Discriminating change in the envelope phase spectrum

Abstract

Models of a modulation filterbank retain envelope phase information for only the lowest rates. To evaluate model predictions, the ability to discriminate change in the envelope phase spectrum of modulated wideband noise was measured. Modulators were narrow-band noises with bandwidth ranging from 5 to 160 Hz. The contrasting envelopes were generated by manipulating the modulator phase spectrum while leaving the amplitude spectrum unchanged. Modulator phase spectrum was varied by either reversing the modulator waveform, randomizing or zeroing phase arguments in the modulator spectrum, or generating a minimum-phase reconstruction through cepstral analysis. For time reversal and randomization, discrimination ability decreased as modulator bandwidth increased, indicating both a loss of phase information with increasing rate and masking of the low-rate information by higher-rate modulation. For zero- and minimum-phase modulators, at some point performance improved with bandwidth. This result implies a sliding temporal window to preserve power fluctuations in the filterbank output. Comparable performance was obtained when equal-bandwidth zero-phase regions were restricted to either a low- or higher-rate region. This result may reflect nonlinearity in modulation processing whereby intermodulation of high-rate components introduces perceptible low-rate modulation. [Work supported by NIH.]