Translation of Divers' Speech Using Digital Frequency Warping

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.