CELP coding at variable rate

This paper describes the general principle of variable rate applied to CELP coding and presents an algorithm conceived for mobile applications based on CDMA technique. The speech coder operates at an average rate of around 6 kbit/s with a peak rate of 16 kbit/s, providing a quality close to the one of the G.728 standard. The algorithm is based on a multi-rate CELP whose operating mode is selected by a combination of open-loop speech classification and closed-loop quality evaluation. The scheme is not fully optimized for the average rate, but attention has mainly been paid to the classification algorithm having the constraint to keep the algorithmic delay equal to 10 ms.

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