Auditory models and human performance in tasks related to speech coding and speech recognition

Auditory models that are capable of achieving human performance in tasks related to speech perception would provide a basis for realizing effective speech processing systems. Saving bits in speech coders, for example, relies on a perceptual tolerance to acoustic deviations from the original speech. Perceptual invariance to adverse signal conditions (noise, microphone and channel distortions, room reverberations) and to phonemic variability (due to nonuniqueness of articulatory gestures) may provide a basis for robust speech recognition. A state-of-the-art auditory model that simulates, in considerable detail, the outer parts of the auditory periphery up through the auditory nerve level is described. Speech information is extracted from the simulated auditory nerve firings, and used in place of the conventional input to several speech coding and recognition systems. The performance of these systems improves as a result of this replacement, but is still short of achieving human performance. The shortcomings occur, in particular, in tasks related to low bit-rate coding and to speech recognition. Since schemes for low bit-rate coding rely on signal manipulations that spread over durations of several tens of ms, and since schemes for speech recognition rely on phonemic/articulatory information that extend over similar time intervals, it is concluded that the shortcomings are due mainly to perceptually related rules over durations of 50-100 ms. These observations suggest a need for a study aimed at understanding how auditory nerve activity is integrated over time intervals of that duration. The author discusses preliminary experimental results that confirm human usage of such integration, with different integration rules for different time-frequency regions depending on the phoneme-discrimination task.