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Abstract

A multi-rate speech codec supports a plurality of encoding bit rate modes by adaptively selecting encoding bit rate modes to match communication channel restrictions. In higher bit rate encoding modes, an accurate representation of speech through CELP (code excited linear prediction) and other associated modeling parameters are generated for higher quality decoding and reproduction. The encoder produces a series of LSF (line spectral frequencies) vectors. For filter stability, each LSF vector comprises an ascending sequence of LSF values. Occasionally, pairs of LSF values are produced (or become through an introduction of channel error) out of ascending order. In response, the encoder performs frame erasure, LSF concealment or pair flipping. With a relatively large number of out of order pairs, frame erasure is applied. With a single out of order pair, within the LSF vector, the pair are flipped. Likewise, with two pairs out of order, the previous LSF vector is used to generate the current LSF vector using concealment. The performance of frame erasure, LSF concealment or pair flipping is also performed in the decoder as well as in the encoder in some embodiments.

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First Claim