Detection and discrimination of mixed modulation signals at low-modulation frequencies

Abstract

Modulation detection thresholds were obtained for a fixed phase difference between the carrier and sidebands and for a fixed amplitude ratio between the lower and upper sidebands. These are three-component signals that, in general, contain both amplitude and instantaneous frequency fluctuations. In a manner similar to Moore and Sek [J. Acoust. Soc. Am. 92, 3119–3131 (1992)], psychometric functions were obtained for pure AM detection and for pure FM detection separately, and predictions made for the thresholds of the general three-component signals. The best fit to the data was obtained when the d’s for AM and FM were added in a linear manner, i.e., d’TOT=d’AM+d’FM. Thresholds for discrimination were then obtained for both AM and FM signals. The standard was a three-component signal with fixed carrier phase and modulation depth set to the detection threshold measured in the first experiment. Discrimination thresholds were elevated and were independent of the standard’s carrier phase, i.e., the ratio of AM to FM in the standard. This implies a common decision statistic for both AM and FM signals near threshold. [Work supported by NIDCD Grant No. DC00683.]