EFFECTS OF CARRIER FREQUENCY AND BACKGROUND-NOISE ON THE DETECTION OF MIXED MODULATION

This article is concerned with the mechanisms underlying the detection of amplitude modulation (AM), frequency modulation (FM), and mixed modulation (MM), i.e., simultaneously occurring AM and FM. In a previous study [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119-3131 (1992)], psychometric functions were measured for the detection of AM alone and FM alone, using a 10-Hz modulation rate and a 1-kHz carrier frequency. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. The detectability of the combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently; Significant effects of relative modulator phase were found when detectability was relatively high, but these effects were not correctly predicted by either of two excitation-pattern models considered. The first experiment reported here was similar to the earlier experiment, but performance was compared for carrier frequencies of 1 and 6 kHz; at the latter frequency, neural synchrony to the stimulus fine structure (phase locking) does not occur. The results at both carrier frequencies were similar to those of our earlier experiment, suggesting that the presence or absence of phase-locking information plays little role in the detection of MM. The second experiment was again similar, but bands of noise were used to mask selectively either the upper or lower side of the excitation pattern of the modulated carrier. The phase effects in this case were in the direction predicted by excitation pattern models. The overall pattern of the results could be predicted reasonably well using a multichannel excitation pattern model based on the assumption that listeners use an unweighted sum of decision variables across all suprathreshold channels with a positive signal-to-noise ratio.