High quality speech at low bit rates makes code excited linear prediction (CELP) the dominant choice for a narrowband coding technique despite the susceptibility to packet loss. One of the few techniques which received attention after the introduction of CELP coding technique is the internet low bitrate codec (iLBC) because of inherent high robustness to packet loss. Addition of rate flexibility and scalability makes the iLBC an attractive choice for voice communication over IP networks. In this paper, performance improvement schemes of multi-rate iLBC and its scalable structure are proposed, and the proposed codec enhanced from the previous work is re-designed based on the subjective listening quality instead of the objective quality. In particular, perceptual weighting and the modified discrete cosine transform (MDCT) with short overlap in weighted signal domain are employed along with the improved packet loss concealment (PLC) algorithm. The subjective evaluation results show that the speech quality of the proposed codec is equivalent to that of state-of-the-art codec, G.718, under both a clean channel condition and lossy channel conditions. This result is significant considering that development of the proposed codec is still in early stage.Index Terms-Discrete cosine transform (DCT), internet low bitrate codec (iLBC), packet loss, scalable coding, speech coding, voice over Internet protocol (VoIP).

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The internet Low Bit-rate Codec (iLBC) inherently possesses high robustness to packet loss which is one of the essential properties of Voice over Internet Protocol (IP) applications. Another important feature is the rate flexibility, which allows the speech codec to adapt its bit rate to constantly changing network condition. Previously, the multi-rate operation of the iLBC was enabled by utilizing the Discrete Cosine Transform (DCT) and entropy coding. In this paper, various approaches to improve performance are presented. The simulation results show that when all the improvement schemes are combined, the performance is improved at all the bit rates compared to the previous results despite the fact that the Huffman table structure is significantly simplified.

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