EFFECT OF THE RELATIVE PHASE OF AMPLITUDE-MODULATION ON THE DETECTIONOF MODULATION ON 2 CARRIERS

This study examined how effectively information about amplitude modula tion (AM) on two carriers is combined, and whether the detection of AM depends on the relative phase of the AM across carriers. Psychometric functions were measured for detecting 5-Hz sinusoidal AM of carriers with frequencies 1100 and 1925 Hz, with a mean level 65 dB SPL for eac h carrier. The carriers had a duration of 400 ms with 50-ms raised-cos ine ramps on either side of this. A single cycle of 5-Hz sinusoidal AM (200 ms in duration) was imposed on the temporal center of the stimul us, with 100-ms steady-state fringes before and after the modulation. The modulators for the two carriers were either in phase or in antipha se. The modulation of each carrier was equally detectable, as determin ed in a preliminary experiment. A continuous pink noise background was used to mask the outputs of auditory filters tuned between the two ca rrier frequencies. There was no effect of relative modulator phase. Ho wever, performance was consistently better than predicted from the ass umption that information about AM from the two carriers is processed i ndependently and combined optimally. The results are discussed in term s of (1) predictions using Dau's ''modulation filter bank model'' [T. Dau et al., in Psychoacoustics, Speech and Hearing Aids, edited by B. Kollmeier (World Scientific, Singapore, 1996), pp. 45-48], and (2)the fact that relative modulator phase does have an effect on the detectio n of frequency modulation on two carriers, as found by Furukawa and Mo ore [J. Acoust. Soc. Am. 100, 2299-2311 (1996)]. (C) 1997 Acoustical S ociety of America. [S0001-4966(97)04012-5].