulation Index predicted an improvement of 0.09, in a scale from 0 to 1. Thus, the improvement at poorest spectral degradation conditions was larger than what would be expected from additional speech information. Therefore, the results implied that the fine temporal cues from the unprocessed low-frequency speech, such as the additional voice pitch cues, helped perceptual integration of temporally interrupted and spectrally degraded speech, especially when the spectral degradations were severe. Considering the vocoder processing as a cochlear implant simulation, where implant users' performance is closest to 4 and 8-channel vocoder performance, the results support additional benefit of low-frequency acoustic input in combined electric-acoustic stimulation for perception of temporally degraded speech.

Figure 1

Spectra of noise matching the long-term speech spectrum of male and female talkers, shown after periodic temporal interruptions and highest (4 channels) and lowest (32 channels) spectral degradations were applied. The spectra of unprocessed and only-interrupted noise are added as a reference.

Figure 2

Spectrograms of one sentence spoken by the male talker and processed with different experimental conditions. (A) shows the original sentence. 500 Hz (the cut-off frequency for LP500) is marked with the straight dashed line as a reference. The solid lines superimposed with the spectrogram are the pitch contours (time-varying F0 frequency) extracted by PRAAT software. The middle and lower panels show the sentence with temporal interruption and spectral degradation with 4 and 32 channels, respectively. The left and right panels show the sentence with NBV and NBV+LP500 processing, respectively.

Figure 3

The top panel shows the mean percent correct scores of interrupted and spectrally degraded sentences, with and without the LP500, as a function of the number of the vocoder channels. The gray symbol shows the baseline score with full-bandwidth interrupted speech with no spectral degradation. The lower panel shows the improvement in performance due to the addition of LP500 for each vocoder-channel condition. The dashed line shows the improvement predicted by the Articulation Index. The error bars denote one standard deviation.