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Multiplication of the instantaneous frequency of a speech signal is studied by computer simulation. Interest centers on a simple means for compressing and expanding the time dimension of the signal, and for scaling the envelope of its short-time spectrum. Simulation of the process indicates that the speech formant dominant in intensity "captures" the multiplication. In the multiplied signal, the frequency of the dominant formant is multiplied, but the frequency spacings between the dominant formant and other formants remain the same as in the original signal. Stretching the time scale of the multiplied signal by the appropriate factor restores the dominant formant to its correct frequency position, but no other formant is correctly restored. By the same result, a scaling distortion of the spectral envelope (for example, as in speech produced in a He-O atmosphere)cannot be restored by instantaneous frequency division of the broad-band signal. A two-format model of the speech signal is used to analyze instantaneous frequency multiplication. Calculations on the model are consistent with spectra obtained from the computer-processed real speech.
Date of Publication: Sep 1969