This paper proposes a variable rate, toll quality CS-ACELP coder that uses coding modes compatible with the three 6.4, 8 and 11.8 kbit/s coding schemes standardised by ITU-T in G.729. In particular we propose a hybrid multimode/ multirate (M3 R) codec presenting four coding categories, with an average bit rate ranging between 3 and 8 kbit/s, that adapts the rate to changes in network conditions. The results indicate that both for the current Internet and for future scenarios hybrid M3 R adaptive coding seems a valid solution to achieve the quality currently guaranteed by PSTN networks.
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