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Any Sound has an Envelope and Fine Time Structure

Any sound can be mathematically factored into the product of a slowly varying envelope (also called modulation), and a rapidly-varying fine time structure (also known as carrier). The figure shows an example of this factorization for a filtered speech signal.

Cochlear implant processing strategies such as continuous interleaved sampling (CIS) present only envelope information and discard the fine time structure (Wilson et al., 1991).  Some patients show excellent performance with these processors.  However, performance might be further improved by modifying the processors to deliver fine structure information.  We are thus interested in assessing the relative importance of envelope and fine structure for speech perception.

The Envelope is Important in Speech Perception

Speech intelligibility in noise and reverberation can be predicted from how well envelope information from different frequency bands is preserved (Houtgast and Steeneken, 1973).
In normal-hearing listeners, envelope information from as few as 3-4 frequency bands suffices for speech reception in quiet (Shannon et al., 1995).


There is Scant Information on the Role of the Fine Time Structure in Speech Perception

Speech processed by peak clipping so that broadband envelope information is eliminated remains fairly intelligible (Licklider, 1949). In this condition, speech recognition must be based on the fine structure in a broad frequency band.
The fine time structure of most speech sounds is precisely coded in the temporal discharge patterns of auditory-nerve fibers (Young and Sachs, 1979). The central nervous system is likely to utilize some of this information.


Houtgast, T., and Steeneken, H.J.M. The modulation transfer function in room acoustics as a predictor of speech intelligibility. Acustica 28: 66-73,1973.                                                                                                   

Licklider, J. Effect of amplitude distortion upon the intelligibility of speech. J. Acoust. Soc. Am. 18: 429-434, 1946.                                         

Shannon, R.V., Zeng, F.-G., Kamath, V., Wygonski, J., and Ekelid, M. Speech recognition with primarily temporal cues. Science 270: 303-304, 1995.
Wilson, B.S., Finley, C.C., Lawson, D.T., Wolford, R.D., Eddington, D.K., and Rabinowitz, W.M. Better speech recognition with cochlear implants. Nature 352: 236-238, 1991.
Young, E.D., and Sachs, M.B. Representation of steady-state vowels in the temporal aspects of the discharge patterns of populations of auditory nerve fibers. J. Acoust. Soc. Am. 66: 1381-1403, 1979.