Abstract
Zwicker [Acustica 6, 356–381 (1956)] proposed that amplitude modulation (AM) and frequency modulation (FM) are coded by a common mechanism. To test this, the detection of simultaneously occurring AM and FM is discussed. In a two-alternative forced-choice task, thresholds for detecting AM alone were determined. Then, thresholds for detecting FM were determined for stimuli which had a fixed amount of AM in the signal interval only. The amount of AM was always less than the threshold for detecting AM alone. The FM thresholds depended significantly on the magnitude of the coexisting AM. For low modulation rates (4, 16, and 64 Hz), the FM thresholds did not depend on the relative phase of modulation for the FM and AM. For a high modulation rate (256 Hz) strong effects of modulator phase were observed, which can be explained by assuming that detection of modulation at high frequencies is based on detection of the lower sideband in the modulated signal’s spectrum. In a second experiment, psychometric functions were measured for the detection of AM alone and FM alone. For each type of modulation, d’ was approximately a linear function of the square of the modulation index. Application of this finding to the results of experiment 1 suggested that, at low modulation rates, FM and AM are not detected by completely independent mechanisms.
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