Random changes in envelope of AM tones and their detection.

The aim of this investigation was to determine the detection thresholds of sinusoidal and random changes in envelope of amplitude modulated (AM) signals. The envelope changes were obtained as a result of modulation of a pure-tone carrier by chosen types of modulators, which gave either sinusoidal, pseudorandom, or random (Gaussian) amplitude modulation. For pseudorandom and random modulation, the detection thresholds were determined when only modulation depth (m) of AM signals was changed randomly (fm const) or when both modulation depth and modulation frequency (fm) were changed randomly (fm random). The data showed that for low modulation frequency (fm≤64Hz) the detection thresholds for sinusoidal amplitude modulation (SAM), pseudorandom amplitude modulation (PAM) and random amplitude modulation (RAM) overlapped one another in the limit of the standard deviation (SD). However, for higher modulation frequencies, the average threshold for the RAM (fm, const) was about 3dB lower (around fm=128Hz) than that for sinusoidal amplitude modulation. In this case the PAM threshold was similar, within the limit of SD, to the SAM threshold. However, when random changes in amplitude were combined with random changes in frequency, the PAM (fm, random), and RAM (fm random) thresholds decreased about 5 dB relative to the SAM threshold.