LOW-COST SMART TRANSCODING ALGORITHM BETWEEN ITU-T G.729 (8 KBIT/S) AND 3GPP NB-AMR (12.2 KBIT/S) (ThuPmPO1)

Author(s) : Mohamed Ghenania (France Telecom R&D, FRANCE) Claude Lamblin (France Telecom R&D, FRANCE)

Abstract : Networks interconnection causes interoperability problems between different speech coding formats. Today, tandem transcoding solutions (decoding/re-encoding) are generally used in communication chains. However intelligent transcod-ing solutions which exploit similarities between formats have been recently proposed in order to overcome tandem draw-backs (computational complexity, algorithmic delay, speech degradation). This paper gives an overview of these intelli-gent transcoding methods for CELP coders. Then, a low-cost intelligent transcoding algorithm between ITU-T G.729 (at 8 kbit/s) and 3GPP NB-AMR (at 12.2 kbit/s) is proposed. It is composed of four parts corresponding to the four CELP parameters conversions: LSP coefficients, fractional pitch lags, fixed codevectors and gains. A novel and computation-ally efficient method is described here for the fixed codevec-tors. The ACELP search is strongly focused on privileged positions, what considerably reduces the number of tested combinations. Objective and subjective quality tests show that this smart transcoding algorithm achieves a quality very close to that of the tandem while strongly reducing complexity with a shorter algorithmic delay.

Menu