Translation of Divers' Speech Using Digital Frequency Warping

Abstract

The composition and pressure of breathing mixtures used by divers translate frequency components of their speech nonlinearly. Recently, a method of forming a new sampled time function from a given finite-duration sampled time function in which the spectra of the two are related through a nonlinear warping in frequency has been discussed [A. Oppenheim, D. Johnson, and K. Steiglitz, “Computation of Spectra with Unequal Resolution Using the Fast Fourier Transform,” Proc. IEEE 59, 299 (1971)]. The technique is based on the expansion of the original time function in terms of a set of functions related to Laguerre functions. The coefficients in the expansion, when interpreted as samples of a new time function, have a Fourier transform that is equal to that of the original function with a nonlinear transformation in frequency. These coefficients are obtained by processing the original signal with a second-order network in cascade with a chain of first-order all-pass networks. To apply the technique to translation of divers' speech, individual pitch periods are processed by the above network. These modified pitch periods are than reconvolved with a train of pitch pulses to synthesize the translated speech.