Transparent speech quality has not been achieved at low bit rates, especially at 2.4 kbps and below, which is an area of interest for military and security applications. In this paper, strategies for low bit rate sinusoidal coding are discussed. Previous work in the literature on using metaframes and performing variable bit allocation according to the metaframe type is extended. An optimum metaframe size compromise between delay and quantization gains is found. A new method for voicing determination from the LPC shape is also presented. The proposed techniques have been applied to the SB-LPC vocoder to produce speech at 1.2/0.8 kbps, and compared to the original SB-LPC vocoder at 2.4/1.2 kbps as well as an established standard (MELP) at 2.4/1.2/0.6 kbps in a listening test. It has been found that the proposed techniques have been effective in reducing the bit-rate while not compromising the speech quality.
Bibliographic reference. Unver, Emre / Villette, Stephane / Kondoz, Ahmet (2009): "Joint quantization strategies for low bit-rate sinusoidal coding", In INTERSPEECH-2009, 2607-2610.