Detection of Random Amplitude Modulation

The main objective of this study was to determine the detection thresholds of random amplitude modulation (RAM) as a function of modulation and carrier frequency (f_c ). The RAM was obtained by amplitude modulation (AM) of pure-tone carriers by specially generated sample functions (modulators). The experiment concerned the detection thresholds of AM stimuli for random amplitude changes at constant modulation frequency (f_m ) and simultaneous random changes in amplitude and modulation frequency. The data showed that, the detection thresholds for random amplitude modulation at constant modulation frequency [RAM(f_m const)] tend to be, in a limited range of f_m , 8–14 dB lower than those for sinusoidal amplitude modulation (SAM). The detection thresholds for random amplitude and modulation frequency [RAM(f_m random)] were 3–6 dB lower than the RAM(f_m const) thresholds for high f_c and f_m . An analysis of the responses of a set of bandpass filters (channels) stimulated by the RAM and SAM signals showed that for the middle and high carrier frequencies, there was a good correspondence between the responses of a set of bandpass filters used and the RAM and SAM detection thresholds.