Effects of modulation rate and rate of envelope change on modulation discrimination interference

Abstract

Modulation discrimination interference (MDI) refers to a reduced ability to detect a change in modulation depth of a target carrier frequency in the presence of other modulated carriers with frequencies remote from that of the target. These experiments examined whether the tuning of MDI in the modulation domain depends primarily on the similarity in modulation periodicity of the target and masking sounds or on the similarity in rate of change of envelope of the target and masking sounds. This question was addressed by manipulating the ac component of the modulation waveform of the target and masker along part of a continuum that ranged from a sinusoid at one end to a square wave at the other; the transition time (rise/fall time) of the modulator was 6.25, 12.5, 25, or 50 ms. When the target and masker carriers were modulated at the same rate [10 periods per second (pps)], MDI decreased with decreasing rise/fall time of the target and with increasing rise/fall time of the masker. Maximum MDI did not occur when the rise/fall times of the target and masker were equal, as might be expected if MDI were caused by perceptual grouping of the target and masker. This pattern of results is consistent with the idea that, for a given carrier, shorter rise/fall times lead to greater salience. When the target and masker had the same rise/fall times, MDI was strongly affected by a difference in modulation rate of the target and masker and slightly affected by the common rise/fall time of the target and masker. Taken together, these results indicate that both modulation periodicity and rate of change of envelope influence MDI.