Discrimination of fundamental frequency of synthesized vowel sounds in a noise background

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Abstract
An experiment was carried out, investigating the relationship between the just noticeable difference of fundamental frequency (jndfo) of 3 stationary synthesized vowel sounds in noise and the signal-to-noise ratio [S/N]. To this end the S/N ratios were measured at which listeners could just discriminate a series of changes in fo in the range from 10 to 0.5%. Similar measurements were obtained for pulse trains and for pure tones as a reference for the results. A measure of S/N ratio based on an approximation of the critical bandwidth appeared to provide a fairly good predictor of the masked threshold of each signal, measured in a 2nd experiment. Using this measure, it was found that a given change in the fundamental of a pulse train could be discriminated at a lower S/N ratio than in a pure tone with a frequency equal to that fundamental. The results for the vowel sounds were found to be in between those for a low-frequency pure tone and those for a pulse train. Owing to the signal-generation method (viz., changing fo by changing the sampling frequency), three cues could in principle be used to discriminate a change in the fundamental of a vowel. A change in the residue pitch, a change in the pitch of a single prominent harmonic, or a change in the spectral envelope of the signal. It can be inferred from the results that the subjects used that particular cue which yielded best performance. Which cue was optimal depended not only on the vowel but also on fo and on the presented change in fo. The pitch of a single harmonic may be the cue most often used. Another interesting result is that changes in fo greater than about 5%, could for each signal be discriminated when the signal was just above masked threshold.

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