Institute for Systems Research

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Author: search.filters.author.Shamma, S.Subject: search.filters.subject.speech processing

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    Ripple Analysis in Ferret Primary Auditory Cortex III. Prediction of Unit Responses to Arbitrary Spectral Profiles
    (1995) Shamma, S.; Versnel, H.; ISR
    We examined whether AI responses to arbitrary spectral profiles can be explained by the superposition of responses to the individual ripple components that make up the spectral pattern. For each unit, the ripple transfer function was first measured using ripple stimuli consisting of broadband complexes with sinusoidally modulated spectral envelopes (Shamma et al. 1994). Unit responses to various combinations of ripples were compared to those predicted from the superposition of responses according to the transfer function. Spectral profiled included combinations of 2-5 ripples of equal amplitudes and random phases, and vowel- like profiles composed of 10 ripples with various amplitudes and phases. The results demonstrate that predicted and measured responses are reasonably well matched, and hence support the notion that AI analyzes the acoustic spectrum in a substantially linear manner.
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    Normalization and Noise-Robustness in Early Auditory Representations
    (1993) Wang, K.; Shamma, S.; ISR
    A common sequence of operations in the early stages of most sensory systems is a multiscale transform followed by a compressive nonlinearity. In this paper, we explore the contribution of these operations to the formation of robust and perceptually significant representation in the early auditory system. It is shown that auditory representation of the acoustic spectrum is effectively a self-normalized spectral analysis, i.e., the auditory system computes a spectrum that is divided by a smoothed version of itself. Such a self-normalization induces significant effects such as spectral shape enhancement and robustness against scaling and noise corruption. Examples using synthesized signals and a natural speech vowel are presented to illustrate these results. Furthermore, the characteristics of auditory representation are discussed in the context of several psychoacoustical findings, together with the possible benefits of this model for various engineering applications.
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    Classification of the Transient Signals via Auditory Representations
    (1991) Teolis, A.; Shamma, S.; ISR
    We use a model of processing in the human auditory system to develop robust representations of signals. These reduced representations are then presented to a neural network for training and classification.

    Empirical studies demonstrate that auditory representations compare favorably to direct frequency (magnitude spectrum) representations with respect to classification performance (i.e. probabilities of detection and false alarm). For this comparison the Receiver Operating Characteristic (ROC) curves are generated from signals derived from the standard transient data set (STDS) distributed by DARPA/ONR.

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    Cochlear Filters Design Using a Parallel Dilating-Biquad Switched-Capacitor Filter Bank
    (1991) Lin, Jyhfong; Ki, Wing-Hung; Thompson, K.E.; Shamma, S.; ISR
    A parallel filter bank is proposed to implement cochlear filters using very large time-constant (VLT) switched-capacitor (SC) filters. Significant hardware reduction is achieved in three ways. First, VLT SC biquads are used where the capacitor spread ratio of each biquad is about inversely proportional to the square root of wT, w is the center frequency of the filter and T is the inverse of the sampling frequency of the biquad. Second, the number of biquads is reduced by biquad sharing where n-biquad per channel cochlear filters are realized with only one additional biquad per channel after the first channel. Finally, LPN-type filter is used to avoid the very small capacitor in the forward path of each biquad. Furthermore, this filter bank s not only parasitics-insensitive but also gain-and-offset compensated using biphase clocking.
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    Realization of Cochlear Model by VLT Switched-Capacitor Filter Biquads
    (1991) Lin, Jyhfong; Ki, Wing-Hung; Thompson, K.E.; Shamma, S.; ISR
    We describe here the realization of a cochlear model using switched capacitor filters (SCF). This approach is made possible by a new design technique, called charge differencing (CD), which reduces by up to 50% the silicon area required to implement very large time-constant (VLT) filter biquads. In this technique, filter time constants are controlled by ratios of capacitor differences making the capacitor spread ratio very small. The new SCF's are also parasitics-free and are stabilized against op-amp inaccuracies, such as input offsets and finite gains, using a two-phase gain-offset-compensation method.