Effects of randomizing phase on the discrimination between amplitude-modulated and quasi-frequency-modulated tones

This study investigated the bandwidth of phase sensitivity. Subjects discriminated amplitude-modulated tones (AM), and quasi-frequency-modulated tones (QFM) in a two-interval, forced-choice task. An adaptive threshold procedure was used to estimate the modulation depth needed to discriminate the stimuli as a function of carrier and modulation frequency. Non-monotonicities in threshold-bandwidth functions were often observed at higher modulation frequencies. The results are discussed in terms of two potential cues: (1) waveform envelope, (2) cubic distortion products. In order to degrade the information obtained from auditory distortions, the phase for the carrier frequency was randomly sampled from a uniform distribution, which diminished the non-monotonicities with minimal effect at lower modulation frequencies. Model simulations demonstrated that phase randomization degrades distortion product cues with only a modest effect on temporal cues. Final results show that maximum bandwidths for phase sensitivity (BWmax) were not proportional to carrier frequencies.

► The bandwidth of phase sensitivity is investigated.
► Amplitude and quasi-frequency-modulated tones are discriminated in a 2IFC.
► Non-monotonicities in threshold functions are found at high modulation frequencies.
► Randomizing the phase of the carrier degrades information from distortion tones.
► Central factors appear to be the limiting factor for carriers greater than 1000 Hz.