Detection of asynchronicity in the amplitude modulation domain

A just noticeable time delay (JNTD) between the onset of a
single sinusoidal amplitude modulation (AM) and a complex modulation applied to
the same carrier was measured in this study. The carrier was a 4-kHz tone and
the modulator was a five-component multitone complex. In the first experiment,
four of five components had constant frequencies, i.e. 160, 170, 180, 190 Hz and
they were turned on synchronously (synchronous components) in the middle of the
carrier duration. The frequency of the fifth component (asynchronous one) varied
from 10 to 150 Hz and it was turned on earlier than the synchronous ones. In the
second experiment, the asynchronous component was situated in the centre of the
synchronous components' spectrum; its frequency was constant and equal to 100
Hz. The spectral separation between the asynchronous component and the
synchronous ones of the modulator varied. The results, i.e. the just noticeable
time delay between the onset of a single sinusoidal amplitude modulation and a
complex modulation (or asynchrony threshold), are analogous to those obtained in
the audible frequency domain. They can be interpreted on the basis of the
auditory system model containing a bank of modulation filters. It seems that two
separate mechanisms are responsible for the JNTD between the onset of the single
component modulation and the complex modulation. The first one results from an
interaction between all the components of a modulator passing a single
modulation filter tuned to the frequency of the asynchronous component. This
sort of interaction (or masking) was most effective when the spectral separation
between the asynchronous component and the synchronous ones was the smallest
one. With an increase in this separation, a significant decrease in the
asynchrony thresholds was observed. The second mechanism determining the
obtained asynchrony thresholds is based on the uncertainty principle: modulation
filters with good frequency selectivity, i.e. filters tuned to low modulation
rates, are characterised by a poor time resolution. Thus, in the case of the
lowest frequencies of the asynchronous component the subjects' performance would
be relatively poor even when there was a significant spectral interval between
this component and the synchronous ones. As in the audible frequency domain, the
pattern of the asynchronicity thresholds was related to the modulation filter
bandwidth. The obtained results suggest the bandwidth of the modulation filters
whose Q factor should be close to 1 or less.