Browsing by Subject "Amplitude modulation"
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Item Coded and filtered quadrature amplitude modulation signaling(Texas Tech University, 2001-12) Harris, JasonIn this thesis, the performance of several modified digital baseband transmission signals will be evaluated against each other with respect to their bandwidth and error probability characteristics. This is accompHshed via a transmission simulator designed specifically for this purpose. Hybrid signals incorporating a combination of signaling techniques such as Walsh Coding, Pre-filtering, and QAM signaling will be simulated and analyzed with a focus on Walsh Coded Gaussian QAM signals.Item Electrophysiological measurement of temporal integration in listeners with normal hearing(2016-05) Chen, Yu-Fu; Champlin, Craig A.; Liu, Chang; Sussman, Harvey M.; Whittaker, Tiffany A.Temporal integration refers to the phenomenon whereby the detection threshold of the stimulus decreases (improves) as the signal duration increases. The majority of studies of temporal integration have relied on behavioral methods. As a result temporal integration can be influenced by the subject’s physical and psychological status and these factors may affect signal detection. In the present study, the measurement of the auditory steady-state response (ASSR) was used to investigate temporal integration in listeners with normal hearing. The stimuli were sinusoidally amplitude modulated (SAM) signals varying in the modulation frequency (40 Hz and 80 Hz) and duration (50 ms, 100 ms, 200 ms, 300 ms, 400 ms, and 800 ms). The carrier was 1-kHz tone, 4-kHz tone, or white noise. The ASSRs were analyzed across different stimulus conditions in terms of amplitude, phase, signal-to-noise ratio (SNR), and percentage of detected responses. The results showed that temporal integration was more clearly revealed when the ASSR was recorded with the 40-Hz modulation frequency as compared to 80-Hz modulation frequency. For 40-Hz modulation frequency, the amplitude of the ASSR increased over the first 200 milliseconds after the stimulus onset until reaching a steady-state plateau, and then dropped rapidly after the stimulus offset. Conversely, the phase strength (i.e., variability) decreased over the first 400 milliseconds and remained relatively stable after that. For 80-Hz modulation frequency, the ASSR amplitude did not increase until approximately 200 milliseconds, beyond which the ASSR amplitude increased at the same rate as for 40-Hz modulation frequency. In addition, the ASSR phase was less stable across subjects, which suggests weaker responses overall. An exponential model fit the electrophysiological data best; however, a significant frequency effect on the time constant was not observed. These results suggest that both auditory midbrain and brainstem are able to integrate auditory information over the first 200 milliseconds of stimulus.Item Physiological and psychoacoustical sensitivity to amplitude and mixed modulation in normally-hearing listeners(2013-12) Pho, Michelle Hsieh; Champlin, Craig A.Researchers previously have found a correlation between the strength of the auditory steady-state response (ASSR) and corresponding auditory behavioral measurements such as speech recognition scores, thus concluding that the ASSR can be used as an objective measurement of auditory supra-threshold properties. In the present study, it was hypothesized that the increase in the strength of the ASSR at different modulation depths would be reflected in corresponding psychophysical measures, specifically, auditory modulation discrimination. These relationships were investigated in normally-hearing listeners at modulation rates of 40 and 80 Hz for both amplitude (AM) and mixed modulation (MM), at several modulation depths. Analyses were completed for two sets of measurements derived from the physiological and behavioral responses. For the first analysis, derived measures of iso-neurometric thresholds were compared to isometric modulation discrimination thresholds. For the second analysis, derived estimates of physiological neurometric slope were compared to estimated psychometric function slopes. Mixed-model analyses for both of these measures revealed significant or near-significant relationships between physiological and psychophysical measures at 40 Hz for AM and at 80 Hz for MM. Bootstrap resampling analyses were completed to estimate the distribution of the resultant statistics. Implications regarding the location of neural encoding for amplitude and frequency modulation were discussed.