Efficient harmonic-CELP based hybrid coding of speech at low bit rates

This paper presents an efficient Harmonic-CELP hybrid coder at 2.4 kbps utilizing the well-known characteristics of the Harmonic and CELP coders. According to frame voicing decision, the proposed hybrid coder switches the RP-VSELP coder as a fast CELP in case of unvoiced, or an improved Harmonic coder in case of voiced. The proposed Harmonic-CELP hybrid coder has several features as follows: fast CELP coding, fast harmonic estimation, variable dimension harmonic vector quantization, perceptual weighting including Bark frequency resolution, fast harmonic synthesis, and naturalness control by band voicing. To demonstrate the performance of the proposed hybrid coder, a 2.4 kbps coder has been implemented and compared with 5.3 kbps ACELP and 4.4 kbps IMBE as reference coders. From results of subjective tests, the proposed hybrid coder showed good quality at about half rates of the reference coders.