This paper presents an enhancement layer coming on top of G.729 Annex A codec. This enhancement layer consists of an optimization of the algebraic fixed codebook pulses that were selected by the Annex A codec during the fixed codebook search. The pulses selected by the ACELP codec have all the same absolute amplitude. Our method optimizes each of these amplitudes in order to increase the quality. This is a low-complexity method that allows the introduction of an intermediate bitrate in ACELP-based scalable coder. On top of this pulse optimization method that requires 1.6 kbit/s we introduce a second fixed codebook search with a limited number of pulses (2) and a bitrate of 2.4 kbit/s.
['M. De Meuleneire', 'Martin Gartner', 'Stefan Schandl', 'H. Taddei']
An Enhancement Layer for ACELP Coder