High-quality harmonic coding at very low bit rates

The paper presents a harmonic vocoder to produce high-quality speech at very low bit rates (below 2 kb/s). Voiced speech is decomposed into forward and backward signals which consist of interpolated harmonics. Unvoiced speech is reconstructed in the time domain with an approach similar to CELP. To remove the “buzzy” quality and avoid the “hoarse” quality, three methods are presented: the randomness of the harmonic phases is controlled according to pitch value and the continuity of synthetic speech is maintained; the spectral envelope determined by the LP model is modified; some noise components can be introduced for voiced synthetic speech. The harmonic vocoder produces quite natural, clear speech. Its perceptual quality is much better than that of the LPC-10 vocoder