Improving the Quality of Low Bitrate LPC Speech Codec Using Gamma-chirp Filterbank

In this thesis, two methods for getting higher speech quality in low bit-rate LPC coder in comparison with the original LPC coder were presented. In order to improve the quality of the speech, the embedded method which can take the place of the traditional synthesis algorithm in LPC coders and the post-processing method that also can be used as the last stage in the LPC-coder based systems were proposed. In the embedded method, the result of MOS test of the synthesized speech increased by 0.8 and in the post-processing method the result of MOS test of the synthesized speech increased by 0.5 in comparison with the original LPC with the cost of increasing the complexity by 5 in terms of MIPS for the embedded method and 16 in terms of MIPS for the post-processing method. Even though the complexity increased in both cases, comparing the bit-rate and the result of MOS test with the other coders in their class (coders that operating at 2.4 kbits/s), the embedded method is still superior while considering the less-complex and high-quality algorithm.