Effects of envelope-vocoder processing on F0 discrimination and concurrent-vowel identification.
OBJECTIVE
The aim of this study was to examine the effects of envelope-vocoder sound processing on listeners' ability to discriminate changes in fundamental frequency (F0) in anechoic and reverberant conditions and on their ability to identify concurrent vowels based on differences in F0.


METHODS
In the first experiment, F0 difference limens (F0DLs) were measured as a function of number of envelope-vocoder frequency channels (1, 4, 8, 24, and 40 channels, and unprocessed) in four normal-hearing listeners, with degree of simulated reverberation (no, mild, and severe reverberation) as a parameter. In the second experiment, vowel identification was measured as a function of the F0 difference between two simultaneous vowels in six normal-hearing listeners, with the number of vocoder channels (8 and 24 channels, and unprocessed) as a parameter.


RESULTS
Reverberation was detrimental to F0 discrimination in conditions with fewer numbers of vocoder channels. Despite the reasonable F0DLs (<1 semitone) with 24- and 8-channel vocoder processing, listeners were unable to benefit from F0 differences between the competing vowels in the concurrent-vowel paradigm.


CONCLUSIONS
The overall detrimental effects of vocoder processing are probably are due to the poor spectral representation of the lower-order harmonics. The F0 information carried in the temporal envelope is weak, susceptible to reverberation, and may not suffice for source segregation. To the extent that vocoder processing simulates cochlear implant processing, users of current implant processing schemes are unlikely to benefit from F0 differences between competing talkers when listening to speech in complex environments. The results provide further incentive for finding a way to make the information from low-order, resolved harmonics available to cochlear implant users.
Qin
Michael K
MK
Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. qin@nsmrl.navy.mil
Oxenham
Andrew J
AJ
D000161:Acoustic Stimulation; D000328:Adult; D000704:Analysis of Variance; D003054:Cochlear Implants; D003198:Computer Simulation; D004056:Differential Threshold; D006801:Humans; D010897:Pitch Discrimination / Q000502:physiology*