Browsing by Author "Shamma, Shihab"
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Item Analog VLSI Implementations of Auditory Wavelet Transforms Using Switched-Capacitor Circuits(1992) Lin, Jyhfong; Pati, Yagyensh; Edwards, Thomas; Shamma, Shihab; ISRA general scheme for the VLSI implementation of auditory wavelet transforms is proposed using switched-capacitor (SC) circuits. SC circuits are well suited for this application since the dilation constant across different scales of the transform can be precisely implemented and controlled by both capacitor ratios and the clock frequency. The hardware implementations are made possible by several new circuit designs. Specifically, extremely area-efficient designs are presented to implement very large time-constant filters such as those used to process speech and other acoustic signals. the designs employ a new charge differencing technique to reduce significantly the capacitance spread ratios needed in the filter banks. Also, a new sum-gain amplifier (SGA-SI) is designed which permits several inputs to be sampled with the same phase. The proposed circuits have been fabricated using a 1 m CMOS double-poly process. Preliminary data and performance measures of the circuits are very encouraging and are presented. Two possible architectures for implementing the wavelet transform are discussed and compared: parallel and cascade filter banks. Responses of both filter banks are simulated using SWITCAP-II. Finally, we shall also briefly discuss the utility, from an implementation point of view, of decomposing the transfer functions of the filter banks into rational form using a recently-developed wavelet system (WS) technique.Item Area-Efficient Switched Capacitor Filters: Very Large Time- Constant Circuits(1992) Lin, Jyhfong; Edwards, Thomas; Shamma, Shihab; ISRThe detailed theoretical analysis of very large time-constant (VLT) integrators which use either charge elimination (T-cell and Huang's integrators) or charge cancellation (Nagaraj's and charge-differencing integrators) and described. Using a new area- efficient design, the charge-differencing (CD) integrator, the capacitance spread ratio can be easily reduced to less than the reciprocal of the square root of the product of the pole frequency and the sampling period. Non-ideal effects of op-amp like finite DC gain and offset voltage can also be compensated by employing offset storing capacitor operating only with the bi- phase clocking scheme.Item Area-Efficient Switched Capacitor Non-Filtering Circuits: Sum- Gain Amplifiers(1992) Lin, Jyhfong; Edwards, Thomas; Shamma, Shihab; ISRSwitched-capacitor rum-gain amplifier (SGAs) are widely used in parallel filter systems. The capacitance spread of such circuits is proportional to the coefficient spread, thus making conventional SGAs unsuitable for high gain applications. SGAs with different input phases and a SGA with the same input phase are discussed, and a new area-efficient SGA, which can reduce the capacitance spread to the square root of coefficient spread ration, is proposed. Furthermore, the non-ideal effects of op- amp like finite DC gain and offset voltage are compensated by using the offset storing capacitor. All the circuits designed here only use the bi-phase clocking scheme.Item The Case of the Missing Pitch Templates: How Harmonic Templates Emerge in the Early Auditory System(1999) Shamma, Shihab; Klein, David J.; ISRPeriodicity pitch is the most salient and important of all pitch percepts.Psycho-acoustical models of this percept have long postulated the existenceof internalized harmonic templates against which incoming resolved spectracan be compared, and pitch determined according to the best matchingtemplates cite{goldstein:pitch}.However, it has been a mystery where andhow such harmonic templates can come about. Here we present a biologicallyplausible model for how such templates can form in the early stages of theauditory system. The model demonstrates that {it any} broadband stimulussuch as noise or random click trains, suffices for generating thetemplates, and that there is no need for any delay-lines, oscillators, orother neural temporal structures.
The model consists of two key stages:cochlear filtering followed by coincidence detection. The cochlear stageprovides responses analogous to those seen on the auditory-nerve andcochlear nucleus. Specifically, it performs moderately sharp frequencyanalysis via a filter-bank with tonotopically ordered center frequencies(CFs); the rectified and phase-locked filter responses are further enhancedtemporally to resemble the synchronized responses of cells in the cochlearnucleus.
The second stage is a matrix of coincidence detectors thatcompute the average pair-wise instantaneous correlation (or product)between responses from all CFs across the channels. Model simulations showthat for any broadband stimulus, high coincidences occur between cochlearchannels that are exactly harmonic distances apart. Accumulatingcoincidences over time results in the formation of harmonic templates forall fundamental frequencies in the phase-locking frequency range.
Themodel explains the critical role played by three subtle but importantfactors in cochlear function: the nonlinear transformations following thefiltering stage; the rapid phase-shifts of the traveling wave near itsresonance; and the spectral resolution of the cochlear filters. Finally, wediscuss the physiological correlates and location of such a process and itsresulting templates.
Item Perception and neural coding of harmonic fusion in ferrets(2004) Kalluri, Sridhar; Shamma, Shihab; Shamma, Shihab; ISR; CAARThe cortical neural correlates for the perception of harmonic sounds have remained a puzzle despite intense study over several decades. This study approached the problem from the point of view of the spectral fusion evoked by such sounds. Experiment 1 tested whether ferrets automatically fuse harmonic complex tones. In baseline sessions, three ferrets were trained to detect a pure tone terminating a sequence of inharmonic complex tones. After the ferrets reached proficiency in the baseline task, a small fraction of the inharmonic complex tones were replaced with harmonic tones. Two out of three ferrets confused the harmonic complex tones with the pure tones and responded as if detecting the pure tone at twice the false-alarm rate, indicating that ferrets can automatically fuse the partials of a harmonic complex. Experiment 2 sought correlates of harmonic fusion in single units of ferret primary auditory cortex (AI), by contrasting responses to harmonic complex tones with those to inharmonic complex tones. The effects of spectrotemporal filtering were accounted for by using the measured spectrotemporal receptive field to predict responses and by seeking correlates of harmonic fusion in the predictability of the responses. Ten percent of units exhibited some correlates of harmonic fusion, which is consistent with previous findings that no special processing for harmonic stimuli occurs in AI.Item Spectro-Temporal Modulation Transfer Functions and Speech Intelligibility(1999) Chi, Taishih; Gao, Yujie; Guyton, Matthew C.; Ru, Powen; Shamma, Shihab; ISR; CAARDetection thresholds for spectral and temporal modulations are measuredusing broadband spectra with sinusoidally rippled profiles that drift up or down the log-frequency axis at constant velocities. Spectro-temporal Modulation Transfer Functions (MTF) are derived as a function of ripple peak density (cycles/octave) and drifting velocity (Hz). MTFs exhibit a lowpass function with respect to both dimensions, with 50 percent bandwidths ofabout 16 Hz and 2 cycles/octave. The data replicate (as special cases) previously measured purely temporal MTFs [Viemeister, 1979] and purely spectral MTFs [Green, 1986].We present a computational auditory model that exhibits spectro-temporal MTFs consistent with the salient trends in the data.The model is used to demonstrate the potential relevance of these MTFsto the assessment of speech intelligibility in noise and reverberantconditions.