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Carrell/Opie research

November 3, 1992

Open letter to Tom Carrell and Jane Opie,

I would like to draw the attention of the members of the AUDITORY
list to your interesting finding just published in Perception &
Psychophysics vol.52, no.4.  You used used sentences made of
sine-wave-analog-speech, which you referred to as "time-varying
sinusoidal" (TVS) sentences.  The sentences were amplitude
modulated at various rates (e.g., 50 Hz) that were fixed over the
course of the sentence.  Modulated sentences (50 Hz and 100 Hz)
were rated as more natural than unmodulated ones and more
phonemes in them were correctly identified.  AM of 200 Hz was
worse than 50 or 100 Hz, and even worse than unmodulated
sentences.  Your discussion relates the results to "comodulation
masking release" (CMR).  In your view the common AM applied to
all the component sinusoidal glides helped them to be heard as a
single auditory object.

This is a noteworthy finding.  One thing to recall is that the AM
creates sidebands that are not necessarily related harmonically
to the 100 Hz modulation rate.  At almost every instant, the
signal would be quite inharmonic.  Even so, the perceptual
integration took place.

You mention in your discussion that the next phase of the
research involves looking at the intelligibility of such signals
in the presence of other sounds.  I don't know whether you have
thought of this, but a natural development of this research would
be to present mixtures of pairs of TVS sentences, and to look at
their intelligibility as a function of the AM frequency
separation between the members of a pair.  This would be the
analog of experiments done by Brokx/Noteboom, Scheffers,
Chalikia/Bregman, Assman/Summerfield, and others, in which the F0
difference between members of a pair of mixed speech sounds was
varied.  These researchers used speech that was either natural or
was synthesized so as to have a natural harmonic structure.  Your
method would be able to show whether it was the fact that the two
speech signals were presented on different HARMONIC SERIES that
was crucial or that at least some part of the within-signal
integration derived simply from the fact that all the spectral
regions showed the effects of a common amplitude modulation.  If
an effect of AM difference were found in your inharmonic stimuli,
this would support the results by Bregman, Abramson, Doehring and
Darwin on the fusion of amplitude modulated tones.  This research
found that the fusion of inharmonic complexes was affected by
common (and in-phase) AM.

Congratulations on your imaginative study.

 - Al Bregman

P.S.  Is anybody else doing research that pertains to these
issues?  If so, would you like to get some feedback from the