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Abstract: Speech coders operating at low bit rates necessitate efficient encoding of the linear predictive coding (LPC) coefficients. Line spectral frequencies (LSF) parameters are currently one of the most efficient choices of transmission parameters for the LPC coefficients. In this paper, an optimized trellis coded vector quantization (TCVQ) scheme for encoding the LSF parameters is developed. When the selection of a proper distortion measure is the most important issue in the design and operation of the encoder, an appropriate weighted distance measure has been used during the TCVQ construction process. Using this distance, we will show that the LSF TCVQ encoder performs better than the encoder conceived with the unweighted distance and a reduction of about 1–2 bits/frame is obtained while maintaining the same performance. We further applied the TCVQ encoder system for encoding the LSF parameters of the US federal standard (FS1016) 4.8kbps code excited linear prediction (CELP) speech coder. At lower bit rates, our objective and subjective evaluation results show that the incorporated LSF TCVQ encoder performs better than the 34 bits/frame LSF scalar quantizer used originally in the FS1016 coder. The subjective tests reveal also that the 27 bits/frame scheme produces equivalent perceptual quality to that when the LSF parameters are unquantized. [Copyright &y& Elsevier]