Effects of harmonicity, modulator phase, and number of masker components on modulation discrimination interference

Abstract

The discrimination of the depth of amplitude modulation (AM) of a signal carrier frequency can be disrupted by the presence of other modulated carriers, an effect called modulation discrimination interference (MDI). MDI may be caused partly by perceptual grouping of the signal and masker carriers. If this is so, then MDI may be greater when the masker carriers are harmonically related to the signal carrier (promoting perceptual fusion) than when they are nonharmonically related. These experiments examine whether this is the case. They also examine the effect of the number of masker carriers, the proximity of the masker carriers to the signal carrier frequency, and the relative modulator phase of the masker and signal. Thresholds for discriminating the depth of 10-Hz AM of a 1000-Hz signal carrier were measured in the presence of a masker that consisted of two to six carrier frequencies, with an equal number of carriers above and below the signal carrier frequency. The masker was either unmodulated or amplitude modulated with a modulation index of 0.5 and a frequency of 10 Hz, and the masker carrier frequencies were either harmonically related to the signal carrier frequency or were nonharmonically related. There was little difference in the size of the interference effect between the harmonic and inharmonic maskers. There was also little effect of the number of masker carriers. For the maskers with harmonically spaced carriers, the interference effect was slightly larger when all carriers were modulated in phase with the signal carrier than when the modulator phase was shifted by 90° for each successive carrier. The results suggest that harmonicity does not play an important role in MDI.