Pre-echo reduction in the ITU-T G.729.1 embedded coder

Pre-echo is a well-known artefact of transform coding at low bit rates. In this paper we present a new method to address this problem. The input signal is assumed to be coded in two stages: in time domain first, and then in transform domain. This is for instance the case in CELP+transform embedded coding. The first stage reconstructs a signal that is usually free of pre-echo. Therefore transform coding can exploit this reconstructed signal as side information for pre-echo detection and reduction. The proposed method is implemented as an adaptive limiter at the decoder side and does not need transmission of auxiliary data. It is part of the recently standardized ITU-T G.729.1 coder, in which it is used in two separate subbands. Experimental test results show that this method has a significant impact on quality in G.729.1 with very small complexity.