Experimental comparison of parameter interpolation in LPC vocoders
Yoh’ichi Tohkura · The Journal of the Acoustical Society of America · 1979
LPC parameter interpolation is effective in smoothing synthesized speech roughness. To choose an appropriate interpolation parameter, LPC parameter interpolation properties were compared experimentally, using: (1) spectral distortion between interpolated spectrum and actual spectrum, and (2) spectral distance relationships among interpolated spectrum and two boundary spectra. Both k parameter and LAR (log area ratio), which are used as transmission parameters because of their good quantization properties, sometimes provide an undesired speech spectrum interpolation. These two parameters cannot interpolate formants correctly, in particular, when the formant bandwidth shows a big change between the two boundary spectra. On the other hand, α parameter provides better speech spectrum interpolation than the former two. Listening tests show that unnatural formant transition due to the k-parameter interpolation can be perceived as slurring and gliding, which are hardly perceived in the α-parameter interpolation.