Modulation discrimination interference for narrow-band noise modulators.

The discrimination of the depth of amplitude modulation of a signal carrier frequency can be disrupted by the presence of other modulated carriers (maskers), an effect called modulation discrimination interference (MDI). This paper examines whether MDI is influenced by the similarity in the envelope pattern of the signal and masker. A narrow-band noise (centered at 10 Hz) was used as the signal modulator. The first experiment used masker modulators that were narrow-band noises identical in spectral characteristics to the signal modulator. The masker modulators were either identical to the signal modulator, negatively correlated with it, or uncorrelated with it. The amount of MDI was similar for all three cases. In experiment 2, the masker was sinusoidally modulated at rates varying from 2 to 64 Hz. The results showed a broad tuning for modulation rate, comparable to that found for sinusoidal modulation of the signal. The maximum amount of MDI produced by the sinusoidally modulated masker was similar to that produced by the noise-modulated maskers when modulation depths were expressed as their root-mean-square values. It is concluded that similarity of the moment-by-moment envelope pattern of the signal and masker modulators plays only a minor role in MDI, although similarity in modulation rate has some influence.