This paper introduces techniques for reducing the computational complexity of re-encoding mixed speech using LPC-based vocoders, with application to conferencing over VoIP and cellular telephony networks. Our methods exploit a priori knowledge of the LPC synthesis filter coefficients, codebook gains and pitch periods for the compressed source speech signals. Experimental results with a modified G.729 vocoder show an average spectral distortion of 1.61 dB compared to reference mixed signals, 1.34 dB less than that introduced by the full encoder. Furthermore, the proposed techniques introduce no additional decrease in speech quality measured using ITU P.862 (PESQ) and informal MOS tests.

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Conference Conference Record of the Thirty-Eighth Asilomar Conference on Signals, Systems and Computers
Gordy, J.D., & Goubran, R. (2004). Reduced-delay mixing of compressed speech signals for VoIP and cellular telephony. In Conference Record - Asilomar Conference on Signals, Systems and Computers (pp. 2270–2274).