Digital Speech: Coding for Low Bit Rate Communication Systems, 2nd Edition

7

Analysis by Synthesis LPC Coding

7.1 Introduction

The broad classification of speech coding techniques that attempt to reproduce the original speech waveform as best as possible can be split into two basic groups, namely analysis-and-synthesis (AaS) schemes and analysis-by-synthesis (AbS) schemes. Although AaS schemes, such as APC [1, 2], ATC [3] and SBC [4], have been successful at rates around 16kb/s and above, below 16kb/s they can no longer reproduce good quality speech. In addition, AaS coders that have been used at bit rates of around 9.6–16 kb/s can not achieve true toll quality performance (MOS≥4). There are two main reasons for their shortcomings: first, the coded speech is not analysed to see if the coding procedure is operating efficiently, i.e. there is no check on or control over the distortions of the reconstructed speech; and secondly, in adaptive schemes, the errors accumulated from previous frames are not usually considered in the current frame of analysis, hence the errors propagate into the following frames without any form of resetting. In AbS schemes, particularly AbS-LPC schemes [5, 6], these two factors are incorporated in the coding process. In AbS-LPC coding systems, a closed-loop optimization procedure is used to determine the excitation signal, which produces a perceptually optimum synthesized speech signal when used to excite the model filter. It is this closed-loop approach which enables AbS-LPC coding schemes to be far more successful at 4.8 to 16 ...