High-frequency regeneration of base-band vocoders by multi-pulse excitation

In this paper, we propose a new method for the regeneration of the high-frequency band in base-band vocoders. This approach takes advantage of the quasi-harmonic structure of voiced speech in the time domain, while existing techniques capitalize on this structure in the frequency domain (squaring, full wave rectifying, interpolation, etc ...). We show that the high frequency part of the excitation signal, which is usually regenerated by a frequency duplication of the harmonic structure of the base-band, always presents time variations which can be modeled by a pulse/noise combination. The position and amplitudes of the pulses are shown to be critical for the good reconstruction of the speech wave-form, in the sense that a slight jitter can produce an audible roughness. We report simulation results on a complete vocoder implemented at 7.2 kbps.