Approaches to adaptive transform speech coding at low bit rates

This paper discusses problems of adaptive transform coding schemes at bit rates of 12 kbit/s and below. Objective and subjective performance reductions, like low-pass filtering effects as one of the main sources of perceptual distortion, are investigated and proposals are made how to improve the performance of the coder at low and medium bit rates. Additionally, the needed transmission of side information reduces the efficiency of the scheme. Various methods to lower the rate of this supplementary data signal are given as well as modifications of the scheme which lead to a more easily implemented coder structure.